PART I. 

 CHAPTER I. 



IMPONDERABLE SUBSTANCES. 



SECTION I. 



CALORIC. 



The term Heat is used in two different senses : one signifying the 

 sensation conveyed by a heated body ; the other, the material cause 

 of heat ; this is called caloric. The effect of the union of caloric 

 with ponderable matter is to communicate a repulsive property to its 

 different molecules, overcoming its cohesion ; and causing, succes- 

 sively, expansion, fusion, and vaporization. Caloric is usually 

 spoken of under the two heads of sensible, or that which is evident 

 to the thermometer, and latent, or that which is insensible to the 

 thermometer. Latent heat may be illustrated by the following ex- 

 periment : — Mix a pound of water at 174°, and a pound of water at 

 32° together ; the temperature of the mixture will be the mean of the 

 two, or 103° ; but if a pound of snow or ice at 32°, be used instead 

 of the water at 32°, the resulting temperature will still be only 32°, 

 but the ice will have melted. Hence, in this last experiment, as 

 much heat has been rendered latent in the melting of the ice, as 

 would have raised an equal weight of water 07ie hundred and forty- 

 two degrees. Again, if a vessel containing water be placed over the 

 fire, the thermometer will indicate the constant increase in its tem- 

 perature up to the boiling point, 212° ; beyo7id this point, however, 

 provided the steam escape, the successive portions of heat received 

 by the liquid will be entirely insensible ; that is, the additional 

 caloric will be rendered latent. This latent caloric may, however, 

 be proved to exist, or may be rendered sensible by conducting the 

 steam into cold water, when it will be found to raise a quantity of 

 water ten times heavier than itself, nearly one hundred degrees ; or, 

 if concentrated in one of these parts, the rise of temperature would 

 be nearly 1000 degrees. Hence it follows, that as much heat is ab- 

 sorbed (rendered latent) in producing steam, as would raise the 

 water of which it consists, 1000 degrees, or to about a red heat, if 

 prevented from assuming the aeriform state. 



