250 



OXYGEN. 



with selenium bases, and tellurium acids with tellurium 

 bases. 



The combinations of oxygen, like those of all other bodies, 

 are attended with the evolution of heat. This result, which is 

 often overlooked in other combinations, in which the propor- 

 tions of the bodies uniting and the properties of their com- 

 pounds receive most attention, assumes an unusual degree of 

 importance in the combinations of oxygen. The economical 

 applications of the light and heat evolved in these combina- 

 tions are of the highest consequence and value, and oxi- 

 dation alone of all chemical actions is practised, not for the 

 value of the products it affords, and indeed without refe- 

 rence to them, but for the sake of the incidental pheno- 

 mena attending it. Of the chemical combinations too, which 

 we habitually witness, those of oxygen are infinitely the 

 most frequent, which arises from its constant presence and in- 

 terference, as a constituent of the atmosphere. Hence, when 

 a body combines with oxygen it is said to be burned ; and 

 instead of undergoing oxidation, it is said to suffer combustion ; 

 and a body which can combine with oxygen and emit heat is 

 termed a combustible. Oxygen, in which the body burns, is 

 then said to support combustion, and called a supporter of 

 combustion. 



The heat evolved in combustion is definite, and can be mea- 

 sured. With this view, it is employed to melt ice, to raise the 

 temperature of water from 32 to 212, or to convert water 

 into steam, and its quantity estimated by the extent to which 

 it produces these effects. The heat from the oxidation of a 

 combustible body is thus found to be as constant as any other 

 of its properties. Despretz obtained, -by such experiments, 

 the results contained in the following table : 



HEAT FROM COMBUSTION. 



1 pound of pure charcoal heats from 32 to 212, 78 Ibs. of water. 



charcoal from wood .... , , 75 



baked wood , , 36 ,, 



wood containing 20 per cent, of water , , 27 



bituminous coal .... , , 60 ,, 



turf , , 25 to 30 



alcohol , , 67.5 



olive oil, wax, &c , , 90 to 95 



ether . 

 hydrogen 



80 

 236.4 



The quantity of heat evolved appears to be connected with 



