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But for what purpose is this azote, of which plants seem to have such an imperious want? M. Payen's researches partly teach us; for they have proved that all the organs of the plant, without exception, begin by being formed of an azotated matter analogous to fibrine, with which at a later period are associated the cellular tissue, the ligneous tissue, and the amylaceous tissue itself. matter, the real origin of all the parts of the plant, is never destroyed; This azotated it is always to be found, however abundant may be the non-azotated matter which has been interposed between its particles.

This azote, fixed by plants, serves, therefore, to produce a concrete fibrinous substance, which constitutes the rudiment of all the organs of the vegetable.

It also serves to produce the liquid albumen which the coagulable juices of all plants contain; and the caseum, so often confounded with albumen, but so easy to recognise in many plants.

Fibrin, albumen, and caseum exist, then, in plants. These three products, identical in their composition, as M. Vögel has long since proved, offer a singular analogy with the ligneous matters, the amidon, and the dextrine.

Indeed, fibrin is, like ligneous matter, insoluble: albumen, like starch, coagulates by heat; caseum, like dextrine, is soluble.

These azotated matters, moreover, are neutral, as well as the three parallel non-azotated matters; and we shall see that by their abundance in the animal kingdom they act the same part that these latter exhibited to us in the vegetable kingdom.

Besides, in like manner as it suffices for the formation of nonazotated neutral matters, to unite carbon with water or with its elements, so, also, for the formation of these azotated neutral matters, it suffices to unite carbon and ammonium with the elements of water; forty-eight molecules of carbon, six of ammonium, and seventeen of water, constitute, or may constitute, fibrin, albumen, and caseum.

Thus, in both cases, reduced bodies, carbon or ammonium, and water, suffice for the formation of the matters which we are considering, and their production enters quite naturally into the circle of reactions, which vegetable nature seems especially adapted to produce.

The function of azote in plants is therefore worthy of the most serious attention, since it is this which serves to form the fibrin which is found as the rudiment in all the organs; since it is this which serves for the production of the albumen and caseum so largely diffused in so many plants, and which animals assimilate or modify according to the exigencies of their own nature.

It is in plants, then, that the true laboratory of organic chemistry resides. Thus carbon, hydrogen, ammonium, and water are the principles which plants elaborate: ligneous matter, starch, gums, and sugars on the one part, fibrin, albumen, caseum, and gluten, on the other, are then the fundamental products of the two kingdoms; products formed in plants, and in plants alone, and transferred by di gestion into animals.

Ashes.--An immense quantity of water passes through the vegetable during the period of its existence. This water evaporates at the surface of the leaves, and necessarily leaves, as residue, in the plant the salts which it contained in solution. These salts compose the ashes, products evidently borrowed from the earth, to which, after their death, vegetables give it back again.

As to the form in which these mineral products deposit themselves in the vegetable tissue, nothing can be more variable. We may remark, however, that among the products of this nature, one of the most frequent and most abundant, is that pectinate of lime discovered by M. Jacquelain in the ligneous tissue of most plants.

IV. If, in the dark, plants act as simple filtres which water and gases pass through; if, under the influence of solar light they act as reducing apparatus which decompose water, carbonic acid, and oxide of ammonium, there are certain epochs and certain organs in which the plant assumes another, and altogether opposite, part.

Thus, if an embryo is to be made to germinate, a bud to be unfolded, a flower to be fecundated, the plant which absorbed the solar heat, which decomposed carbonic acid and water, all at once changes its course. It burns carbon and hydrogen; it produces heat; that is to say, it takes to itself the principal characters of animal life.

But here a remarkable circumstance reveals itself. If barley or wheat is made to germinate, much heat, carbonic acid and water are produced. The starch of these grains first changes into gum, then into sugar, then it disappears in producing carbonic acid, which the germ is to assimilate. Does a potato germinate? here, also, it is starch which changes into dextrine, then into sugar, and which at last produces carbonic acid and heat. Sugar, therefore, seems the agent by means of which plants develope heat as they need it.

