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of summer added to that incident on it during the similar portion of winter be denoted by 2, and suppose

that the excess of the heat incident in the portion of THE ASTRONOMICAL THEORY OF THE

summer above that incident in the similar portion of

winter be denoted by 2a ; then it is clear that i ta is GLACIAL PERIOD.

proportional to the amount of heat received by the The Cause of an Ice Age. By Sir Robert Ball, Astro specified area during the specified portion of its summer,

nomer-Royal for Ireland. Pp. 180. (London : Kegan and I - a is proportional to the amount of heat received Paul and Co., 1891.)

by the area during the similar portion of winter. Thus THIS THIS book gives a popular account of the theory of we may say that the contrast between the summer and

Adhémar and Croll as to the causes of glacial winter supplies of heat (for given area and given periods in geological history.

portions of summer and winter) is represented by the The author's power as a popular expositor is well fraction (1 + a) = (1 - a). known, and this little book shows him at his best. He knows when to drive a point home, and yet is never sphere, and the whole of summer and winter, are contedious in his reiteration But he has given here some sidered, and Sir Robert Ball shows that a is then equal thing more than a lucid explanation, for he makes a to 2 sin 23°27' = ; (1 + a) = (1 - a) is found to be valuable contribution to the subject, and the book may almost exactly as 5 to 3. Using percentages he gives be read with advantage by those who are already the ratio as 63 to 37, but the simple numbers 5 to 3 acquainted with the literature bearing on the theory.

afford a closer approximation to accuracy. The theory itself may be sketched in outline as It is clear that if the specified portions of summer and follows:

winter embrace the solstices, and if the specified area is It is known that, under the perturbations of Venus and tropical, a will be small, and if it is polar it will be large. Jupiter, the eccentricity of the earth's orbit varies within The fraction (1 +a)= (1 a) continually increases as certain limits. When the eccentricity is large, and when we go northward, and it may be taken as a measure of the precession of the equinoxes brings the perihelion to the severity of a climate. It is quite uncertain how far near the middle of, say, the northern winter, the annual the climate of any one place depends on the heat supplies supply of solar heat is so distributed that there will be a of the whole hemisphere on which it lies, and therefore it glacial period in the northern and a mild climate in the is uncertain how large an area and how long a season southern hemisphere. Two or three maxima of glacia- we ought to take into consideration in the present intion and mildness will usually succeed one another at vestigation. But I should have thought it legitimate, in intervals of 10,500 years, because the eccentricity varies treating of the causes of glaciation, only to consider the with extreme slowness. When the eccentricity is small, semi-annual heat supply of a polar cap, comprising, say, as at present, a moderate climate will prevail in both all the area north of latitude 30°; thus would have made hemispheres, whatever be the position of the perihelion. (1 + x) = (1 - a) much greater than 5 to 3. It does not

The keynote of Sir Robert Ball's presentation of this seem to me, however, that we are bound to find an theory is given in a short mathematical appendix. I am answer to this almost insoluble problem. disposed to dissent to some extent from the manner in So far we have considered the supply of heat whilst which this view is set forth, but the general argument the earth describes so many degrees of longitude round will, I think, do much to convince the scientific world of the sun, but climate depends on the supply of heat during the truth of the theory, even where Croll's more elaborate a given time. discussions failed to do so.

When the earth's orbit is circular, summer and winter I will now give a paraphrase of the argument, and will are of equal length, and so also are similar portions of point out where it appears to me open to objection. summer and winter ; thus the two ways of estimating

The time taken by the earth to describe a degree of the heat supply coalesce, and the contrast between the longitude round the sun varies as the square of its dis- summer and winter daily supplies of heat is also repretance from the sun, and the intensity of solar radiation sented by the fraction (1 + a) = (1 - a). The present varies inversely as the square of the same distance. condition of affairs differs but little from this standard Hence the amount of heat received by the whole earth case, and we know that the contrast between the summer during the description of a degree of longitude is and winter daily supplies of heat is such as to produce constant.

