MOLECULAR FORCES. 175 



with in the organism were regarded as beyond the limits of vital 

 force. Assimilation and reproduction, like growth, were regarded 

 as inexplicable, according to physical laws. But although many 

 individual points may defy all attempts at explanation, we cannot 

 doubt that these phenomena must be susceptible of a general 

 explanation ; for if we limit ourselves to the known phenomena of 

 motion, we shall find that there is not any indispensable necessity 

 to assume that this kind of molecular motion requires the control 

 of any such agent as vital force. 



We meet with very many cases in which several bodies seem 

 to induce in other bodies an action similar in force to the one they 

 exhibit, although there is no appearance of a relation of affinity 

 between the products of decomposition and those bodies which 

 are still undecom posed. Organic chemistry is rich in cases of 

 this kind, and similar instances are not wanting in inorganic 

 chemistry ; the most frequent and striking of these occur in the 

 processes of fermentation ; for we here find that a small quantity 

 of a substance undergoing a definite metamorphosis, can induce a 

 special form of decomposition or metamorphosis in an infinite 

 quantity of some other substance. As the slightest contact with 

 any individual point of matter in the molecules of iodide of mer- 

 cury, arsenious acid, metallic iron, or fulminate of mercury, and in 

 a hundred other similar substances, gives rise to an endless series 

 of definite motions ; so the smallest amount of a putrifying body 

 is able to impart to the chemical molecules a definite motion, 

 which is propagated in an uninterrupted sequence from atom to 

 atom, and may thus call into existence new forms and new 

 qualities. All these phenomena, which were formerly referred to 

 a specific catalytic force, not amenable to any law, and which were 

 first referred by Liebig to their true physical relations of causality, 

 indicate the point of view from which we ought in a physical 

 light to examine many of those vital phenomena which at an 

 earlier period were ascribed solely to the vis vitalis. 



The primary origin of all vital phenomena is as unfathomable 

 a mystery as the first impulse by which suns with their planets 

 and satellites were impelled in their orbits ; but if we direct our 

 attention to the motion once imparted to organic molecules, we 

 shall be able to trace the co-operation of the laws of the impulse or 

 propagation of motion in the development, growth, reproduction, 

 and secretion observable in organised bodies. From our experience 

 on these points we shall frequently see how it is possible that sub- 

 stances which appear in all chemical points to be opposed to one 

 another, may present similarities. The germ in the egg and in the 



