I jaw of Density -Numbers. 197 



constant coefficient) expresses the volume in cubic centimetres 

 of the molecular weight in grams. 



In explaining this I may be permitted to restate a few 

 well-known facts. 



Two grams of hydrogen at 760 millim., and at 0° C, 

 occupy 22327 cubic centim. 



(sf grams of any substance in the state of gas under the same 

 conditions of temperature and pressure also occupy 22327 

 cubic centim., both by convention and in accordance with 

 Avogadro's law. 



This constant volume 22327 cubic centim. is multiplied at 

 the boiling-point S° by the fraction T/273 ; and consequently 

 the volume (molecular) of a substance in vapour at its boiling- 

 point is 



^=22327 T/273 = 81-8 T cubic centim., 



and the vapour-density is 



D s =a/^=a/81*8T. 



As the coefficient 81*8 is the same for all bodies, it may be 

 omitted if it be thought fit to do so. 



Relation between the Gaseous Molecular volume Y s and the 



Liquid Molecular volume vs. 

 When we take equal volumes [e.g. cubic centims.) of dif- 

 ferent liquids at S° and convert them into vapour by heating, 

 we obtain a certain number of cubic centim. of vapour at 

 760 millim. and at S°; this number is for any substance what- 

 ever. 



81-8 T/v s (cubic centim.). 



In this connection I wish to draw attention to a special 

 case which appears of particular interest : it is as follows : — 

 We find that in certain groups of analogous substances 



81;8T/^=constant. 



This phenomenon is independent of the new law, and may be 

 expressed as follows : — 



V -7^- = constant = v r . 



(The constant 81*8 is omitted here.) 



I shall call the constant v r the " reduced volume " of the 

 substances in question. 



When a cubic centimetre at s° of the liquids (which have 

 the same v r ) is volatilized, one obtains a definite volume of 

 vapour which is the same for all these substances, and is 



vol. const. = 22327/*y cubic centim. 



