26 INTRODUCTION. 



from expanding, the increase of heat will manifest itself as pressure, which 

 rises with the same regularity as shown for expansion, viz., 0.3665 per cent. 

 for every degree centigrade. 



Melting 1 and boiling-. The temperature at which a solid sub- 

 stance is converted into a liquid and this into a gas, is of a certain 

 fixed degree or point for every substance, and the temperatures at 

 which this conversion takes place are known as melting- (fusing-) 

 and boiling-points. 



Some forms of matter appear incapable of existing in the three 

 states of aggregation, like water. As yet, we know carbon in the 

 solid state only, and the conversion of some gases, as, for instance, 

 oxygen and hydrogen, into liquids or solids, has been accomplished 

 only recently and in small quantities. 



Other substances, again, may assume two, but not the third state. 

 Some substances pass from -the solid directly into the gaseous state 

 (ammonium chloride, calomel), and the process of converting a solid 

 into a gas directly, and this back again into a solid, is called sublima- 

 tion. 



(Distillation is the conversion of a liquid into a gas, and the recon- 

 densation of the gas into a liquid. 



Many liquids, and even some solids, evaporate or assume the gaseous state at 

 nearly all temperatures. Water and ice, mercury, camphor, and many other 

 substances vaporize at temperatures which are far below their regular boiling- 

 points. This fact is to be explained by the assumption that during the rapid 

 vibratory motion of the particles of these masses, some particles are driven 

 from the surface beyond the sphere to which the surrounding molecules exert 

 an attraction, and thus intermingle with the molecules of the surrounding air. 



This evaporation, which takes place at various temperatures and at the 

 surface only, is not to be confounded with boiling, which is the rapid conver- 

 sion of a liquid into a gas at a fixed temperature with the phenomena of ebulli- 

 tion, due to the formation of gas in the mass of liquid. Bojling-point may 

 therefore be denned as the highest point to which any liquid nan baJieated 

 under the normal pressure of one atmosphere. 



Thermometers are instruments indicating different temperatures. 

 Use is made in their construction of the change in volume of dif- 

 ferent substances by the action of heat. The most common ther- 

 mometer is the mercury thermometer. This instrument may be 

 easily constructed by partly filling with mercury a glass tube having 

 a bulb at the lower end, and placing it into boiling water. The 

 point to which the mercury rises is marked B. P. (boiling-point), 

 and the tube sealed by fusion of the glass. It is then placed in 



