150 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the closeness of the combination is inversely proportional to its com- 

 plexity. 



The degree of potential energy of a combination may be measured 

 by the amount of heat liberated in its combustion ; thus, a heat unit 

 is the amount of heat required to raise one gramme of water one 

 degree C. So one gramme of carbon yields 8000 heat units, or calorics ; 

 one gramme of hydrogen equals 34,000 heat units. These numbers are 

 greatly modified when the carbon and hydrogen are combined or enter 

 into combination with other elements. Thus, when pure carbon is 

 oxidized to C0 2 , the heat developed is less than when, with an equal 

 quantity of O, CO, or combinations of CH and O and H 2 are burned 

 to form C0 2 . The higher the atomic weight of compounds, the greater 

 the amount of heat given off in combustion. Thus, fats yield more heat 

 than sugar and alcohol ; but while equal weights of such high atomic 

 bodies yield more heat in proportion to their weight, when compared 

 with equal quantities of oxygen consumed the}' yield less than simpler 

 bodies, as sugar or alcohol. 



The elementary compounds which are found in animal and vegetable 

 cells in no way differ from those found in inorganic nature. Similar 

 elementary substances are found in the earth and atmosphere, and 

 become constituents of animal and vegetable organisms. In organisms 

 chemical affinity exerts the same sway as in inanimate nature. Acids 

 unite with bases to form salts within cells just as without; no one of 

 the elementary constituents of cells has lost its power of uniting with 

 oxygen, and the products so formed are identical with similar bodies 

 formed elsewhere. 



We have already traced the processes occurring in animal and 

 vegetable cells, by which these bodies are converted in the former from 

 simple elementary substances to complex organic bodies, and in the 

 latter again reduced to their simple elementary form. It is evident that 

 the animal and vegetable cells differing in the chemical processes which 

 occur within them will also differ in the transformations of energy which 

 occur within them. Thus, we have seen that vegetable cells containing 

 chlorophyll convert stable oxygen compounds of carbon and hydrogen, 

 C0 2 and H 2 0, with liberation of oxygen, into the looser organic com- 

 pounds, such as starch, or, less frequently, glucose or fat. They therefore 

 return to atoms of these combinations a portion of their potential energy, 

 which in the original transformation into C0 2 and H a O they had lost in 

 actual energy in the form of heat. To accomplish this, plant-cells require 

 the assistance of an external force, namely, the heat and light of the sun, 

 which they convert into a chemical potential force in the resulting 

 organic compounds. The condition is more complicated in cells which 

 possess no chlorophyll. Here, also, the reduction processes require an 



