254 OXYGEN. 



But if the oxide of mercury, so produced, be then put into a 

 small retort, and reconverted by a red heat into oxygen and 

 fluid mercury, the quantity of the oxygen emitted is found to 

 be the same as had combined with the mercury *in the first part 

 of the operation, thus proving that oxygen is really present in 

 the oxidized body. 



The evolution of heat, which is the most striking pheno- 

 menon of combustion, still remains to be accounted for. It 

 has been referred to the loss of latent heat by the combustible 

 and oxygen, when, from the condition of gas or liquid, they 

 become solid after combustion ; to a reduction of capacity 

 for heat, the specific heat of the product being supposed to be 

 less than that of the bodies burned ; to a discharge of the elec- 

 tricities belonging to the different bodies, occurring in the act 

 of combination. But the first two hypotheses are manifestly 

 insufficient, and the last is purely speculative. The evolution 

 of heat during intense chemical combination, such as oxidation, 

 may be received at present as an ultimate fact ; but if we choose 

 to go beyond it, we may suppose that the heat exists in a com- 

 bined and latent state in either the oxygen or combustible, or 

 in both, that each of these bodies is a compound of its material 

 bases with heat, the whole or a definite quantity of which they 

 throw off on combining with each other. Heat, like other ma- 

 terial substances, is here supposed, not to evince its peculiar 

 properties while in a state of a combination with other matter, 

 but only when isolated and free. This view gives a literal cha- 

 racter to the expressions, liberation, disengagement, and evo- 

 lution of heat during combustion. The phenomenon, it is to 

 be remembered, is not confined to oxidation, but occurs in an 

 equal degree in combinations without oxygen, and indeed to a 

 greater or less extent in all chemical combinations whatever. 



Uses. Pure oxygen has not as yet found any considerable 

 application in the arts. But by the chemist it is applied to 

 support the combustion of hydrogen gas, in producing intense 

 heat. A more considerable application of it is likely to arise 

 in the combustion of oil in the lamp of Mr. Gurney, to produce 

 an intense light suitable for marine light- houses. In this lamp, 

 which is an Argand with several concentric wicks, oxygen gas 

 from a gasometer is admitted into the centre of the flame, and 

 is found to produce so much more light than air does, from the 

 combustion of the same quantity of oil, as fully to compensate 



