30 
COMBUSTION. 
Chap. V. 
whence comes the heat that manifests itself during 
combustion? Is this heat merely a property of light? Dr 
Black proved that heat is capable of combining with, or 
becoming fixed in bodies which are not combustible, as 
in ice and water; and concluded of course, that it is not 
a property but a body. This obliged philosophers to 
take another view of the nature of phlogiston. 
According to them, there exists a peculiar matter, 
extremely subtile, capable of penetrating the densest 
bodies, astonishingly elastic, and the cause of heat, light, 
magnetism, electricity, and even of gravitation. This 
matter, the etber of Hooke and Newton, is also the 
substance called phlogiston, which exists in a fixed 
state in combustible bodies. When set at liberty, it 
gzives to the substances called caloric and light those 
peculiar motions which produce in us the sensations of 
heat and light. Hence the appearance of caloric and 
light in every case of combustion; hence, too, the rea 
son that a body after combustion is heavier than it was 
before ; for ae phlogiston is itself the cause of gravita 
tion, it would be absurd to suppose that it possesses 
gravitation ; it is more reasonable to consider it as en 
dowed with a principle of levity. 
Some time after this last modification of the phlo¬ Alteredby 
Priestley. 
gistic theory, Dr Priestley, who was rapidly extending 
the boundaries of pneumatic chemistry, repeated many 
experiments formerly made on combustion by Hooke, 
Mayow, Boyle, and Hales, besides adding many of his 
own. He soon found, as they had done before him, 
that the air in which combustibles had been suffered to 
burn till they were extinguished, had undergone a very 
remarkable change; for no combustible would after 
wards burn in it, and no animal could breathe it with¬