ity of vegetable digestion, or the amount of carbonic acid decomposed, by the quantity of oxygen gas evolved. And to eliminate the heat and chemical rays, he also employed those colored media which absorb in their passage the greater portion of these constituents of the sunbeam. The results he obtained confirm those of Morren and Daubeny, namely, that yellow or luminous light is the agent in vegetable digestion. They may, perhaps, be considered decisive on this point; for Mr. Hunt's experiments relate rather to the influence of light upon germination and the development of the green color, than directly upon the formation of vegetable tissue. In whatever way this question may be settled, it advances us but a short distance towards the solution of the great problem, "how light acts in producing organization," to which Dr. Draper's volume is professedly devoted. But the whole that it really contains on the subject all that it contributes to our former stock of knowledge or opinion - is embraced in the following hypothesis, which, so far as we know, has the merit of entire originality. "The very remarkable qualities which certain nitrogenized substances are known to exhibit, acting as ferments as they are undergoing decay, might lead to the supposition, that the decomposition of carbonic acid by leaves is due to the action of some nitrogenized body, the eremacausis of which is promoted by the rays of the sun. p. 182. And again, "There are many facts which go to prove, that the decomposition of carbonic acid is a secondary result, brought about by the action of nitrogenized ferment in a state of eremacausis, the sunlight operating in the first instance upon the ferment itself." p. 183. These facts, however, all resolve themselves into one, namely, the evolution of a quantity of nitrogen by leaves; which has long been known, but which has always been supposed to arise from the mechanical escape of a portion of the free nitrogen gas, necessarily absorbed, as we have seen, with the water which the roots imbibe. But Dr. Draper endeavours to show, that the quantity exhaled is much too large for such origin; and the results of his experiments (paragraphs 812-818) are certainly not only extraordinary in themselves, but in more than one respect apparently ir reconcilable with those of Boussingault. "Its true source,' he concludes, "is only to be sought in some nitrogenized compound present in the leaf, which is undergoing decomposition in some regulated way "; and upon this ferment he supposes that the light acts, to incite the decomposition of carbonic acid in vegetable digestion. Surely, a bold theory is here built upon a very narrow, not to say an unsubstantial, basis; and when constructed, it helps us very little in the way of explanation. With due deference, therefore, we must still continue to believe, that the essential phenomenon of vegetable growth is something very different from the working of beer. No hypothesis could well involve a higher intrinsic improbability. It consists of Liebig's doctrine of fermentation, misapplied, as we think, to a case of a radically different order. Fermentation, germination, &c., are destructive processes; vegetable digestion is a creative process. In the former, vegetable tissue and vegetable products are decomposed and carbonic acid formed; in the latter, on the contrary, carbonic acid is decomposed and vegetable tissue and products are formed. Is it probable, that two such diametrically opposite processes as the formation and the destruction of organic matter are owing to the same or similar causes? that the gluten or albumen, which in the germinating grain of wheat incites the decomposition of vegetable tissue, starch, and sugar, and their resolution into carbonic acid and water, should in the leaf give rise to exactly the reverse effects, the decomposition of carbonic acid and water, and the production of vegetable tissue, starch, and sugar? Besides, the formation of the nitrogenized products themselves is one of the essential results of vegetable digestion. Can their formation, then, be due to a process which in the first place involves their destruction? We will only add, that this hypothesis is open to another, by no means uncommon, objection; that of leaving the matter to be explained less plain than before. Framed purposely to explain the action of light on the foliage, it renders the phenomenon more obscure than ever, by gratuitously connecting it with a process (that of fermentation) which goes on quite as well in the dark. - We have taken special notice of this point, because it is the only one in the chemico-physiological portion of Professor Draper's volume, excepting the experiments upon vegetable digestion with the solar spectrum, that strikes us as new, and with the purely physical part we are not disposed to meddle. We must except, however, the annunciation of the discovery, that "the rays of the sun are the true nervous principle of plants!" (pp. 102, 206, etc.,) an idea which is seriously maintained in a full and eminently verbose chapter. Thus this obscure subject is rendered as clear as daylight. Some modern physiologists have been vainly searching for the traces of a nervous system in the plant itself. They should have looked to the sunshine; unless, indeed, moonshine would do as well. Noteworthy, also, is the intimation, that our Indian summer-long a puzzle to philosophers—is owing to the heat evolved by the decaying foliage of the season! p. 93. Dutrochet's explanation of the rise and circulation of the sap in plants by what he called endosmosis, and the reduction of this latter power to the general laws of capillarity and interdiffusion by Professor Graham and others, leave no novelty in Dr. Draper's very detailed chapter upon that subject; except in the proposition,—the truth of which we must still be permitted to doubt, that