How is it possible not to be struck from this with the coincidence of the following facts? Fecundation is always accompanied by heat. Flowers as they breathe produce carbonic acid: they therefore consume carbon; and if we ask whence this carbon comes, we see in the sugar cane, for example, that the sugar accumulated in the stalk has entirely disappeared when the flowering and fructification are accomplished. In the beet root, the sugar continues increasing in the roots until it flowers; the seed-bearing beet contains no trace of sugar in its root. In the parsnip, the turnip, and the carrot, the same phenomena take place.

Thus, at certain epochs, in certain organs, the plant turns into an animal; it becomes, like it, an apparatus of combustion; it burns carbon and hydrogen; it gives out heat.

But, at these same periods, it destroys in abundance the saccharine matters which it had slowly accumulated and stored up. Sugar, or starch turned into sugar, are then the primary substances by means of which plants develope heat as required for the accomplishment of some of their functions.

And if we remark with what instinct animals, and men too, choose

for their food just that part of the vegetable in which it has accumulated the sugar and starch which serve it to develope heat, is it not probable, that, in the animal economy, sugar and starch are also destined to act the same part, that is to say, to be burned for the purpose of developing the heat which accompanies the phenomenon of respiration?

To sum up, as long as the vegetable preserves its most habitual character, it draws from the sun heat, light, and chemical rays; from the air it receives carbon; from water it takes hydrogen; azote from the oxide of ammonium, and different salts from the earth. With these mineral or elementary substances, it composes the organised substances which accumulate in its tissues.

They are ternary substances, ligneous matter, starch, gums, and sugars.

They are quatenary substances, fibrin, albumen, caseum, and gluten.

So far, then, the vegetable is an unceasing producer; but if at times, if to satisfy certain wants, the vegetable becomes a consumer, it realises exactly the same phenomena which the animal will now set before us.

(To be continued).

Observations on Mr. DONOVAN's Reflections on the Inadequacy of the principal Hypotheses to account for the Phenomena of Electricity. By G. A. DE LUC, Esq., F. R.S., &c.*

1. THIS paper of Mr. Donovan will, I hope, be very useful in settling the doctrine of electricity, against which he finds the objections detailed in his paper, upon its true foundation.

2. The doctrine of positive and negative electricities, first published by Franklin, is true in itself; but by the manner in which he had expressed it it was involved in many difficulties which he had not foreseen. I shall have occasion hereafter to relate the opportunity which I had, long after, to demonstrate to him personally, by an experiment, what kind of influence the air has in the propagation of the electric fluid.

3. Franklin's theory was first attacked by Dr. Peart, who pointed out some phenomena of electric motions absolutely inconsistent with his statement of his own system. I shall not enter into that discussion, but point out directly the source of an insurmountable difficulty in that theory, as it was first expressed by its author, which circumstance (as I shall successively prove) has been the only cause of its rejection by Mr. Donovan. The error was this: Dr. Franklin considered as the standard of plus and minus, or middle point between them, a certain natural quantity of electric matter belonging to all the bodies of the earth: he called negative the bo• From Tilloch's "Philosophical Magazine."

B b

dies from which some part of the quantity was abstracted, and positive those to which a new quantity was added.

4. This error of Franklin has created all the just objections of Mr. Donovan; but from this circumstance I may judge that the latter has not had the opportunity of knowing two works which I have published, one in England, in 1787, in two volumes, under the title of "Idées sur la Météorologie:" the other was published at Paris, in 1804, also in two volumes, under the title of “ Traité élémentaire sur le Fluide Electro-galvanique." In both these works I have applied Volta's system to the motions of a pair of balls, which motions are the most immediate test of electrical theories.

