already have passed through a rotation similar to that above detailed, and that the seeds deposited by former preparatory growths may retain their vitality, and be called into life by the favorable conditions existing after a fire. This is a point, however, requiring for its establishment a series of experiments which I have not yet been able to undertake. If, as already suggested, forest fires, in the uncultivated state of the country, be a provision for removing old and decayed forests, then such changes as those above detailed, must have an important use in the economy of nature, since by their means different portions of the country would succeed each other in assuming the state of "barrens," producing an abundance of herbs and wild fruits suitable for the sustenance of animals which could not subsist in the old forests; and these gradually becoming wooded, would keep up a succession of young and vigorous forests. 3dly, The progress of restoration may be interrupted by successive fires. These are most likely to occur soon after the first burning, but may happen at any subsequent stage. The resources of nature are not, however, easily exhausted. When fires pass through young woods some trees always escape; and so long as any vegetable soil remains, young plants continue to spring up, though not so plentifully as at first. Repeated fires, however, greatly impoverish the soil, since the most valuable part of the ashes is readily removed by rains, and the vegetable mould is entirely consumed. In this case, if the ground be not of great natural fertility, it becomes incapable of supporting a vigorous crop of young trees. It is then permanently occupied by shrubs and herbaceous plants; at least these remain in exclusive possession of the soil for a long period. In this state the burned ground is usually considered a permanent barren; a name which does not, however, well express its character, for though it may appear bleak and desolate when viewed from a distance, it is a perfect garden of flowering and fruit-bearing plants, and of beautiful mosses and lichens. There are few persons born in the American colonies, who cannot recall the memory of happy youthful days spent in gathering flowers and berries in the burnt barrens. Most of the plants already referred to as appearing soon after fires, continue to grow in these more permanent barrens. In addition to these, however, a great variety of other plants gradually appear, especially the Kalmia angustifolia or sheep laurel, which often becomes the predominant plant over large tracts. Cattle straying into barrens deposit the seeds of cultivated plants, as the grasses and clovers, as well as of many exotic weeds, which often grow as luxuriantly as any of the native plants. Lastly, When the ground is permanently occupied for agricultural purposes, the reproduction of the forest is of course entirely prevented. In this case, the greater number of the smaller plants found in the barrens disappear. Some species of the Solidago and Aster, and the Canada thistle, as well as a few smaller plants, remain in the fields, and sometimes become troublesome weeds. The most injurious weeds found in the cultivated ground, are not, however, native plants, but foreign species, which have been introduced with the cultivated grains and grasses; the ox-eyed daisy or white weed, and the crowsfoot or buttercup, are two of the most abundant of these. When a district has undergone the last change, when the sombre woods and the shade-loving plants that grow beneath them, have given place to open fields, clothed with cultivated plants, the metamorphosis which has taken place extends in its effects to the indigenous animals; and in this department, its effects are nearly as conspicuous and important as in relation to vegetation. Some wild animals are incapable of accommodating themselves to the change of circumstances; others at once adapt themselves to new modes of life, and increase greatly in numbers. It was before stated that the barrens, when clothed with shrubs, young trees, and herbaceous plants, were in a condition highly favorable to the support of wild animals; and perhaps there are few species which could not subsist more easily in a country at least partially in this state. For this reason, the transition of a country from the forest state to that of burned barrens is temporarily favorable to many species, which disappear before the progress of cultivation; and this would be more evident than it is, if European colonization did not tend to produce a more destructive warfare against such species than could be carried on by the Aborigines. The ruffed grouse, a truly woodland bird, becomes, when unmolested, more numerous on the margins of barrens and clearings than in other parts of the woods. The hare multiplies exceedingly in young second growths of birch. The wild pigeon has its favorite resort in the barrens during a great part of the summer. The moose and cariboo, in summer, find better supplies of food in second growth and barrens than in the old forests. The large quantities of decaying wood, left by fires and wood-cutters, afford more abundant means of subsistence to the tribe of woodpeckers. Many of the fly-catchers, warblers, thrushes, and sparrows, greatly prefer the barrens to most other places. Carnivorous birds and quadrupeds are found in such places in numbers proportioned to the supplies of food which they afford. The number of instances of this kind might be increased to a great extent if necessary; enough has, however, been stated to illustrate the fact. SECOND SERIES, Vol. IV, No. 11.-Sept., 1847. 