Page:The Scientific Monthly vol. 3.djvu/187

Rh are only about 1/1800 of the mass of the hydrogen atom: they are liberated from metals on which ultra-violet light falls, and can be released from atoms of matter by a variety of agencies. Hydrogen is present in all acids and in most organic compounds. It also has the highest power of combustion. Its ions are very important factors in artificial respiration and in gastric digestion. It is very active in dissociating or separating oxygen from various compounds, and through its affinity for oxygen forms water, the principal constituent of protoplasm.

Oxygen, like hydrogen, has an attractive power which brings into the organism other elements useful in its various functions. It makes up two thirds of all animal tissue as it makes up one half of the earth's crust. Beside these attractive and synthetic functions its great service is as an oxidizer in the release of energy; it is thus always circulating in the tissue. Through this it is involved in all heat production and in all mechanical work, and affects cell division and growth.

Nitrogen comes next in importance to hydrogen and oxygen as structural material and when combined with carbon and sulphur gives the plant and animal world one of the chief organic food constituents, protein. It was present on the primordial earth, not only in the atmosphere but also in the gases and waters emitted by volcanoes. Combined with Hydrogen it forms various radicles of a basic character (e. g., amino acids, in ammonium compounds); combined with oxygen it yields acidic radicles such as  in nitrates. It combines with carbon in radicles and in  and  forms, the latter being particularly important in protoplasmic chemistry. This life element forms the basis of all explosives, it also confers the necessary instability upon the molecules of protoplasm because it is loath to combine with and easy to dissociate from most other elements. Thus we find nitrogen playing an important part in the physiology of the most primitive organisms known, the nitrifying bacteria.

Carbon also exists at or near the surface of cooling stars which are becoming red. It unites vigorously with oxygen, tearing it away from neighbouring elements, while its tendency to unite with hydrogen is less marked. At lower heats the carbon compounds are remarkably stable, but they are by no means able to resist great heats; thus Barrell observes that a chemist would immediately put his finger on the element carbon as that which is needed to endow organic substance with complexity of form and function, and its selection in the origin of plant life