5. The essential and characteristic difference of Volta's theory, compared to that of Franklin, consists in the standard between plus and minus. Volta has demonstrated that there is no other standard or middle point between these two opposite electric states, than the actual electric state of the air, which possesses electric matter as well as the bodies which it embraces. But I must first relate the opportunity which I have had to learn that system from its author himself. Being at Paris in the year 1782, I made that very interesting acquaintance, and M. Volta was so good as to explain to me completely his system. The same year he came to London, and directed me in the construction of a more extensive set of electrical instruments, with which I made all the experiments related in the above mentioned works.

6. The principle of Volta with respect to electric motions, being that they have for their true standard the actual electric state of the ambient air, it came into my mind to submit that principle, as the foundation of the whole theory, to an experimentum crucis.

7. This experiment is related from p. 55 to 57 of the first volume of the "Traité élémentaire sur le Fluide Electro-galvanique:" where I first mention, as an indispensable condition for the success of that experiment, that the air be very dry; and I had the means, by my hygrometer, to determine the necessary degrees of dryness, which is about 43° of my scale. As these experiments require some time, a greater quantity of aqueous vapour mixed with the air, dissipates too fast the electric fluid accumulated upon bodies, and transmits too fast some electric fluid to those which have been rendered nega

tive.

8. I made this series of experiments in two contiguous rooms, separated by a short passage and a door. In one of the rooms I placed a very strong electric machine, by which the air could be modified without affecting that of the other room. I had a pair of pith balls with long conducting threads, suspended to a brass cap at the extremity of a varnished glass rod, and thus thoroughly insulated, as well as the ferule to which the balls were suspended. Before the electric machine was put in motion in its room, the balls had no divergence in either of them.

9. In one of the experiments, I fixed a point at the extremity of the prime conductor of the machine, the rubber being placed in

commucication with the ground: a luminous brush appeared at the point, indicating that the electric fluid escaped from it and was communicated to the air of the room. Now, when I brought the pair of balls from the next room, where they did not diverge, they strongly diverged in the room of the machine; and by the test of a rubbed stick of sealing-wax, they were found to diverge negatively, because the air had acquired some electric fluid by the action of the positive point, and thus was made positive; but when the balls were brought back to the other room, they ceased to diverge.

10. I made the inverse experiment, by fixing a point to the rubber of the machine, its prime conductor being placed in communication with the ground. The machine being worked in this state, a luminous point was seen at the extremity of the point fixed to the rubber, indicating that some electric fluid passed from the air of the room to the rubber, and thus that air was made negative. Now the pair of balls, which did not again diverge in the next room, being brought into that wherein the machine was, they strongly diverged, and by the test they diverged positively, being brought into au atmosphere rendered negative by the point fixed to the rubber.

11. These experiments cannot leave any doubt, on Volta's theory, that in the divergence of a pair of balls, the standard of plus and minus is not, as Franklin had determined it, a natural electric state of bodies; but that it is the actual and variable electric state of the ambient air. Had Mr. Donovan been acquainted with that theory of Volta, still retaining the fundamental theory of positive and negative, first announced by Franklin, but changing the standard from a fixed to a changeable state well defined, he would certainly have retained the fundamental doctrine of positive and negative with that correction.

12. As to the theory of Epinus, that electric atmospheres do not exist, which Mr. Donovan seems to prefer to that of Franklin corrected by Volta, I have shown in one of my works, that Epinus, instead of simplifying the theory, has fallen into a greater complication of hypotheses, even contrary to some general laws of nature. But I shall not enter into that discussion, as Volta's system renders it unnecessary.

13. It is by this influence of the air, receiving its share of the electric fluid possessed by the bodies which it surrounds, that Volta explained the motions of electrified balls, and the following is his explanation." When two balls suspended near each other are in a positive state comparatively to the air of the place, they both communicate some electric fluid to the air between them; while each of them communicates alone the electric fluid to the outward air. Each of the balls therefore moves towards that outward air on each side, not by repelling each other, but by moving towards the air possessing less electric fluid."

14. During the course of my experiments to demonstrate, in various manners, the certainty of Volta's system on the cause of motion of electrified balls, it came into my mind that an analogous

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