the exhalation of the sap in the leaves exerts no essential influence upon its rise in the stem. Chemists are naturally prone to magnify their office, and to explain almost every thing in the animal and vegetable economy upon chemical principles. In their views, frequently, the vital force the organizing principle that animates the living body and controls its formation plays nearly the same subordinate part as the Supreme Being in the system of the Brahmins. The latter admit the existence of a supreme deity, but suppose that he remains in a state of holy obscurity and inertness. The chemists kindly allow, that there may be such a thing as a vital force in the abstract, but are too apt to consider it a superfluity. At least, they would leave it nothing to do. But Dr. Draper is more hardy, and denies its existence altogether. "In this work, the existence of the vital force of physiologists as a homogeneous and separate force is uniformly denied. Living structures, far from being the product of one such homogeneous power, are rather the resultants of the action of a multitude of natural forces. Gravity, cohesion, elasticity, the agency of the imponderables, and all other powers which operate both on masses and atoms, are called into action, and hence it is, that the very evolution of a living form depends on the condition, that all these various agents conspire. There is no mystery in animated beings, which time will not at last reveal." ―p. 2. Not only are living beings the result of the action or equilibrium of a great variety of common purely physical forces, -if we rightly apprehend the statement, but the living body offers no more or other resistance to these physical forces than a dead carcass. For it is a vulgar error," says our author, "that a living being possesses a principle of resistance to external agents, while a dead one submits to them; both equally change, or, of the two, the living one putrefies and changes the more rapidly." (p. 6.) "Life and vitality figure away in these visionary speculations [the theories of physiologists] as though they were realities." (p. 41.) If, indeed, vital, as distinguished from common physical, forces have no existence, we may as well admit the doctrine with which the following sentence concludes. "What, then, are the final impressions left upon our minds by these general considerations? They teach us, that life never occurs except in regions to which the imponderable agents can have access, an observation which is equally true of vegetable and of animal forms; that elementary organization directly or indirectly arises from the plastic agency of those all-pervading forces." (p. 14.) Surely, the author would disclaim the inference which he here seems to invite. The scientific readers of Dr. Draper's large volume will find themselves as much mistaken as we were, if they suppose, that, although addressed to chemical philosophers, it is actually filled with original contributions to science. These, if separately presented, would occupy but a moderate space. Indeed, from the discursive and tropically luxuriant style, and from the particularity with which the author enters upon the most detailed explanations of perfectly trite and elementary matters, we should imagine that the treatise may have been originally prepared, and have done service, in the form of popular lectures to medical students. The introductory chapter, in particular, though well enough adapted for that purpose, an ill-natured critic might compare to an omelette soufflée, of which Dumas and Cuvier may have furnished the egg, Humboldt, Lyell, and others, the condiment, and Dr. Draper the intumescence. The author's ingenious views on the rise of the sap in plants, and the circulation in animals, are prefaced by an elementary treatise on capillary attraction. Before we reach his proper observations on the action of light upon foliage, we have two very diffuse chapters to undergo, on the solar spectrum in general, Newton's theory of colors, the calorific and the chemical rays, and a great variety of elementary matters, with which a philosopher of the lowest acquirements might be presumed familiar, or for which, if he would refresh his school-boy recollections, he might turn to his forgotten text-books. And when at length the interesting experiment comes to be described, (the substance of which we have given in a former page,) the reader's attention is directed, not so much to the point which it was intended to prove, as to the gaudy tints that are reflected by the apparatus upon the wall of the room. "This prismatic experiment is one of the most beautiful objects which organic chemistry can offer, carried on in a chamber which would be totally dark, were it not for the intensely colored curves which are cast upon the walls by reflection from the tubes, curves which often are many yards in length, indicating by their gaudy tints and brilliancy the intensity of the sun's light. The tubes and the vegetable leaves glow with the colors in which they are immersed. Meantime the most interesting phenomenon which can be witnessed is silently going forward; dead and inanimate matter is, under the influence of the plastic beam, putting on the form of organization and life.”— p. 64. This reminds us of one of the newspaper articles, emanating from an American observatory, upon the great comet of 1843, about the time of its first appearance. The observer, unfortunately, did not see the comet for which he was searching; but he filled his page with a glowing account of the twilight, and with some miscellaneous observations on the fixed stars. And, finally, when the nervous principle of plants is considered, and what that is our readers are already informed, we are treated, by way of introduction, with a particular account of the five senses; of which the following may serve as a specimen. -- |