certain known climates, differing according to latitude. Let the year be divided into only two seasons, viz. the northern summer or southern winter when the sun is

*181+be the sun's hour angle at sunset on any day in summer, and

on the corresponding day in winter, and if the sun's parallax on those days be north of the line, and the northern winter or southern proportioual to E, then it is easy to show that the amount of heat summer when the sun is south of the line. Also let received by unit area in the course of the day is proportional to

(1+E(0 + coto) + 3#] sin 8 sina, similar days in summer and winter be defined as days on

where £8 is the sun's declination, + in summer and - in winter, and à is which the sun sets (say at Greenwich) as much after the latitude of the place of observation.

It follows that, what is called in the text, the contrast for uvit area in 6 p.m. as before 6 p.m. ; similar parts of summer and latitude A, for this pair of days iswinter will mean parts limited by similar days.

( + E)”, where a = Now consider the solar heat incident on any specified

2(0 + coto) area of one hemisphere, during any specified portion of

The expression for the heat supply on unit area during any portion of

summer or winter involves elliptic integrals, which might be given if it the summer and during the similar portion of the winter. were worth while.

A triple integral is required to express the heat supply of any specified Suppose that the heat incident on the area in the portion !

area during any specified portion of the year.

I + a


The question we have to ask is, If the orbit becomes he tries to trace in detail the action and reaction of the eccentric, how will the contrast of daily supplies be astronomical cause, for in doing so he is led to maintain affected?

various theses which are not susceptible of proof, and are In order to answer this, let us go at once to the even highly doubtful. He thus takes as the central point extreme, when the eccentricity of orbit is a maximum. of his position one at which it appears to me to be We learn that if aphelion is at midsummer, summer will weakest. In 1886 I wrote :be 199 days, and winter 166 days; and the converse is

“Adequate criticism of Mr. Croll's views is a matter of true when the perihelion is at midsummer.

great difficulty, on account of the diversity of causes Since 199 is to 166 nearly as 6 to 5, we see that with which are said to co-operate in the glaciation. In the midsummer perihelion there are 5 days of summer to 6 case of an effect arising from a number of causes, each of winter, and with midsummer aphelion there are 6 of of which contributes its share, it is obvious that if the

amount of each cause and of each effect is largely consummer to 5 of winter.

jectural, the uncertainty of the total result is by no means Hence, with midsummer perihelion, the short summer

to be measured by the uncertainty of each item, but daily supply of heat may be taken as proportional to is enormously augmented. Without going far into }(1 + a), and the long winter daily supply as propor- details, it may be said that these various concurrent tional to d(i - a). Hence the contrast between the causes result in one fundamental proposition with regard short summer and long winter daily supplies is repre- the whole argument. That proposition amounts to this,

to climate, which must be regarded as the keystone of 6(1 +a). sented by

; that is to say, the standard contrast that climate is unstable. 5(1 a)

“ Mr. Croll holds that the various causes of change of is augmented in the ratio of 6 to 5. Next, with mid- climate operate inter se in such a way as to augment their summer aphelion, the long summer daily supply of heat several efficiencies. Thus, the trade-winds are driven by may be taken as proportional to ](1 + a), and the short the difference of temperature between the frigid and winter daily supply as proportional to }(í – a). Hence torrid zones, and if from the astronomical cause the

northern hemisphere becomes cooler, the trade-winds on the contrast between the long summer and short winter that hemisphere encroach on those of the other, and the

5(1 +a) daily supplies is represented by that is to say, part of the warm oceanic current, which formerly flowed

6(1 – a)

into the cold north zone, will be diverted into the southern the standard contrast is diminished in the ratio of 5 to 6. hemisphere. Thus the cold of the northern hemisphere

