24 INTRODUCTION. L.ECT. I. 



identical with that of the lungs and the liver. Molecules, 

 composed of at least three elements, into each of which a 

 great number of elementary atoms enter, must necessarily 

 form chemical systems, whose affinities differ from those 

 which are possessed by molecules chiefly composed of two 

 elements, and in which the number of elementary atoms is 

 smaller. And if the general chemical actions, by showing 

 us that combinations become weaker in proportion as the 

 number of the elementary atoms increases, are sufficient to 

 explain the tendency of organic bodies to resolve themselves 

 into more simple combinations ; if chemistry furnishes us 

 with many instances of this same tendency in some inor- 

 ganic compounds whose composition has many analogies 

 with that of organic bodies, it is not, therefore, to be in- 

 ferred that the laws of inorganic chemistry are sufficient to 

 give us a complete explanation of all the chemical pheno- 

 mena of life. We must then conclude that organization 

 and the molecular structure of living beings effect impor- 

 tant modifications in the action of physical and chemical 

 agents. 



.Actions of Contact or Catalysis. We must not, however, 

 omit to add that each successive day increases the number 

 of a particular class of chemical phenomena whose expla- 

 nation is impossible by the ordinary laws of affinity only ; 

 I refer now to actions of contact or catalysis. In the greater 

 number of these we find that a substance, usually in very 

 small quantity, excites in other compounds, without itself 

 undergoing any modification, considerable changes either 

 of chemical composition or of physical properties. To this 

 category of phenomena belong the various kinds of fermen- 

 tation. We shall find that the number of catalytic actions 

 in living beings is immense. Admitted. We can also 

 produce them in our laboratories ; they are of the same 

 nature as those which platinum black effects on a mixture 



