Action of a to embarrass the brief exposition of vegetable life now to be given by any historical details, since these will come with more propriety subsequently. It is sufficient to mention that the chemical explanations of vegetable physiology rest essentially on the discovery of oxygen gas by Priestley, of the constitution of carbonic acid by Lavoisier, and of water by Cavendish and Watt. While the sun is shining, the green parts of plants, especially the leaves, decompose carbonic acid, one of the ingredients of the atmospheric air. This substance is composed of two elements, carbon and oxygen; the former is appropriated by the plant, and enters into the composition of elaborated or descending sap, from which forthwith organic products, such as starch, sugar, wood fibre, acids, and bases are made. The other element, the oxygen, is for the most part refused by the plant, and returns to the air. As the process of decomposition goes on, new portions of carbonic acid are presented through mechanical movements, the trembling of the leaf, breezes, and currents rising from the foliage warmed by the solar beams giving place to other cool currents that set in below. air. The action of a plant upon the air is therefore the separation of combustible material from that medium. Carbon is thus obtained from carbonic acid; from water, hydrogen. Plant life is chemically an operation of reduction, for in like manner ammonia is decomposed into its constituents, which are nitrogen and hydrogen; and sulphuric and phosphoric acids, which like ammonia, may have been brought into the plant through its roots in the form of salt bodies, are made to yield up the oxygen with which they had been combined, and their sulphur and phosphorus, combustible elements, are appropriated. Composition Every plant, from the humblest moss to the oak of a thousand years, is thus formed by the sun from and resolution material obtained from the air-combustible of matter and material once united with oxygen, but now force. separated from that body. It is of especial importance to remark that in this act of decomposition, force, under the form of light, has disappeared, and become incorporated with the combustible, the organizing material This force is surrendered again, or reappears whenever the converse operation, combination with oxygen, occurs. Vegetable products thus constitute a magazine in which force is stored up and preserved for any assignable time. Hence they are adapted for animal food and for the procuring of warmth. The heat evolved in the combustion of coal in domestic economy was originally light from the sun appropriated by plants in the Secondary geological times, and locked up for untold ages. The sun is also the source from which was derived the light obtained in all our artificial operations of burning gas, oil, fat, wax, for the purposes of illumination. My own experiments have proved that it is the light of the sun, in contradistinction to the heat, which Correlation occasions the decomposition of carbonic acid, of physical furnishing carbon to plants and oxygen to the forces. atmosphere. But such is the relation of the so-called imponderable principles of chemistry to each other, and their mutual convertibility, that that which has disappeared in performing its function as light may reappear as heat or electricity, or in the production of some mechanical effect. Food is used by all animals for the sake of the force it thus contains, the remark applying to the car- The nature of nivora as well as the herbivora. In both cases the source of supply is the vegetable kingdom, indirectly or directly. The plant is thus indispensable to the animal. It is the collector and preserver of that force the expenditure of which constitutes the special display of animal life. From this point of view, animals must therefore be considered as machines, in which force obtained as has been described, is utilized. The food they take, or the tissue that has been formed from it, is acted upon by the air they breathe, and undergoes partial or total oxydation, and now emerges again, in part as heat in part as nerveforce, in some few instances in part as light or electricity, the force that originally came from the sun. and force There is, therefore, a cycle or revolution Cycle through through which material particles suitable for which matter organization incessantly run. At one moment pass. they exist as inorganic combinations in the air or the soil, then as portions of plants, then as portions of animals, then they return to the air or soil again to renew their cycle of movement. The metamorphoses feigned by the poets of antiquity have hence a foundation in fact, and the vegetable and animal, the organic and inorganic worlds are indissolubly bound together. Plants are reducing, animals oxydizing, machines. Plants form, animals destroy. Thus, by the light of the sun, the carbonic acid of the atmosphere is decomposed-its oxygen is set free, its carbon furnished to plants. The products obtained serve for the food of animals, and in their systems the carbon is reoxydized by the air they respire, and, resuming the condition of carbonic acid, is thrown back into the atmosphere in the breath, ready to be decomposed by the sunlight once more, and run through the same cycle of changes again. The growth of a plant and the respiration of an animal are dependent on each other. imperish Material particles are thus the vehicles of force. They The duration undergo no destruction. Chemically speaking, of matter and they are eternal. And so, likewise, force