In the first case, the heat supply is less evenly dis- is augmented, and this in its turn displaces the iradetributed through the year than at present, and we have a

winds further, and this again acts on the ocean currents,

and so on; and this is neither more nor less than inmuch more severe climate; in the second, it is more stability. evenly distributed, and we have a much milder one. It “ But, if climate be unstable, and if from some of those follows also that, if we compare the two extreme cases temporary causes, for which no reasons can as yet be together instead of both with the mean case, the change assigned, there occurs a short period of cold, then surely of contrast is represented by the ratio of 62 to 5, or of

some even infinitesimal portion of the second link in the

chain of causation must exist ; and this should proceed, 36 to 25.

as in the first case, to augment the departure from the I must refer the reader to the able discussion in the book original condition, and the climate must change."* of the effects which we have reason to suppose would flow

I see no reason to depart from what I said five years from a change of contrast represented by the numbers 36 to 25; and it must suffice to say here that it seems enough ago, but I now learn from this book how it is that Croll to explain on the one hand the occurrence of the glacia- more forcible proof of it may be contained in a short

mistook the strong points of his own theory, and that a tion of England, and on the other hand the occurrence of work than in an elaborate volume. After expressing this sub-tropical plants in Greenland, Now, the above seems to me to be substantially the opinion, it is but fair to quote and indorse the following

passage (p. 112) on Croll's famous work on "Climate and argument in the book, but I dissent from the stress laid

Time": on the numerical determination of the quantity a.

On p. 90 Sir Robert says :

“I was greatly struck,” says the author,“ by this work

when I first read it many years ago. Subsequent ac“ This theory will be entirely misunderstood unless the quaintance with this volume :. has only increased facts signified by these numbers (the evaluation of (1 + a) my respect for the author's scientific sagacity, and my = (1 – a)) are borne in mind. No one can discuss the admiration for the patience and the skill with which he astronomical theory of the Ice Age unless the figures 63 has collected and marshalled the evidence for the theory and 37 (5 and 3 are more accurate) form a portion of his that he has urged so forcibly." consciousness, and the refrain of his every argument.”

There are a few other points in the "Ice Age,” not It may be admitted that it might have been more difficult involved in the main line of argument, on which I should to present the argument in a popular form without assigning like to comment. a numerical value to a, but Sir Robert Ball is fully equal

The method adopted of stating the disturbing forces to such a task; and I contend that the numerical value of of the planets on the earth appears to me unduly sensaa is beside the mark, even if a valué, appropriate to the tional. We learn (p. 74) that the disturbing force of investigation in hand, were attainable.

Venus is 130 million million tons, and it is impossible After presenting his own view of the question, Sir not to be impressed with the magnitude of the force. Robert Ball says (p. 134) that Croll does not seem to

But if we had been told that the disturbing force on each have been really aware of the full strength of the astro-pound of the earth's mass was only 1/7000 of a grain, nomical theory, and in this I entirely agree. Croll, in

1 Ball (p. 134) fails to see the force of this argument, fact, rather weakens than strengthens his position when

2 Brit. Assoc. Report, 1886, Address to Section A.

we should have been equally impressed with its insigni- certainly commend the book better than any description ficance-and yet the two statements are virtually the of its contents. First, then, as a bit of serious biology, same. In fact, the unscientific reader is not likely to we will give a passage on the nesting-habits of the realize the prodigious number of pounds in the earth's ostrich. mass.

“The nest is scooped out in the sand, and two or It may be remembered that Croll computes, in “Climate

three hen-birds may combine to lay their eggs in it, to and Time," the value of the eccentricity of the earth's the number of about twenty. It is said, and that by orbit from Leverrier's formulæ, and endeavours thus to several observers, that, besides the eggs laid in the nest, assign actual dates to various glacial periods. Now, Sir each hen lays several in the neighbourhood, and that Robert Ball very justly will not admit that our knowledge these are broken when the young are hatched, and the of the solar system is accurate enough to justify the ap- regard these statements with some suspicion. The hens

contents are given them as food. But I am inclined to plication of these formulæ to the enormously long inter- take turns in sitting during the day, never leaving them vals of time involved. I think, however, that it would long in the scorching heat of the South African sun. But have been of interest to the general reader to be told in at sun-down the cock-bird takes charge of the eggs, and round numbers the kind of intervals which we have

sits throughout the night. He is not going to be bound reason to believe may have elapsed between one glacial by any conventional rules as to the proper division of

labour between the sexes. period and the next; in fact, to learn whether the intervals

