112 ELEMENTS OF LABORATORY WORK 



Avogadro's theory, then, assumes that equal volumes at the 

 same temperature and pressure of those gases which undergo 

 the same volume-change, when they are equally changed in 

 temperature or pressure, contain an equal number of particles. 



It is obvious, if this be true, that the relative quantities 

 of matter contained in equal volumes of different gases under 

 the same conditions of temperature and pressure, will be the 

 same as the relative quantities of matter contained in separate 

 particles of these gases. If the theory be true, we may learn 

 the masses of relative particles which are so small as to be 

 far removed from the possibility of direct observation. 



If we admit that the observations of changes of pressure 

 and temperature in gases lead to this conclusion, it remains to 

 be shown how our conceptions of these changes are aided. 

 Other observations have indicated to us that these particles 

 are in constant motion. They must, therefore, approach the 

 sides of the containing vessel with a frequency which is directly 

 proportional to their number. If the volume of a system of 

 these particles is halved, the number of their impacts on the 

 sides of the vessel is doubled. When we speak of the pressure 

 being doubled, this appears to be what is meant. 



Again, when the temperature of a gas changes, the speed of 

 its particles is supposed to change, an increase of speed being 

 the real change known to us hitherto as a rise of temperature. 

 An acceleration in the average speed of a system of particles 

 will increase the number of impacts against the sides of the 

 containing vessel just as much as a diminution in the capacity 

 of the vessel. Hence, the volume remaining the same, the 

 pressure increases with rise in . temperature ; or the pressure 

 remaining the same, the volume increases. 



We are driven to the conclusion that the expansion of a 

 gas during a rise of temperature, while its pressure is un- 

 changed, is a change of the same nature as that which occurs 

 when a moving body, coming in contact with another body, 

 sets that body in motion. If this be so, the converse changes 

 which have been observed are such as might be expected. 

 That is, the forcible diminution of the volume of a gas should 

 cause its temperature to rise, and, using the same reasoning, 



