90 PROGRESS OF SCIENCE IN THE CENTURY. 



in definite proportions of their volumes (Gay-Lus- 

 sac's law) ; then, since density of a gas means the 

 amount of matter measured by weight in the same 

 volume, it follows that the combining weights of gases 

 bear a simple numerical proportion to their densities. 



Avogadro's Law. Another important and closely 

 related result was expressed in 1811 by the Italian 

 chemist, Amadeo Avogadro (1776-1856). He was 

 impressed by the fact that, when there is chemical 

 interaction between gases, there is observable a very 

 simple relation between the volumes concerned. A 

 pint of oxygen combines with two pints of hydrogen 

 to form two pints of steam. Such a simple fact, com- 

 bined with others relating to the physical properties 

 of gases, led him to suggest that a given volume of 

 any gas (elementary or compound) contains the same 

 number of molecules as the same volume of any 

 other gas measured at the same temperature -and pres- 

 sure. Equal volumes of gases, equal numbers of 

 molecules is Avogadro's law, another foundation- 

 stone of modern chemistry. It should be noted that 

 similar views were stated by Ampere in 1814, but 

 neither he nor Avogadro found contemporary recogni- 

 tion or even attention. 



Avogadro distinguished between molecules inte- 

 grantes and molecules elementaires, or, as would now 

 be said, between molecule and atom. " The physi- 

 cal properties of the gases (especially the similarity 

 in their behaviour towards changes of pressure and 

 of temperature) led Avogadro to assume in equal 

 volumes of all gases the same number of molecules; 

 and the distances of the latter from one another he 

 considers to be so great in proportion to their 

 masses, that they no longer exercise any attraction 

 upon one another. These molecules are not sup- 



