LATENT HEAT. 89 



not the fire be increased ; all the extra heat goes to melt the remainder of 

 the substance. The heat only produces changes of state. So this heat above 

 fusing point disappears apparently, and is called Latent Heat. This can 

 easily be proved by melting ice. Ice melts at 32 Fahr., or o Cent., and at 

 that temperature it will remain so long as any ice is left ; but the water at 

 32, into which the ice has melted, contains a great deal of latent heat, for 

 it has melted the ice quickly, and yet the thermometer does not show it. It 

 is just the same with boiling water. 



When substances are fused they expand as a rule, but ice contracts ; so 

 does antimony. On the other hand, when water solidifies it does not 

 contract as most things do. It expands, as many of us are aware, by finding 

 our water pipes burst in the winter ; and the geologist will tell us how the 

 tiny trickling rills of water fall in between the cracks of rocks and there 

 freeze. In freezing the drops expand and split the granite blocks. Type- 

 metal expands also when it becomes solid, and leaves us a clear type ; but 

 copper contracts, and won't do for moulding, so we have to stamp it when 

 we want an impression on it. 



There is no doubt that chemical combinations produce heat, as we can 

 see every day in house-building operations, when water is poured upon lime; 

 but there are also chemical combinations which produce cold. Fahrenheit 

 produced his greatest cold by combining snow and salt, for in the act of 

 combining, a great quantity of heat is swallowed up by reason of the heat 

 becoming latent, as it will do when solid bodies become liquid. Such 

 mixtures or combinations are used as Freezing Mixtures when it is necessary 

 to produce intense cold artificially. Sulphate of Sodium and Hydrochloric 

 acid will also produce great cold, and there are many other combinations 

 equally or even more efficacious. 



Heat is communicated to surrounding objects in three well-known, 

 ways by conduction, by radiation, and by convection. Conduction of heat 

 is easily understood, and is the propagation of heat through any body, and 

 it varies very much according to the substance through which it passes. 

 Some substances are better conductors of heat than others. Silver has a 

 far greater conductivity than gold, and copper is a better heat-conductor than 

 tin. Flannel is a non-conductor, or rather a bad conductor, for no substance 

 can be termed actually a non-conductor. Flannel, we know, will keep ice 

 from melting, and a sheep's wool or a bird's feathers are also bad conductors- 

 of heat ; so Nature has provided these coverings to keep in the animal heat 

 of the body. A good conductor of heat feels cold to the touch of our fingers, 

 because it takes the heat from our hands. This can be tried by touching 

 silver, lead, marble, wood, and wool. Each in turn will feel cold and less 

 cold, because they respectively draw away, or conduct less and less heat from 

 our bodies. So our clothes are made of bad-conducting substances. The 

 bark of a tree is a bad conductor, and if you strip off this clothing the tree 

 will die. 



Solids conduct heat the better the more compact they are. Air being 



