Human Physiology. 91 



the bodies of minute creatures. The limestone that forms vast 

 mountain ranges, or covers continents, has been separated 

 atom by atom from the water in which it was dissolved, by the 

 agency of animal life. 



The surface of the earth is formed of metals, oxygen and 

 carbon, the latter uniting first with oxygen and forming carbonic 

 acid gas, which always exists in minute proportions in the 

 atmosphere, enters into the composition of all vegetables, and 

 is given out by all animals. Hydrogen, oxygen, nitrogen, 

 chlorine, carbon, sulphur, phosphorus, calcium, sodium, mag- 

 nesium, iron, enter into the composition of vegetables and 

 animals. 



It will be seen that the most important elements of nature 

 exist in the lightest, or most attenuated forms. Hydrogen* is 

 the lightest of gases eight times lighter than oxygen yet it is 

 the chief element of water, without which there could be no 

 soil, no life; and it enters largely into the composition of oil, 

 fat, woody fibre, and animal tissues. Oxygen is the vital 

 element of air, is eight-ninths by weight of water, in union with 

 metallic bases it forms the whole crust of the earth, and enters 

 into the composition of all vegetables and animals. Nitrogen 

 forms four-fifths of the bulk of the atmosphere, and in combi- 

 nation with hydrogen, as ammonia, is the basis of manures, a 

 component of vegetables and an important element of animal 

 life. Chlorine unites with sodium to form the important 

 substance, common salt. Carbon, forming a gas with oxygen, 

 is the basis of vegetable and animal matter, is stored in the 

 earth in coal, and crystallised in the diamond. 



There are physicists who deny the existence of matter 

 that is, of solid atoms and material molecules, and refer all the 

 phenomena of matter to points of force. Hydrogen, oxygen, 

 carbon, etc., are different kinds of force, acting from mathema- 

 tical points. Every object in nature is simply a collection of 

 such points of force acting upon our senses. A still more 



