24 INTRODUCTION. 



says, pieces of matter of measurable dimensions, with shape, motion, 

 and laws of action, intelligible subjects of scientific investigation. 



Before leaving the molecular theory, I will mention the Law (more 

 correctly, the hypothesis) of Avogadro, which may be stated as follows : 

 All gases or vapors, without exception, contain, in the same volume, the 

 same number of molecules, provided temperature and pressure are the 

 same. Or, in other words : Equal volumes of different gases contain, 

 under equal circumstances, the same number of molecules. The correct- 

 ness of this law has good mathematical support deduced from the law 

 of Mariotte, many other facts and considerations leading to the same 

 assumption. We shall learn, hereafter, that the law of Avogadro is 

 one of the greatest importance to the science of chemistry. 



Motion of molecules. Heat. If we place over a gas-flame a 

 vessel containing a lump of ice of the temperature of C., or 32 F., 

 the ice gradually melts and becomes converted into water ; but if we 

 measure with a thermometer the temperature of the water at the 

 moment when the last particle of ice is melted, we still find it at the 

 freezing-point, 0'C. or 32 F. From the position of the vessel over 

 the flame, as well as from the fact that the ice has been liquefied, we 

 know that the vessel and its contents have absorbed heat. Yet vessel 

 and water show the same temperature as before. If the heat of the 

 flame is allowed to continue its action on the ice-cold water, the ther- 

 mometer will soon indicate a rapid absorption of heat until it reaches 

 100 C. or 212 F. Then the water begins to boil and escapes in the 

 form of steam, but the temperature again remains stationary until the 

 last particle of water has disappeared. 



There must be, consequently, some relation between the state of 

 aggregation of a substance and that agent which we call heat. It was 

 the heat which liquefied the ice, it was the heat which converted the 

 liquid water into steam or gaseous water. Yet the water, having 

 absorbed considerable heat during the process of melting, shows a 

 temperature of C. (32 F.), and the steam also having absorbed 

 large quantities of heat, shows 100 C. (212 F.), the temperature of 

 boiling water. A certain amount of heat has consequently been lost 

 or at least hidden. What has become of it ? 



According to our present theory /heat is a result of the motion of 

 molecules\ All molecules of any substance are in a constant vibratory 

 motion, and the velocity or amplitude of this molecular motion deter- 

 mines the degree of what we call heat. 



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



