Chemical Effects of Caloric, $c. 63 



considered as warm ; but they have no heat in 

 themselves, for they keep in fact any body cool 

 better than other substances, as ice in ice-houses 

 is kept under straw. These bodies hinder the 

 caloric from escaping, their interstices being 

 filled with air, which is the rarest of fluids. 

 Snow keeps the ground warm also, from its 

 being of a soft and spongy texture. It is true 

 it cannot keep the ground warmer than the 

 freezing point, but this is warm when compared 

 with the intense cold felt in several climates, fre- 

 quently 32 below frost ; the freezing point be- 

 ing as much warmer than this, as our summer 

 weather is hotter than frost. 



Fluids convey heat very readily. Air cools 

 bodies extremely fast. This, however, may de- 

 pend upon the expansibility of air when heated, 

 which will produce the effects of a constant 

 change. Thus, if I expose a hot body to the 

 air to cool, the air that is in contact with it ex- 

 pands and becomes lighter, consequently it is 

 driven upwards; and thus there is a constant 

 succession of cold air applied to the body : and 

 if we place the heated body between the sunshine 

 and a wall, we shall see the rarefied vapour 

 rising like an undulating smoke upon the wall. 

 The reason why we see it is, because the rays of 

 light which pass through these steams are turned 

 aside ; by this means the wall is less illuminated 

 in this part than the rest, and therefore we see 

 the shadow, for the same reason that we see the 



