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base; capsule two-celled, two-valved. There are four species.

HYDKOMANCY, a method of divination by water, practised by the ancients in this manner: they filled a cup or bowl of water; then fastening a ring to a piece of thread tied to their finger, held it over the water, and repeated a certain form of words, desiring to be satisfied with regard to their enquiry; and if the question was answered in the affirmative, the ring would strike the sides of the bowl of its own accord.

HYDROMETER. The best method of weighing equal quantities of corrosive volatile fluids, to determine their specific gravities, appears to consist in enclosing them in a bottle with a conical stopper, in the side of which stopper a fine mark is cut with a file. The fluid being poured into the bottle, it is easy to put in the stopper, because the redundant fluid escapes through the notch or mark, and may be carefully wiped off. Equal bulks of water and other fluids, are by this means weighed to a great degree of accuracy, care being taken to keep the temperature as equal as possible, by avoiding any contact of the bottle with the hand or otherwise. The bottle itself shows, with much precision, by a rise or fall of the liquid in the notch of the stopper, whether any such change has taken place. See Gravity, specific.

But as the operation of weighing requires considerable attention and steadiness, and also a good balance, the floating instrument called the hydrometer, has always been esteemed by philosophers, as well as men of business. It consists of a hollow ball, either of metal or glass, capable of floating in any known liquid ; from the one side of the ball proceeds a stem, which terminates in a weight, and from the side diametrically opposite proceeds another stem, most commonly of an equal thickness throughout. The weight is so proportioned that the instrument may float with the last mentioned stem upright. In the less accurate hydrometer this stem is graduated, and serves to show the density of the fluid, by the depth to which it sinks; as the heavier fluids will buoy up the instrument «hor>- than such as ara lighter. In this way, however, it is clear, that the stem must be comparatively thick, in order to possess any extensive range; for the weight of vitriolic ether is not equal to three-fourths of the same bulk of water, and therefore such an hydrometer, intended to exhibit the comparative

densities of these fluids, 'must have its stem equal in bulk to more than one fourth oftfee whole instrument. If this bulk be give: chiefly in thickness, the smaller differences of density will not be perceptible, and it cannot, with any convenience, be given a length.

To remedy this imperfection, various contrivances have been proposed, for the mos: part grounded on the consideration, that i change in the ballast, or weight employed to sink the ball, would so far change the instrument, that the same short range a gradations on a slender stem, which were employed to exhibit the densities of ardent spirit, might be employed in experiments upon water. Some have adjusted weigh b to be screwed upon the lower stem, and others, with more neatness and accuracy, have adjusted them to be slipped upon the extremity of the upper stem. But the method of Fahrenheit appears to be on all accounts, the simplest and most accurate.

The hydrometer of Fahrenheit consists of a hollow ball, with a counterpoise below, and a very slender stem above, terminating in a small dish. The middle, or half length of the stem, is distinguished by a fine hoe across. In this instrument every division of the stem is rejected, and it is immersed, in all experiments to the middle of the stem, by placing proper weights in the little dish above. Then as the part immersed is constantly of the same magnitude, and the whole weight of the hydrometer is known; this last weight, added to the weights in the dish, will be equal to the weight of fluid displaced lay the instrument, as all writers on hydrostatics prove. And accordingly the specific gravities for the common form of the tables will be had by the proportion. As the whole weight of the hydrometer and its load, when adjusted in distilled water, is to the number 1,000, &c. so is the whole weight, when adjusted in any other fluid, to the number expressing its specific gravity.

In order to show the degree of accuracy an instrument of this kind is capable of, it may in the first place be observed, that the greatest impediment to its sensibility arises from the attraction or repulsion between the surface of the fluid and that of the stem. If the instrument be carefully wiped with a clean soft linen cloth, the metallic surface will be equally disposed to attract or repel the fluid. So that if it possess a tendency to descend, there will be a cavity surrounding the stem; or if, on the contrary, its tendency be to rise, the fluid will stand round the stem in a small protuberance. The operator must assist this tendency by applying the pincers, with which he takes up his weights to the rim of the dish. It is very easy to know when the surface of the fluid is truly flat, by observing the reflected - image of the window, or any other fit object seen near the stem in the fluid. In this way the adjustment of the weights in the dish may, without difficulty, be brought to the fiftieth part of a grain. If, therefore, the instrument displace one thousand grains of water, the result will be very true to four places of figures, or even to five. This will be as exact as most scales are capable of affording.

Some writers have spoken of the adjustment of an hydrometer of this kind, so that it shall at some certain temperature displace one thousand grains of water, as if this were a great difficulty. It is true, indeed, that the performance of a piece of workmanship of this nature would require both skill and judgment on the part of the artist; but it is by no means necessary.

Nothing more is required on the part of the workman, than that the hydrometer ■hall be light enough to float in ether, and capable of sustaining at least one-third of its own weight in the dish, without eversetting in a denser fluid. This last requisite is obtained by giving a due length to the stem beneath, to which the counterpoise is attached. With such an instrument, whatever may be its weight, or the quantity of water it displaces, the chemist may proceed to make his experiments, and deduce his specific gravities by the proportion before laid down. Or to save occasional computation, he may once for all make a table of the specific gravities, corresponding to every number of the load in the dish, from one grain up to the whole number of grains, so that by looking for the load in one column, he may always find the specific gravity in the column opposite.

