DIVISIBILITY. 25 



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 

 noticed also 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 (more correctly speaking, 

 perhaps, a certain amplitude of molecular motion) is required to con- 

 vert 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 thej heat which is necessary to convert 

 solids into liquids and liquids into gases, and which is not indicated 

 by the thermometer, is called latentjieat. j 



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

 of being indicated by a thermometer), by reconverting the gasjnto a 

 liqjoidy-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 surround- 

 ings, 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 othgr 

 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 exception expand or contract 

 alike when the temperature is raised or lowered an equal number of 

 degrees. 



This expansion or contraction of gases is 0.3665 per cent., or -yfa of their 

 volume for every degree centigrade ; thus 100 volumes of air become 100.3665 

 volumes when heated 1 degree C., or 136.65 when heated 100 degrees C. This 

 regularity in the expansion and contraction of gases is expressed in the Law of 

 Charles, which says : If the pressure remain constant, the volume of a gas increases \ 

 regularly as the temperature increases, and decreases as the temperature decreases. 

 If heat be applied to a gas confined in a closed vessel and be thus prevented I 



