Energy requirements of animals have been carefully investigated by three methods: First, by determining the balance between the intake of energy in the food and the output in the excrement; second, by observing the amount of oxygen consumed during respiration and the amount of carbon dioxide exhaled; third ,ff by direct measurement of the heat given o by the body in a calorimeter. The three meth ods are combined in experiments made with the respiration calorimeter devised by Atwater. By these means feeding standards have been established, especially by Armsby, for different animals under different conditions such as growing, working, fattening and giving milk.
Besides the importance of the required amount of different food substances it is now known that the quality of the protein has some effect on its availability for animal use. Work by Osborn has shown that some proteins do not furnish nitrogen in a form that can be used by growing or adult animals. Analyses of these proteins show them to be deficient in certain ones of the amino acids of which pro teins are composed.
Carbon and Nitrogen The study of agricultural chemistry shows interesting cycles for carbon and nitrogen. Carbon is taken from the air as carbon dioxide by plants and synthesized into organic compounds which are then used by animals and returned to the air by oxidation during respiration or decom position. Some carbon is returned to the air by plants by oxidation during respiration or decomposition or combustion, and there are other branches to the cycle, but in the main it may be represented as air plant animal air plant animal, etc.
In the same way combined nitrogen as ni trates is taken from the soil by plants and synthesized into organic compounds which are then used by animals and returned to the soil in manure or the dead body. Here also some of the nitrogen is returned to the soil by de caying plants, but the main cycle is soil plant animal soil plant animal, etc.
Dairy Dairying is an import ant agricultural industry not only commer cially but also because dairy farming tends to keep the soil fertile by returning to it all the plant food taken from it except for the small amount used in producing the milk. Chemical
investigations have been directed to the com position and character of milk from different cattle and under different conditions, to the chemical changes taking place during souring and curdling, to the processes of malting but ter and cheese and to the effect of pasteuriza tion on the composition of milk.
Every State in the country now has an agricultural college or an agricul tural station where experiments in agricultural chemistry are carried on, and the United States Department of Agriculture is investigating many of the problems in this field. The As sociation of Official Agricultural Chemists, organized in 1880, is composed of the Federal and State officials who are enforcing the laws regarding foods, drugs, feeding stuffs, fertili zers, insecticides and fungicides, and those who are carrying on investigations along these lines under the Federal or State governments. The main work of this association is the im provement of the methods of analysis of agri cultural products and it meets annually to re port the results of collaborative work carried out on these methods. Its analytical methods are official in many of the Federal and State laws. The proceedings of the meetings and the methods of analysis are published in the Journal of the Association of Official Agri cultural Chemists.
Chamberlain, J. C, 'Or ganic Agricultural Chemistry' (1916) ; Fraps, G. S.
of Agricultural Chemistry' (1913); Hall, A. D., 'The Book of the Roth amsted Experiments' (1905); Hart, E. B., and Tottingham, W. E., 'General Agricultural Chemistry' (1910); Hilgard, E. W.,