4 ELEMENTS OF BIOLOGY. 



tendency to pass through a series of cyclical changes, which 

 follow one another in a regular and determinate sequence. 



The above points are the leading characters by which 

 living bodies are fundamentally separated from dead matter. 

 There are, however, a few subordinate points in which 

 some or all living bodies differ from those which are dead : — 



a. Chemical Composition. — Dead bodies are composed of 

 numerous elements, which exist either in an uncombined 

 condition, or in a state of union. The combinations of 

 these elements may be said to be naturally in a state of 

 stable equilibrium, and they show no tendency to spon- 

 taneous decomposition. Further, the combining elements 

 unite with one another in low combining proportions, and 

 the resulting compounds for the most part consist of no 

 more than two or three elements. 



Living bodies, on the other hand, are composed of few 

 chemical elements, and these are almost always in a state 

 of combination. Furthermore, the combinations are always 

 complex, consisting of three or four elements, and these 

 elements are united with one another in high combining 

 proportions. Finally, the chemical compounds of living 

 bodies are invariably characterised by the presence of 

 water, and are prone to spontaneous decomposition. Thus, 

 the great organic compound, albumen, is composed of 144 

 atoms of carbon, no of hydrogen, 18 of nitrogen, 42 of 

 oxygen, and two atoms of sulphur. Iron, however, exists 

 in the blood, possibly in its elemental condition, and cop- 

 per has been detected in the liver of certain of the Mam- 

 malia, and largely in the colouring-matter of the feathers of 

 certain birds. It is to be remembered, also, that certain 

 mineral salts, well known as occurring in dead nature, are 

 apparently absolutely indispensable to living bodies, at any 

 rate as a general rule. Living bodies, therefore, whilst 

 certainly presenting us with a peculiar group of chemical 

 compounds, are to a certain extent built up of substances 

 which commonly occur dissociated from vitality. 



