XXX INTRODUCTION 



when burned, while the inorganic carbon compounds cannot be burned. The 

 heat of combustion, measured in calories, serves as an index of the energy 

 content of organic compounds. By a large calory, or kilogram-calory (Cal., 

 or kg.-cal.) is meant the amount of heat necessary to raise the temperature of 

 iooo g. of water from o° to i°C; by a small calory, or gram-calory (cal. or 

 g.-cal.) is meant the amount of heat necessary to raise the temperature of i g. 

 of water the same amount. 



The following table shows the amounts of heat obtained from the combustion 

 of i g. of various substances, expressed in kilogram-calories. 



Hydrogen 34-6 



Carbon 8 . о 



Linseed oil 9.3 



Ethyl alcohol (C 2 H 6 0) 7.1 



Gluten flour 5.9 



Ammonia (NH ») 5.3 



Starch (C 6 H 10 O 5 ) 4.1 



(Hucose (C6G12OG) 3.7 



Asparagin (C4H2N2O3) 3.3 



It is evident from this table that hydrogen develops much more heat 

 during combustion than does carbon. The more oxygen the molecule of a 

 substance contains, the less is its heat of combustion, and it is for this reason 

 that ethyl alcohol develops more heat than starch. The introduction of hy- 

 drogen into the molecule, on the contrary, produces a great increase in the heat 

 of combustion; thus, oil develops more heat than does pure carbon, while 

 ammonia, without any carbon at all — but because of its high hydrogen content 

 — produces a far greater amount of heat than does either starch or glucose. 



Wohler's discovery led to a great advance in the physico-chemical interpreta- 

 tion of physiological processes. But there were still other difficulties to 

 overcome. Many chemical reactions go on in plants and animals at the tempera- 

 ture of the organism (i.e., about ordinary room temperatures), while the same 

 reactions outside the organism occur only at much higher temperatures or with 

 the aid of strong acids. For instance, as will be seen later, plant respiration 

 is a process of oxidation or combustion, but it proceeds at medium temperatures, 

 while ordinary combustion requires a very high temperature. While plant 

 and animal substances outside of the organism generally undergo oxidation 

 slowly at ordinary temperatures, with the oxygen of the air, they are oxidized 

 much more rapidly in the organism, at the same temperatures. This dis- 

 crepancy was explained by the theory of catalysis, advanced by Berzelius in 

 1836. Catalytic action, according to this author, is a process wherein certain 

 substances (called catalyzers) are capable of accelerating chemical reactions be- 

 tween other substances, by the presence of the catalyzer alone, independently 



a The gram-calory is frequently defined as the heat required to raise the temperature of,a 

 gram of water one degree Centigrade, but this is not precise, since the specific heat and the 

 heat of vaporization of water vary with its temperature. The definition given in the text 

 is that of the o-degree gram-calory. Other calories are in use, as the 15-degree gram-calory, the 

 heat needed to alter the temperature of a gram of water from 14.5 to i5.5°C, etc. — Ed. 



