LATENT HEAT. 43 



or steam. These three states of water are familiar 

 to every one, but few are aware what causes the great 

 difference between them. 



46. When ice is placed before the fire, or exposed 

 to the sunshine, or in any other way warmed, it ab- 

 sorbs heat, it melts and becomes water; and when 

 water is heated, it assumes the form of steam or 

 vapor. The difference between these three forms of 

 water is entirely caused by the quantity of heat they 

 contain ; and we may truly say that water is a com- 

 pound of ice and heat, and that steam is a compound 

 of water and heat. 



47. Although this seems very like chemical action, 

 it is really quite different, and must not be confounded 

 with it. Chemical action can only take place between 

 material substances, or those that have weight; now 

 heat is not a substance — it is not a thing we are able 

 to weigh, like all the chemical elements, and conse- 

 quently when it combines with any substance it only 

 alters the appearance and outward characters of that 

 substance, but does not at all change its chemical 

 properties or nature. 



48. When heat is thus combined with a substance, 

 it is said to be latent (hidden), which means that it is 

 not sensible to the feel. This will be easily under- 

 stood from a very simple experiment. If we put some 

 water in a kettle on the fire, we find that it will soon 

 begin to feel warm to the hand if immersed in it; the 

 warmth which we then feel is called free or sensible 

 heat: but if we put some ice into the kettle in place 