“A very careful observer, Mrs. Barber, has drawn are probably millions of millions of years, or hundreds attention to the fact that the indistinct grey colours of of thousands of years. I conjecture that our knowledge the hen ostrich are wonderfully adapted for purposes of of the planetary movements is sufficient to enable us to concealment. These birds while upon their nests do not say that such an interval may be something comparable erect their necks, but place them at full length in front of with 200,000 years. I should like, further, also to ask

them upon the ground; and the grey-brown body might,

Mrs. Barber says, be easily mistaken for some other Sir Robert Ball whether he does not consider that Le- object, such as, for instance, an ant-hill, so common verrier's formulæ may probably be relied on to give at on the plains of South Africa. That so large a bird least a rough approximation for about 100,000 years in should be inconspicuous may seem surprising; but the past; and, if this is so, whether we might not conclude, another observer, Mr. W. Larden, tells us of his exwith fair probability, that the last glacial period occurred perience with the rhea, or South American ostrich, which about that number of years ago ? I must, however, dis

seems quite to bear this out. 'One day,' he says, “I

came across a rhea in a nest that it had made in the dry claim any special knowledge on this point, and I should

weeds and grass. Its wings and feathers were loosely gladly see his opinion, or that of any other physical arranged, and looked not unlike a heap of dry grass ; at astronomer, on the matter.

any rate the bird did not attract my attention until I was In conclusion, I wish to say that, in making the fore

close on him. The long neck was stretched out close going criticisms and suggestions, I have no intention of along the ground, the crest feathers were flattened, and

an appalling hiss greeted my approach. It was a pardondisparaging the book; on the contrary, it is only because

able mistake if for a moment I thought I had come across it is a good book that it is worth while to consider it a huge snake, and sprang back hastily under this imcarefully. I have found it profoundly interesting from pression.' end to end, and I am convinced that it will be widely “ The male oštrich, with his splendid black and white read, as it deserves to be.


feathers, would not be thus inconspicuous by day. But he sits at night, and his strength and pugnacity would induce most other creatures to let him alone. Mrs.

| Barber describes the careful manner in which the female POPULAR ZOOLOGY.

bird approaches the nest in the morning, when her turn

for incubation has come. In wide circles, and apparently Amrit Sketches. By C. Lloyd Morgan, F.G.S., Prin. in the most unconcerned manner, she will feed round the cipal at University College, Bristol. (London : Edward | nest, never once looking towards it, but gradually apArnold.)

proaching nearer and nearer to it by diminishing each THIS is one of those delightful books of natural tions have brought her to within a yard or so of the nest,

circle as she walks round, until at length her perambulaTHI

history for young people which their parents never when the birds will rapidly change places, the male had the benefit of, and for which they ought to be duly walking swiftly away, and not remaining in the vicinity thankful. A competent naturalist here gives them the of the nest during the day. The wonderful rapidity with result of his full and varied knowledge, but gives it so it is impossible to see the exact manner in which it is

which the change is effected is perfectly astonishing, and blended with imagination and humour, so intermingled done, so swiftly do they change places.” with anecdote and personal adventure or observation, as to make it a real story-book about animals, by reading

As an example of Mr. Lloyd Morgan's lighter manner, which we learn much of their lives and habits, their

what can be more attractive than the opening sentences peculiarities of structure and their relations to each other,

of his chapter entitled “Long-nose, Long-neck, and while we seem to be only reading for amusement. There Stumpy”? is nothing systematic in this volume. It is merely a ““ And which of all the animals in the Zoo do you like collection of miscellaneous chapters on a variety of best ?' I said to a bright, fair-haired little girl whom I had animals, beginning with the lion and ending with the assisted in her descent from the elephant.

