22 Elementary Biology. 



plex compound known, is said to contain no fewer than 

 1,897 atoms (C GO oH 060 N m FeS 3 O 179 ). 



The more complex unstable compounds are chiefly 

 formed as products of the vital processes, and are found 

 associated for the most part with living organisms. They 

 have, therefore, been termed organic. Similarly, owing to 

 the fact that the simple stable compounds are found, as a 

 rule, in the outside world, and in great measure independent 

 of living things, they have been termed inorganic. 



The general tendency of elements is to unite to form, 

 and of complex compounds to decompose into, simple 

 stable compounds. 



SECTION IV. LAWS OF CHEMICAL CHANGE. 



The chemical changes taking place in an organism are 

 of three types, synthesis, isomeric change, and decompo- 

 sition. 



1. By synthesis is understood the building up of com- 

 pounds out of elements, or out of simpler compounds, in 

 obedience to their several chemical affinities. 



2. Isomeric change frequently occurs in the plant and 

 animal organism, and consists in the rearrangement of the 

 atoms of a compound. Hence we have to distinguish 

 between the composition and the constitution of a com- 

 pound, i.e. between simply the number and nature of the 

 atoms, and the relation of the atoms to one another. 



3. Decomposition. A compound is decomposed when 

 (a) the complex molecule breaks up into two or more simpler 

 molecules, the sum of the atoms in all of which is iden- 

 tical with the sum of those in the original molecule. Thus 

 grape-sugar, when subjected to a certain treatment, decom- 

 poses or dissociates into two molecules of alcohol and two 

 molecules of carbonic acid. 



(/>) By dehydration, or the separation of a certain 

 number of atoms of hydrogen and oxygen in the form of 



