144 HEAT. 



ice. When water is thus cooled down, the least agita- 

 tion causes it to become ice instantaneously. 



Any kind of salt dissolved in water, lowers its freezing 

 point. Hence, sea water does not freeze so soon as fresh 

 water. 



OF THE CAUSE OP HEAT. 



There are many natural agencies, the causes of 

 which remain unknown at the present day, and probably 

 always will. Learned men have been unremiting in 

 their investigations for ages, to discover the hidden 

 causes of natural phenomena, but in many cases 

 without effect, among which is the cause of heat. 

 Philosophers and learned men have, as yet, been unable 

 to determine whether heat is caused by a peculiar subtile 

 fluid which enters into, and is expelled from bodies, and 

 thus produces the sensation of heat or cold ; or whether 

 the cause is owing to a motion of the particles of the 

 body, vibratory or rotatory. There have been numerous 

 arguments adduced in support of both suppositions, all 

 of which, however, are inconclusive ; and as our object is 

 rather to explain the laws and effects of heat, than to deal 

 with ingenious theories, we shall omit any detailed account 

 of the arguments and experiments in favor of either. 



OF THE MOTION OF HEAT. 



When we hold our hand near a piece of hot iron we 

 feel the sensation of heat, which is caused by the heat of 

 the iron flying off in all directions, in the same manner 

 as sparks fly from heated iron when hammered upon the 

 anvil. This peculiar motion of heat is called radiation. 



When we hold a piece of iron with one end of it in the 

 fire, and the other in the hand, that part of the iron which 

 is in the hand will in time become hot ; which is in 

 consequence of the heat being conducted through the 

 iron, from one end to the other : this property of heat is 

 called conduction. 



Radiation. Every person must have observed that 

 when a hot body is exposed to the air, that it cools in a 

 short time, and that bodies do not cool in the same time ; 

 now is this difference owing to the form or dimensions of 

 the body ? or the nature or color of its surface 1 the 

 answers to these questions lead to some highly important 

 practical results. 



