300 NATURAL HISTORY OF ORGANIZED BODIES. 



The best known of all vegetal)le functions, the respiration of plants, presents 

 this first experimental idea, tliat the green matter of plants, under the influence 

 of solar light, decomposes the water and carbonic acid, thus setting at liberty 

 the hydrogen and oxide of carbon. Now, these latter substances are the ele- 

 ments which chemical synthesis employ's to form the ternary compounds, which 

 may all be derived from the action of nascent hydrogen on the oxide of carbon. 



If the chemist, in his laboratory, must proceed by a series of transfonnations 

 in order to realize substances in which the elements are more condensed, 

 nature attains the same end in a more direct manner, without, on that account, 

 violating the ordinary laws of chemistry. In nature all the elements are in con- 

 tact in a nascent state, so that the simpler compounds which result therefrom 

 remain not long in their first phase of evolution, having close at hand every 

 principle necessary for the formation of more complex bodies. Organic bodies 

 an-ive, therefore, with immediate eft'ect at their highest degree of condensation, 

 while, in the chemical reactions of the laboratory, we are obliged, in following 

 up the conditions of the fc^nnation of these bodies, to create artilicial and succes- 

 sive phases. 



In the study of the functions of life, the physiologist finds himself confronted 

 with phenomena so complex that he cannot at once comprehend the laws which 

 govern them. But he is struck with certain characters wliich seem to him more 

 constant than others. From these he deduces the existence of certain vital laws, 

 an ephemeral hypothesis which disappears soon or late before a more profound 

 investigation of the phenomena, and is absorbed in the more comprehensive 

 generalizations of physical or chemical laws. 



First of all, the production of heat and that of movement seem to him to be 

 attributes of the animal kingdom. If some species appear to form an exception 

 to this sort of general law Avliich he has established, the physiologist explores 

 the facts more attentively, and perceives that the animals which he had at first 

 distinguished from others by calling them animals tcifh cold blood, constitute but 

 an apparent exception, and that they also produce heat, though in less quantity 

 than others, besides that they have not the property of preserving this heat, but 

 allow it to escape \vhen they are placed in a cold medium. Eventually it is 

 recognized that the chemical actions which take place in the organism are the 

 cause of the production of heat in animals, and that the quantity of heat disen- 

 gaged increases or decreases according to the intensity and nature of those 

 actions. Thenceforth the production of animal heat presents itself only as a 

 particular case of the disengagement of heat in chemical reactions. 



Movement in animals was at first considered a direct result of life ; in its appa- 

 rent spontaneity, a character was even supposed to have been found which dis- 

 tinguished it from tlie movements whose laws are determined by m(?chanical 

 principles. But it was at length recognized that the production of movement, 

 like that of heat, requires in animals a chemical action ; that its production, 

 therefore, is not unlimited, but must be assimilated to the labor of our machines, 

 which transform into movement the heat derived from the combustion of carbon. 

 Considered under this point of view, the animal organism would not differ from 

 our machiner}', except in its more advantageous capacity of production, but can 

 yioM, on the whole, in labor only what the chemical actions exerted on the 

 .absorbed aliments will admit of. This extension of physical laws to the 

 functions of organized beings commends itself so strongly to reason, that no hesi- 

 tation is at present felt in pushing conclusions to their last consequences, and in 

 seeking, for example, in the animal economy the verification of the law of the 

 equivalence of heat and of mechanical lal)or. 



Nothing can be more legitimate than this tendency to reduce all the phenom- 

 ena of nature to sinq^le and general laws ; to me it even seems that this mode of 

 procedure has every chance in its favor of being the right one ; still, from prob- 

 able hypothesis to demonstration is a long stride. On this account it is that we 



