SOURCES OF CARBON IN NATURE. 297 



derived from water is an essential part of the nutrition of 

 plants. It is in this way that hydrogen comes to enter into 

 organic structure. Neither water nor hydrogen is accounted 

 an aliment of animals, because, as already said (p. 286), though 

 water is essential to the life of animals, it is in order to fur- 

 nish in its entire state a basis for the tissues, and not that 

 it may be decomposed to supply hydrogen to the organic 

 structure. The hydrogen which so universally is present in 

 the organic structure of animals is derived from their organic 

 food, into the composition of which the hydrogen enters by 

 the decomposition of water in the vegetable kingdom. 



Carbon. Carbon, the most familiar form of which is char- 

 coal of wood, exists only in the solid state. It cannot be 

 volatilised by any intensity of heat. Hence it is to be re- 

 marked that smoke, which essentially, or at least most com- 

 monly, is carbon, is really in the solid form, however much it 

 may be comminuted, and that it ascends merely because the 

 heated air in which it is suspended rises upwards. Carbon 

 uncombined exists but sparingly in the mineral kingdom, but 

 combined with oxygen it abounds in the form of carbonic acid 

 gas. Besides what exists in the atmosphere, it is a component 

 in the numerous varieties of carbonate of lime, as chalk, marble, 

 limestone, marl, of which it forms nearly an eighth part, and 

 of the carbonate of magnesia, of which it constitutes more 

 than a seventh part. It is the characteristic constituent of 

 all substances termed organic. The solid parts of plants, 

 shrubs, and trees owe their form and solidity to this element. 

 In the tissue of wood the proportion of carbon is nearly 

 three-sevenths, while in dried muscular flesh the proportion 

 of carbon by weight is not far from one-half. 



Nitrogen. Nitrogen is also called azote, and this last name 

 is so frequently used, both in the simple form and in com- 

 pound words, that the knowledge of it cannot be dispensed 