"I think I like Long-nose, Long-neck, and Stumpy oyster, every chapter of which is both pleasant and

best, because they are so big and curous, and Long-nose instructive.

best of all because he has given me a ride. Did you The best way to notice a book of this kind is to give a know it was his nose ? few examples of the author's style, which in this case will “Of course I affected the most extreme surprise and delight at the novel suggestion that the big, patient ani- both with pictures and diagrams; and we may especially mal's trunk was really his nose, and said that I had always note that the structure of the elephant's tooth and that of thought it was his proboscis. 6. No, it isn't that, it's his nose. Auntie says so.

the bee's compound eye are clearly elucidated by the cuts That's Auntie over there, waiting for me. I suppose

that accompany the descriptions. you's seen Stumpy?'

Lastly, there is a pervading tone of sympathy with all " I inquired who Stumpy was, and whether I might that lives, as well as a general love and admiration of not know him by another name.

Nature, that renders it a most suitable work for the “I think they sometimes call him Pottums. But we call him Stumpy. Now I must go to Auntie.'”

young. The cover and general get-up are attractive, and

every school should add this charming volume to its list And then our author tells us much about those three of prizes, with the certainty that it will be highly apprestrange and remote types, the elephant, hippopotamus, ciated for its own sake by the recipients, and that its and giraffe, in his own pleasant manner-their singular influence will be altogether wholesome and good. structure and habits, their external diversities concealing

A. R. W. so much internal resemblance-devoting, however, most attention to the elephant, and correcting some exaggerated statements that have been made respecting that animal. PHYSIOLOGICAL CHEMISTRY FOR MEDICAL One of the most interesting chapters is that on snakes.

STUDENTS. It is full of information, and there is an almost fascinat- Outlines of Practical Physiological Chemistry. By F. ing account of the whole process of capturing and de

Charles Larkin, F.R.C.S., and Randle Leigh, M.B., vouring its prey by a python, as observed at the Antwerp B.Sc. Second Edition. (London: H. K. Lewis, Zoological Gardens. Prof. Lloyd Morgan has visited, or

1891.) lived in, many lands, and often enlivens his pages with personal anecdotes, of which the following is by no means THE authors state in their preface that this edition of the most remarkable :

in teaching the subject to medical students,” from which "My first experience of South African death-dealing we gather that the medical student is being treated in snakes was somewhat different.

One of my pupils the physiological laboratory in much the same spirit as brought me, in a large cigar-box, a ‘ring-hals-slang,' a deadly and courageous snake not uncommon at the Cape,

he has long been dealt with in that of the chemist. The and turned him out on the verandah for our delectation.

work before us is constructed upon an essentially similar He was a spiteful little fellow, with an ominous hood, principle to those numerous little treatises, the be-all and dark glossy skin, and glistening brown eye. He struck the end-all of which is to instruct the medical student in viciously at the cigar-box held up before him, indenting three months how to analyze simple salts. For such the wood, and moistening it with venom and saliva. I treatises, and the unedifying kind of instruction to which was particularly anxious to dissect out the poisongland and examine the poison-fang of the snake, so

they give rise, neither teacher nor student is to be my friend kindly presented it to me, replacing it in blamed: the fault lies with the authorities who frame the the cigar-box, which he tied securely. After examin- medical curriculum and the syllabus for the subjects of ing the fastenings, I placed the box on the window- examination. The root of the mischief lies in having to sill of my bedroom, which looked out into the veran- treat the medical student during his preliminary scientific dah, and left it there for the night. Next morning I training as a separate genus from the student of general procured a large washing-pan, big enough to drown a small python, placed the cigar-box therein, loaded it science, a course which is rendered necessary through the with a couple of bricks, and poured in water to the brim. attempt to crowd such a large number of subjects into a I gave the ring-hals’ three good hours to get thoroughly period of time which is wholly inadequate for the purpose : drowned, removed the bricks, took out the box, gently whilst another evilitending to degrade the standard of the cut the string, lifted the lid—and found that I had been examinationsis the existence of competing corporate bodies drowning with the utmost care an empty cigar-box. It had been securely tied, and how a creature more than possessing the power of granting medical qualifications. For thrice the girth of my thumb had managed to escape

these ills the obvious remedies are, on the one hand, exwas, and still is, a mystery to me.