This method is very ready and convenient in practice; but if it be preferred, the weights may be adjusted to the hydrometer, so as to show the specific gravity, without computation or reference. For this purpose the hydrometer must be properly counterpoised in distilled water, at the assumed standard temperature; suppose 60", and the whole weight of the instrument and its load called 1.000, &c. Then the weight of the instrument aud its load must be separately determined in grains and parts, or other weights, by a

good pair of scales, and as the whole weight of the instrument and its load is proportioned to the weight of the instrument alone, so will be the number 1.000, Sec. to a fourth term expressing the weight of the instrument in such parts as make the whole 1.000, &c. Make an actual set of decimal weights of which 1,000, he, shall be equal to the hydrometer and its load; and it is clear, that whatever may be the load in these weights, if it be added to the number denoting the weight of the instrument, the sum will denote the specific gravity of the fluid, wherein the instrument floats with that load.

By following the above easy method, it will be found that every hydrometer, wheresoever made, must give the same results. The subject is indeed in itself sufficiently simple, and would require scarcely any discussion, if it had not happened that many philosophers, for want of requisite attention, have made their experiments with hydrometers graduated on the stem by no certain rule by which operators, at a distance from each other, might compare their experiments. The hydrometers, or peseliqueurs of Baume, though in reality, comparable with each other, are subject, in part, to the defect, that their results, having no independent numerical measure, require explanation to those who do not know the instruments. Thus, for example, when a chemist acquaints us that a fluid indicated fourteen degrees of the peso-liqueur of Baume, we cannot usefully apply this result, unless we have some rule to deduce the correspondent specific gravity ; whereas we should not have been in any respect at a loss, if the author had mentioned the specific gravity itself. As a considerable number of French philosophers refer to this instrument, it will be of use to explain its principles.

M. Baume appears to have directed his attention chiefly to the acquisition of a means of making hydrometers with a graduated stem, which should correspond in their results, notwithstanding any differences in their balls or stems. There is little doubt but he was led into the method he adopted, by reflecting on that by which thermometers are usually graduated. See Thermometer.

As thermometers are graduated, independent of each other, by commencing with an interval between two stationary points of temperature, so M. Baume adopted two determinate densities, for the sake of marking an interval on the stem of his hydrometer. These densities were those of pure water, and of water containing £ parts of its weight of pure dry common salt in solution. The temperature was ten degrees of Reaumur above freezing, or 54.5° of Fahrenheit. His instrument for salts was so balanced, as nearly to sink in pure water. When it was plunged in this saline solution, the stem arose in part above the surface. The elevated portion was assumed to be fifteen degrees, and he divided the rest of the stem with a pair of compasses into similar degrees.

It is unnecessary to inquire in this place, whether this interval be constant, or how far it may be varied by any difference in the purity, and more especially the degree of dryness of the salt. Neither will it be requisite to inquire how far the principle of measuring specific gravities by degrees, representing equal increments, or decrements, in the bulk of fluids, of equal weight, but different specific gravities, may be of value, or the contrary. It does riot seem probable, that Baum's instrument will ever become of general use, for which reason nothing further need be ascertained, than the specific gravities corresponding with its degrees, in order that such experiments as have this element among their data may be easily understood by chemical readers.

M. Baum, in his "Elemens de Pharmacie," has given a table of the degrees of his hydrometer for spirits, indicated by different mixtures of alcohol and pure water, where, he says, the6pirit made use of gave 37 degrees at the freezing point of water; and in a column of the table he states the bulk of this spirit, compared with that of an equal weight of water, as 35£ to 30. The last proportion answers to a specific gravity of *>.842, very nearly. A mixture of two pails, by weight, of this spirit, with thirty of pure water, gave twelve degrees of the hydrometer at the freezing point. This mixture, therefore, contained 6j parts of Blagden's standard to 100 water ; and by Gilpin's excellent tables, its specific gravity must have been 0.9915. By the same tables, these specific gravities of 0.845! and 0.9915 would, at 10° Reaumur, or 55° Fahrenheit, have fallen to 0.832 and 0.9905. Here then are two specific gravities of spirit corresponding with the degrees 12 and 57, whence the following table is considered.

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One of the principal uses of the hydrometer in common life being to determine the specific gravity of vinous spirits on the mixtures of alcohol, which consist of water, an article of no value in a commercial light, and alcohol, which is of considerable price, it becomes of importance to determine how much of each may be contained in any mixture. The following tables, extracted from the large table of Gilpin in the "Philosophical Transactions," may be considered as of the first authority. They were made with mixtures of water and alcohol, of 0.825 at 60". The alcohol was obtained from malt.

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HYDROPHILUS, in natural history, a genus of insects of the order Coleoptera. Antenna' clavate, the club perfoliate; feelers four, filiform; the hind legs are formed for swimming, fringed on the inner side, and nearly unarmed with claws. The insects of this genus, like those of the Dytiscus, which see, are inhabitants of ponds and stagnant waters, where they swim with much dexterity, turning round, with great velocity; they fly abroad by night in search of other waters. The males are distinguished from the females, by having a horny concave flap or shield on the fore legs, near the setting on of the feet; the hind legs are peculiarly fitted for their aquatic situation, being furnished on the inner side with a series of long and close-set filaments, resembling a fin, by which they are enabled to swim with great ease. The larva remain about two years and a half before they change into pupae, forming a con

venient cell, and secreting themselves in some bank. They are very voracious, and destructive to the more tender aquatic insects, worms, and young fish, which they seize with their forked jaws, and destroy by sucking out their juice. Here are upwards of thirty species. The principal European species is the H. piceus, waterclock, which is not uncommon in our on~n country. The female of this species affords an example of a faculty, which seems to be exercised by no other insect of this order, viz. that of spinning a kind of web, or flatfish circular case of silk, which it leaves floating on the water, and in which it deposits its eggs. This case, says Dr. Shaw, is terminated on its upper surface by a lengthened conical process, resembling a horn, of a brown colour, and of a much stronger nature than the case itself, which is white. The larvae, as soon as hatched, make their escape from the cnvelopement

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