24 INTRODUCTION. 



having also absorbed large quantities of heat, shows 100, the 

 temperature of boiling water. A certain amount of heat has con- 

 sequently been lost or at least hidden. What has become of it? 



According to our present theory, heat is caused by the motion of 

 molecules. All molecules of any substance are in a constant 

 vibratory motion, and the velocity of this motion determines 

 the degree of what we call heat. 



An increase of heat is equal to an increase of the vibratory 

 motion of the molecules and a decrease in temperature is caused 

 by slower motion. The transfer of heat is a transfer of the 

 motion of some particles to other particles. 



One of the effects of increased heat is in nearly all cases an 

 increase in volume, or, in other words, all substances expand 

 when heated, and contract on cooling. 



Another effect of the application of heat is, as we have just 

 learned, the conversion of solids into liquids, and of liquids 

 into gases. We also noticed the apparent loss of heat during < 

 this conversion, and can easily account for it now by saying, 

 that a certain amount of vibratory motion, or a certain velocity 

 of the molecules, is required to convert solids into liquids and 

 liquids into gases. The molecules of steam vibrate with a much 

 greater velocity than those of water of the same temperature, 

 and the molecules of water move with greater velocity than 

 those of ice of the same temperature. In other words, the 

 different states of aggregation depend on the rapidity of the 

 motion of molecules and the heat which is necessary to convert 

 solids into liquids and liquids into gases, and which is not indi- 

 cated by the thermometer, is called latent heat. 



This latent heat may again be converted into free heat (heat 

 capable of being indicated by a thermometer) by reconverting 

 the gas into a liquid, or this latter into a solid. In both cases a 

 liberation of heat, which is a transfer of the motion of the 

 molecules upon the surroundings, will be noticed. 



Increase of volume by heat. The increase of volume by heat 

 is not alike for all matter. Gases expand more than liquids, 

 liquids more than solids, and of the latter the metals more than 

 most other solid substances. Whilst the expansion of any two 

 or more different solids or liquids is not alike, gases show a 

 fixed regularity in this respect, namely, all gases, without ex- 



