DIVISIBILITY. 23 



each molecule being magnified in the same proportion, these 

 molecules would represent balls of about two inches in diameter. 



Whilst molecules consequently are exceedingly small particles, 

 yet they are not entirely immeasurable ; they are, as Sir W. 

 Thomson 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 

 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 correctness 

 of this law may be mathematically proved by the law of 

 Mariotte, but many other facts and considerations lead to the 

 same assumption. We shall learn, hereafter, that the law of 

 Avogadro is one of the greatest importance for 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 or at 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. Here the water begins to boil and 

 escapes in the form of steam, but the temperature again remains 

 stationary until the last particle 6*f 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 0, and the steam, 