tension of the minimum time occupied by the curriculum, I leave the reader to imagine the detailed search of whilst, on the other hand, a uniform standard for qualievery cranny of our bedroom, on which my wife insisted. fication is required for the whole of the United Kingdom: For several days every boot had to be hammered with a stick before it was put on ; I stood on a chair and shook Considering the necessarily technical and empirical charac

fortunately, both of these changes are already in progress. every pair of trousers, and other analogous garments: ter of the greater part of medical education proper, it is, in lest they should be already occupied. But no 'ring-hals was forthcoming. And I suppose it must have been a our opinion, of the greatest importance that in the teachweek or so afterwards that I was summoned to the ing of the pure sciences medical students there should kitchen to expel an unwelcome intruder---the black cook be as little empiricism and rule-of-thumb as possible ; being, so far as her skin permitted, pale with terror-which proved to be none other than the missing 'ring-bals. I

and it is, therefore, just in his study of chemistry that despatched him promptly, but not by drowning.”

the future physician and surgeon should receive an in

sight into the scientific use of the understanding. Among the specially good chapters are those on “Cousin Now, it is in this respect that the work before us, which Sarah," the chimpanzee ; on the sparrow as typical of contains a large number of facts arranged in a handy birds, under the title “ Master Impertinence"; on chame form, falls short of what is required. The subject of leons, frogs, sticklebats, crayfish-but it is useless to par- physiological chemistry is still at best such a very emticularize when all are good. The book is well illustrated, pirical one, that it becomes the more necessary to give an

explanation of phenomena whenever they are properly of reasoning, although all the necessary points have understood. It will be urged by some that such informa previously been stated. tion is out of place in a practical manual, and that it

The examples include a series selected from the Debelongs to theoretical works and lectures on the subject ; | and list of answers are included.

partment examination papers, and a table of contents in our experience, however, the great difficulty in labora

If the working of elementary problems in chemistry is tory teaching is to make the student associate his prac- to be an intellectual process, founded upon an appreciatical work with what he hears in the lecture-room or tion of fact and theory, which may be supplemented but reads in his study, and that it is only by continually has not to be corrected by the student as he progresses,

books such as the above fail to fulfil this end. drawing his attention to the bearing of his experiments that the latter are made to have any great educational value. In short, it is only in this way that the instruction

LETTERS TO THE EDITOR. in a chemical laboratory materially differs from that ob- [The Editor does not hold himself responsible for opinions ex. tainable in a kitchen, and that a work on practical pressed by his correspondents. Neither can he undertake physiological chemistry will be raised above the level of to return, or to correspond with the writers of, rejected a hand-book on cookery. By amplifying their work on

manuscripts intended for this or any other part of NATURE,

No notice is laken of anonymous communications.] the lines indicated, we believe that its value to the student would be much enhanced, for in its present form

The Theory of Solutions. it can only be used to much purpose under the guidance

IN NATURE of December 31 (p. 193) occurs a review by

J. W. R.” of my book on “Solutions," which gives me of an accomplished and energetic demonstrator.

occasion to enter, in a few words, upon the questions there In conclusion, we must point out an error which should brought forward, and to set right some errors, which have hardly occur in a work prosessing to be the result of ex- recently appeared in other places as well. perience, still less in a second edition. On p. 29 the

First of all I wish to express to Mr. J. W. R. my cordial

thanks for the thorough and careful manner in which he has student is told to prepare lactic acid as follows :-“Place

made himself acquainted with the contents of my book. I 50 c.c. of milk in a warm chamber for several weeks have no intention to discuss the objections made, some of which until it becomes strongly acid. Shake the ether, and

