VIII. 



CHANGES I\ MATTER PRODUCED BY HEATING. 



We lia^e already referred to two of the changes heat can produce 

 in matter, namely, an increase of size and a rise of temperature; but 

 if you try to think of the most striking change that heat gives rise 

 to, you will probahly decide that it is the change from solid to liquid 

 or from liquid to vapor. To illustrate the former change, suppose you 

 rake two similar cans, fill one with crusjied ice well packed in, and in 

 the other pour ice-cold water until the two cans seem equally heavy 

 (you may weigh them if you have a balance). >;<i\\', put them on a 

 stove, taking care to see that they are on nearly equally hot places. 

 You will find that when the water is so hot that you can barely put 

 your finger in it, the ice has juerely melted and the water it has 

 formed is nearly ice cold. Xow, they must have taken in about equal 

 quantities of heat from the stove, and mi you see that to melt ice 

 requires nearly as much heat as to raise the same quantity of water 

 from the melting point to the boiling point. This heat that goes to 

 melt ice does not ])roduce the ordinary effect of making the substance 

 warmer and hence the heat so used is called hidden or latent heat. 



Now, let us turn to the other case, that in which water turns into 

 vapor. This is somewhat more difficidt to experiment on, but you 

 will see into it to some extent by putting a canful of water on the 

 stove and ])lacing a thermometer in it. You must take care not to 

 use for this ])ur]iose a thermouu'ter such as is used for findino- tem- 

 perature out (if doors, but one that cooks soiuetiuu's use for finding 

 the heat of the nven will do for the puipose. You would only break 

 the former, fur it is not supposed to go as high as the temperature of 

 boiling ^\•ater. Sii]i]Hising. however, that you have the right kind of 

 thermometer, you will notice that after the water boils its tempera- 

 ture does not rise any higher, in spite of the I'act that it must all the 

 time 1)0 taking in heat from the stove. This heat has been taken 

 away with the vapor and is necessary for the turning of water into 

 vapor. Heie, then, we have another case of hidden heat; the heat 

 requireil to make va]ior from water is called the latent heat of vapor. 

 Yini will gCT. another example of it if you wet a finger and hold it up 

 in the wind (as claimed in Leaflet 1".'); you will find that as the 

 water eva])(irat('s the side to the wind bi'Cdmes much cooler than the 

 rest, showing that heat is taken up in vaporizino- the water, and as 

 this heat is taken from the finger it lea\'es that side colder. You will 

 see the same thing much more clearly if you let a drop of ether fall 

 on the back of vour hand. 