22 Nearly all the animals above noticed, and many others, disappear when the country becomes cultivated. There are, however, other species which increase in numbers and at once adapt themselves to the new conditions introduced by man. The robin (Turdus migratorius) resorts to and derives its subsistence from the fields, and greatly multiplies, though much persecuted by sportsmen. The Fringilla nivalis, a summer bird in Nova Scotia, becomes very familiar, building in out-houses, and frequenting barns in search of food. The song sparrow and Savannah finch, swarm in the cultivated ground. The yellow bird (Sylvia æstiva) becomes very familiar, often building in gardens. The golden-winged woodpecker resorts to the cultivated fields, picking grubs and worms from the ground. The cliff-swallow exchanges the faces of rocks for the eaves of barns and houses, and the barn and chimney swallows are everywhere ready to avail themselves of the accommodation afforded by buildings. The acadian or little owl makes its abode in barns during winter. The bob-lincoln, the king bird, the waxwing or cherry bird, and the humming bird, are among the species which profit by the progress of cultivation. The larger quadrupeds disappear, but the fox and ermine still prowl about the cultivated grounds, and the field-mouse (Arvicola Pennsylvanica) which is very abundant in some parts of the woods, is equally so in the fields. Many insects are vastly increased in numbers, in consequence of the⚫ clearing of the forests. Of this kind are the grasshoppers and locusts, which, in dry seasons, are very destructive to grass and grain; the frog-spittle insects (Cercopis) of which several species are found in the fields and gardens, and are very injurious to vegetation; and the lepidoptera, nearly the whole of which find greater abundance of food and more favorable conditions in the burned barrens and cultivated fields, than in the growing woods. It may be remarked, in general, that there is no animal, frequenting in Europe the cultivated grounds, and either beneficial or noxious to man, which has not, in the indigenous species of America, an exact representative, filling its place in the economy of nature, and often, in a natural historical point of view, closely related to it. This results from the general sameness of arrangement in the system of nature in the old and new world; and if studied in its details, would form a subject of great interest to the zoologist and physical geographer. ART. XIV.-Review of the Organic Chemistry of M. CHARLES GERHARDT. (Concluded from p. 100.) We have already seen that M. Gerhardt halves the equivalents of most substances, taking the equivalent of hydrogen to be represented by the weight of its atom. Chlorine, bromine and iodine which unite with hydrogen, volume for volume, are also divided so that their volumes correspond to that of oxygen. In many reactions in which carbonic acid and water are evolved, they are observed to be in the proportions C, O, and H, O,, or in quantities double those which are regarded as equivalents in mineral chemistry. It will also be observed that in the formulas of all those substances which like alcohol and its derivatives are ordinarily represented by four volumes of vapor, the equivalents of carbon and oxygen are divisible by two and those of hydrogen by four. This has led many chemists to consider the oxygen in organic compounds as having double the equivalent ascribed to it in mineral combinations. If we regard C, O, and H, O, as representing single equivalents, it will then be necessary to double the formulas of mineral chemistry in order to harmonize the two. If on the other hand these represent two equivalents, the formulas of organic compounds must be divided; and this last course has been adopted by M. Gerhardt. 2 2 The protoxyds of the metals corresponding to water in their composition, will hence be expressed by M, O, and the equivalents of metals themselves will be one-half the number usually adopted. The equivalents of organic acids are generally determined from their silver salts, and in the monobasic acids the weight corresponding to one atom of silver is taken as the equivalent of the salt; thus the acetic acid is C, H, O,, and the acetate of silver C, (H, Ag) O,, in which it is impossible to imagine the existence of water or oxyd of silver, which are H, O and Ag, O. 2 The The equivalents of chlorine, bromine and iodine, will by this arrangement, be like oxygen represented by a single volume; those of nitrogen, phosphorus and arsenic, are also divided, while carbon with sulphur and selenium are retained unaltered. equivalent of water is represented by H, O and equals two volumes of vapor, hydrochloric acid is HCl = 2 vol. vapor, and ammonia in like manner is N H, and its equivalent is expressed by two volumes. (Précis, Vol. I, pp. 47-53.) 3 Mode of Combination.-Many compounds have the power of exchanging one or more of their equivalents of hydrogen for a metal, thus producing a series of compounds known as salts. All of those metals which unite with chlorine in single equivalents, are able to exchange themselves for hydrogen, equivalent for equivalent. The acids are then to be regarded as salts of hydrogen, and the view which regards them as compounds of an anhydrous acid with water, is inadmissible, as the monobasic acids contain but one equivalent of hydrogen, which is replaced by a metal, while water contains two equivalents of that element. A second mode of combination is that designated by title of metalepsis or equivalent substitution. In these as in the salt compounds, certain elements are capable of being replaced by others, without altering the molecular constitution of the organic substance. The phenomena of metalepsis are divided into two classes, those in which hydrogen is replaced by chlorine, bromine or iodine, and those where oxygen is exchanged for sulphur, selenium or tellurium. The two metaleptic groups are illustrated by the following examples. Metalepsis of Hydrogen. CH, Formene, (marsh gas.) C (H, CI) Chlorinized formene, (chlorid of methyle.) 3 C (H, Cl,) Bichlorinized formene. C (HCl) Trichlorinized formene, (chloroform.) 2 C2 (HCl)O, Trichlorinized acetate, (chloracetic acid.) C. H 10 2 Metalepsis of Oxygen. O Ether, (sulphuric ether.) C. H S Sulphuretted ether, (sulphuret of ethyle.) 4 10 C. H. Se Seleniuretted ether, (seleniuret of ethyle.) 4 10 C, H, Te Telluretted ether, (telluret of ethyle.) 4 10 In these reactions the substitutions are always equivalent, and since one equivalent of chlorine corresponds to one of hydrogen, and one of oxygen to two of hydrogen, it follows that oxygen cannot like chlorine replace hydrogen. The metaleptic hydrogen is to be carefully distinguished from that which is replaceable by a metal. The action of chlorine upon acetic acid, removes three equivalents of its hydrogen and substitutes three of chlorine in their place, forming chloracetic acid C, (HCl ̧) O2, but the fourth equivalent can only be removed by substituting a metal in its place. Substitution of Residues.-The action of nitric acid upon many organic substances, results in the formation of water and a new compound which contains the elements of the acid; thus benzene C, H., with nitric acid NHO, forms an equivalent of water and the new substance C, H, NO,. In this and analogous com 8 |