On Fire. 351 



fire, during the strong frosts, we should become inactive, and sub- 

 ject to many unpleasant sensations; the aged and the weak would 

 perish; and what would become of the little infant, if the chilly 

 blasts were not tempered to its delicate limbs? 



Fire dilates such bodies as are exposed to its influence. A 

 piece of iron made to fit a hole in a plate of metal, so that it easily 

 passes through when cold, being heated, cannot be made to enter; 

 but upon being again cooled, readily passes into the hole as at first. 

 This dilation caused by the heat, is still more perceptible in fluid 

 bodies; as spirits, water, and more particularly air; and upon this 

 principle our thermometers are constructed. If we observe the 

 effects of fire upon compact and inanimate substances, we shall 

 find that they soon begin to melt, and change in appearance, part 

 becoming fluid, or remaining solid, but of a different nature. 

 Heat communicates fluidity to ice, oil, and all fat substances, and 

 most of the metals. Some sohd bodies undergo other changes; 

 sand, flint, slate, quartz, and spar, become vitrified in the fire; 

 clay is converted into stone; marble, calcerous stones, and chalk 

 are changed into lime. The diversity of these effects does not 

 proceed from the fire, but from the different properties of the bod- 

 ies upon which it acts. It may produce three kinds of effects 

 upon the same body; it may melt, vitrify, and reduce it to lime 

 provided that it possesses the three necessary properties, of being 

 metallic, vitrifiable, and calcerous. Thus fire of itself produces 

 nothing new; it only developes in bodies those principles, which, 

 before its action, were not perceptible. Upon fluids, fire produces 

 two effects, it makes them boil, and converts them into vapor. 

 These vapors are formed of the most subtile particles of the fluid 

 separated by the fire ; and they ascend in the air because they are spe- 

 cifically lighter than that fluid. In living creatures fire produces 

 the sensation of heat in every part of the body; without this ele- 

 ment man could not preserve Hfe; a certain degree of heat is 

 necessary to give vitality and motion to the blood; for which pur- 

 pose we are constantly inhaling fresh air, which always contains the 

 matter of heat, and imparts it to the blood in the lungs, while this 

 organ of respiration expels the air that has lost its vivifying prop- 

 erties. 