I am quite willing to recognize as well founded, but to make decant the ethereal extract. Evaporate the ether, add

clear one important question in which I do not seem to be

properly understood by my critic. extract residue with water. It is strongly acid, and Mr. J. W. R. begins his discussion with the words, “To vields crystals [sic] of lactic acid.” The passage ob- the fundamental question—' Is solution a physical or a chemical viously contains several printers’errors, but the crystalline process?'-the answers are various ”; and out of this variety

he evidently finds against me a reproach. I have intentionally nature of lactic acid is new to us.

neither set up this question nor sought to answer it, for I hold it to be unclear and therefore very harmsul. To the question, “Is gas-formation a chemical or a physical process ? ” would be

answered, “In certain cases, as in the development of carbon OUR BOOK SHELF.

dioxide out of champagne, a physical one ; in others, as the

development of carbon dioxide from limestone, a chemical Problems in Chemical Arithmetic. By E. J. Cox, F.C.S.

one; and in many cases, as in the development of hydrogen Pp. 76. (London: Percival and Co., 1891.)

from palladium hydride, one would be in doubt what to This book contains a series of arithmetical examples

answer.” The question set up is faulty in implicitly assuming chosen to meet the requirements of the examinations

that solution must be either a physical or a chemical process, held by the Science and Art Department in the ele

and by this prepossession he is hindered from recognizing that I mentary stage of chemistry. There does not seem to be

was entirely justified in placing the physical or chemical side of

the question in the foreground according to the nature of the any outstanding feature to distinguish the book from

However, in case Mr. J. W. R. is not satisfied with this others of its kind ; indeed, setting aside the actual exer- explanation, and insists upon setting up this question, I must cises, which may be useful to the teacher, the explana- postpone further discussion upon it until he shall have given tions of the principles involved in the calculations are, a sufficient definition of the ideas “physical and as a rule, meagre, and frequently inaccurate. Informa- “chemical " processes, and of their distinction. I know of no tion such as the following is, to say the least of it, such definition, and have consequently not made use of the faulty : “ Whatever may be weight of any given expressions. volume of water, an equal volume of mercury under

From the definition of solution given by me, Mr. J. W. R. similar conditions will be 136 times as heavy." This concludes that I am a representative of the “physical theory conclusion was made to follow as a result of an apparently of solutions, in contrast to which he places the chemical practical method of obtaining specific gravity, although theory. I cannot repeat energetically enough that I have no mention whatever was made of temperature or its the existence of such a contrast.

never recognized such a contrast, and that I cannot at all admit

It has never been maintained, effects in the description of the process.

either by me or by any other representative of the newer theory The author keeps on repeating, without any qualifying of solutions, that no interaction takes place between the solvent clause, that the formula of a compound represents the and the dissolved substance ; on the contrary, I have for years molecule of the compound; and the student is led to directly encouraged research work directed towards making clear infer (pp. 19 and 20) that the empirical formula as found the nature of such interactions. What distinguishes the new by analysis serves to fix molecular weight. As a con- theory of solutions, founded by van 't Hoff, from the others is sequence of the above idea, several problems are set

that it has succeeded in discovering and bringing into connection such as, Find the molecular weight of starch, of fluor

a series of properties of solutions, which can be treated entirely spar, &c.—which are misleading, since they cannot be independently of the question of a possible interaction between the solved in practice.

parts of the dissolved substance and of the solvent. The relationship between vapour-density and mole- making of a solution or in the altering its concentration there is

All these properties hang together with the fact that in the cular weight is given, but the use of the former as a

developed or absorbed a definite amount of free or available means of determining the latter is not even hinted at;

energy, which is equal for equimolecular quantities of different and the author prefers to find the value of the ratio, substances, and is independent of the nature of the solvent. The molecular weight to vapour-density, by the use of amount of this free energy is the same as in the analogous numerical examples, rather than by a general process processes with gases. These are purely experimental facts,



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