never ability of deteriorates or becomes lessened. It may asforce. sume new phases, but it is always intrinsically unimpaired. The only changes it can exhibit are those of aspect and of distribution; of aspect, as electricity, affinity, light, heat; of distribution, as when the diffused aggregate of many sunbeams is concentrated in one animal form. It is but little that we know respecting the mutations and distribution of force in the universe. We cannot tell what becomes of that which has characterized animal life, though of its perpetuity we may be assured. It has no more been destroyed than the material particles of which such animals consist. They have been transmuted into new forms-it has taken on a new aspect. The sum total of matter in the world is invariable; so, likewise, is the sum total of force. These conclusions resemble in many respects those of the Theory of philosophy of Averroes, but they are free from Averroes. the heresy which led the Lateran Council, under Leo X., to condemn the doctrines of the great Spanish Mohammedan. The error of Averroes consisted in this, that he confounded what is here spoken of under the designation of force with the psychical principle, and erroneously applied that which is true for animals to the case of man, who is to be considered as consisting of three essentially distinct parts-a material body, upon which operate various physical forces, guided and controlled by an intelligent soul. In the following paragraphs the distinction here made is brought into more striking relief. mining posi The station of any animal in the organic series may be determined from the condition of its nervous Anatomical system. To this observation man himself is not mode of deteran exception. Indeed, just views of his position tion in the in the world, of the nature of his intellect and animal series. mental operations, can not be obtained except from the solid support afforded by Anatomy. The reader has doubtless remarked that, in the historical sketch of the later progress of Europe given in this book, I have not referred to metaphysics, or psychology, ness of the or mental philosophy. Cultivated as they have metaphysical been, it was not possible for them to yield any other result than they did among the Greeks. A lever is no mechanical power unless it has a material point of support. It is only It is only through the physical that the metaphysical can be discovered. The useless sciences. Physiology. An exposition of the structure, the physical forces, and the intellectual operations of man must be Necessity of founded on anatomy. We can only determine resorting to the methods of action from the study of the Anatomy and mechanism, and the right interpretation of that mechanism can only be ascertained from the construction of its parts, from observations of the manner in which they are developed, from comparisons with similar structures in other animals, not rejecting even the lowest, and from an investigation of their habits and peculiarities. Believing that, in the present state of science, doctrines in psychology, unless they are sustained by evidence derived from anatomy and physiology, are not to be relied on, I have not thought it necessary to devote much space to their introduction. They have not taken a part in the recent advances of humanity. They belong to an earlier I have social period, and are an anachronism in ours. referred to these points heretofore in my work on Physiology, and perhaps shall be excused the following extract: "The study of this portion of the mechanism of man brings us therefore in contact with metaphysical science, and some of its fundamental dogmas we have to consider. Nearly all philosophers who have cultivated in recent times that branch of knowledge, have viewed with apprehension the rapid advances of physiology, foreseeing that it would attempt the final solution of problems psychological which have exercised the ingenuity of the last questions. twenty centuries. In this they are not mistaken. Certainly it is desirable that some new method should be introduced, which may give point and precision to whatever metaphysical truths exist, and enable us to distinguish, separate, and dismiss what are only vain and empty speculations. Solution of "So far from philosophy being a forbidden domain to the physiologist, it may be asserted that the time has now Uncertainty of come when no one is entitled to express an metaphysics. opinion in philosophy unless he has first studied physiology. It has hitherto been to the detriment of truth that these processes of positive investigation have been repudiated. If from the construction of the human brain we may demonstrate the existence of a soul, is not that a gain? for there are many who are open to arguments of this class on whom speculative reasoning or a mere dictum falls without any weight. Why should we cast aside the solid facts presented to us by material objects? In his communications throughout the universe with us, God ever materializes. He equally speaks to us through the thousand graceful organic forms scattered in profusion over the surface of the earth, and through the motions and appearances presented by the celestial orbs. Our noblest and clearest conceptions of his attributes have been obtained from these material things. I am persuaded that the only possible route to truth in mental philosophy is through a study of the nervous mechanism. The experience of 2500 years, and the writings of the great metaphysicians attest, with a melancholy emphasis, the vanity of all other means. |
