where, in consequence of an operation, that is, in a great measure, mechanical, a certain portion of the blood is, by a kind of filtration, strained off from the mass; while the fourth will constitute the excretions, substances which consist of the residual mass of the blood, after the secretions and transudations have been removed from it. If we apply this principle to the secretions as we have found them actually to exist, we must consider the solid albuminous, the gelatinous, and the simple oleaginous, as the only substances belonging to the first class; the mucous, the fibrinous, and the compound oleaginous to the second; the liquid albuminous will belong to the third; while the aqueous and the resinous will be placed in the fourth.

A very curious and important physiological question here presents itself respecting the origin of these salts, and more especially concerning the earth of bones, whether it is actually formed in the body, or whether it is, in the first instance, received into the system along with the aliment, and after being conveyed into the blood is separated from it, and gradually accumulated in the different organs, of which it afterwards forms a constituent part. The question becomes particularly interesting as it respects the physiology of some of the inferior orders of animals, and the connexion which they have with certain geological phenomena. A great proportion of the substance of several of the testacea and crustacea, consists of carbonate of lime, and it appears probable that many of the large calcareous strata which exist in

different parts of the world, have originated from the detritus or decomposition of these animals. We are then naturally led to inquire what is the origin of this lime? Did it exist in some other form previous to the creation of these animals, did they receive it into their system and organize its particles, so as to mould it into their shells and crusts, or have their digestive and secreting organs the power of actually generating lime? The former opinion is the one that appears the most obvious, and accords the best with our ideas of the usual operations of nature; but it is rendered improbable from the immense quantity of matter which the animals must have appropriated to themselves, and it is not very easy to conceive in what state the lime could have existed previous to its reception into their system.

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The same kind of question occurs with respect to vegetables, and although they differ so essentially from animals as to render it dangerous to extend the analogy from the one to the other, yet in this ticular point, they appear to be placed in precisely a similar situation. A considerable part of the solid matter of vegetables consists of carbon, and they also contain small quantities of various earths and metals, but as these substances are insoluble in water, which is the only medium through which plants are supposed to obtain their nourishment, it has been asked, how are they procured by the plants? Are they suspended in a state of minute division in the water which is absorbed by their vessels? can they be derived from the atmosphere? or do the plants actually

generate them? The question, as it respects both animals and vegetables, has been attempted to be resolved by direct experiment.

A set of experiments were performed on this subject by Vauquelin. They consisted in ascertaining the exact quantity of earthy matter which entered into the composition of the shell of the eggs, and also of the excrement of fowls; he carefully analyzed the food which they received, so as to learn precisely the quantity of earthy or saline matter which it could derive from this source, and then compared this with the quantity of lime and other earths which was found in the egg shells or the excrements." Another train of experiments was performed by Dr. Prout. He examined, with his usual accuracy, the composition of recent eggs, ascertained the nature and amount of their elements, and compared these with the composition of eggs after incubation, when the chick was fully developed. There are considerable difficulties attending experiments of this kind, and they require a degree of continued accuracy which few individuals are disposed to devote to them; but in the case of the two chemists above mentioned, no

3 He examined the relation which these quantities bore to each other in the male and female, and in the latter during the period of laying her eggs, and other analogous circumstances, and was led to draw the following conclusion, that a quantity of lime had been formed by the digestion, and the assimilation of the oats, that a portion of phosphoric acid had also been formed, that a certain quantity of carbonate of lime had been produced, and that a small quantity of silex had disappeared; Ann. de Chim. t. xxix. p. 3. et seq.

deficiency of this kind can be suspected, and in both cases the results appeared to indicate that the animal had acquired a greater quantity of earthy matter than could be accounted for, except by supposing that, in some way, a quantity of it had been developed by the vital powers.*

4 Prof. Berzelius, referring to Vauquelin's experiments, un hesitatingly concludes, that the earthy substances, which are evacuated by the animals, "must be capable of being composed or decomposed, as occasion requires, by the processes of organic chemistry;" View, p. 73, et seq. Dr. Prout is led to think that the earthy matter of the bones of the chick" does not pre-exist in the recent egg;" Phil. Trans. for 1822, p. 399. The average quantity of lime in the shells of eggs was found to vary so much, that it appeared impossible to determine positively whether the earth was derived from this source by chemical analysis; but he observes, there are "very strong reasons for believing that the earthy matter is not derived from the shell." He, however, adds with philosophic caution, “I by no means wish to be understood to assert, that the earth is not derived from the shell; because, in this case, the only alternative left me is to assert, that it is formed by transmutation from other matter; an assertion which I confess myself not bold enough to make in the present state of our knowledge, however strongly I may be inclined to believe that, within certain limits, this power is to be ranked among the capabilities of the vital energies." p. 400. The experiments of Fordyce, in which gold fish were kept in water that was supposed to be pure, and by being merely supplied with air, not only lived for many months, but increased very considerably in size, prove that the functions of these animals may be maintained in a perfect and healthy state for an indefinite length of time, merely by means of atmospheric air and water, but they do not exactly bear upon the question discussed in the text; Treatise on Digest. p. 78,.0.

The experiments on vegetables were conducted upon similar principles: the composition of certain plants, seeds, or bulbs, was accurately ascertained; they were placed in distilled water, or planted in clean washed sand, sulphur, or some substance whence they could not be supposed to derive any extraneous matter, and were moistened with distilled water. After they had grown for some time they were carefully analyzed, and a comparison was made of the elements which they contained before and after their germination and growth. The results of these experiments, like the former, seemed to prove, that the solid matter which entered into the composition of the vegetables, in the more advanced periods of their growth must, in part at least, have been produced by some action of the vital powers, and could not have been obtained ab extra. For although it might be possible, which however does not appear to be the case, to refer the whole of the carbon to the decomposition of carbonic acid, as dissolved in the water employed, or diffused through the atmosphere to which they were exposed, and although a part of the earthy matter which was found in them, might be derived from the soil, and suspended in the fluid which entered into their vessels; it seems very difficult, if not impossible, to explain the whole of it upon this principle."

5 Dr. Thomson has given us a very good summary of the experiments that have been performed on this subject in his section on the "food of plants;" Chem. v. iv. p. 320, et seq. The conjoined evidence of the experiments of Braconnot, Shrader,