74 JUNIOR GRADE SCIENCE 



the same reason the ends of iron bridges are not fixed to the supports 

 upon which they rest. Iron tyres are put on carriage wheels by first 

 heating the tyre, and while it is hot, slipping it over the wheel. As 

 the tyre cools it contracts and clasps the wheel tightly. 



The common occurrence in domestic life of the cracking of thick 

 glasses, when boiling water is poured on them, may be explained by 

 this expansion of solids by heating. The part of the glass with which 

 the hot water comes in contact is heated and expands ; but the effect 

 is quite local ; the heating is confined to one spot, because glass does 

 not allow heat to pass through it readily. It is this local expansion 

 of the glass which results in the cracking of the vessel. 



It is not to be supposed, however, that substances always expand 

 on being heated ; it will be learned later that water, under certain 

 conditions, shrinks in volume as its temperature rises. A piece of rubber 

 too, stretched by a weight, contracts considerably on being heated. 

 This result is, however, deceptive. Unstretched rubber acts in the 

 usual way and expands as its temperature rises. The explanation is 

 that hot rubber is not so easily stretched as cold. The amount of 

 stretch due to the weight is lessened, and masks the expansion duo to 

 the heat. 



Measurement and change of temperature. Change of temperature 

 means change in the state of hotness or coldness of a body. The 

 change of size which takes place when a thing is heated gives a good 

 way of measuring the change of temperature which it undergoes. 

 Think of the experiment with the coloured water in the flask with a 

 long tube attached to it. Suppose the coloured water in the tube 

 rises through a certain number of inches after the water has been heated, 

 and that when the flask is placed into some other liquid, or some more 

 water, the coloured water is found to rise up the tube to just the same 

 place, we should have every right to say that the second liquid is exactly 

 as hot as the first was. This plan proves a means of measuring tem- 

 perature. The flask and tube with the water have become a " tem- 

 perature measurer," that is, a thermometer. 



