446 NATURE'S TEACHINGS. 



been filled too much to allow for the expansion of the heated 

 liquid. 



Advantage has been taken of this principle in the formation 

 of the Thermometer, a word which signifies " heat -measurer." 

 Liquid of some kind is placed in an hermetically sealed tube, 

 generally terminating with a bulb, and in proportion to the 

 heat the liquid expands, and is forced up the tube. 



Any liquid will answer to a certain extent, but, as water 

 freezes at 32, it would be useless for measuring degrees of cold 

 below the freezing point. Coloured spirits of wine are used ; 

 but the very best liquid is mercury, which is a metal in a state 

 of fusion. 



This expansion by heat is so powerful in iron, that it is 

 utilised in several ways. 



Take, for example, wheel-making. The iron tire is made 

 rather smaller than the wheel, and is then placed in a fire until 

 it is red-hot. It then expands so much that it can be easily 

 slipped over the wheel as it lies on the ground. Cold water is 

 then dashed on it, and the tire contracts with tremendous force, 

 binding the parts of the wheel firmly together. 



In all buildings where iron is much used, such as iron 

 bridges, iron beams, &c., it is necessary to make allowance at 

 both ends, so as to permit the iron to expand on a hot day and 

 contract on a cool one. Buildings formed of stone and iron 

 were once thought to be safe in case of fire. They are now 

 known to be just the contrary, the stone flying with the heat, 

 and the iron expanding. 



