( B29 ) 



'Jlio character of this „change 

 of pressure by substitution" is il- 

 lustrated in the annexed figure : in 

 this diagram the abscissae repre- 

 sent the compositions, and the 

 ordinates the pressures for the 

 volume 0,020 and at the tempe- 

 rature 18° C. From this figure it 

 appears that this change of pres- 

 sure is not proportional to that of 

 the composition, the pressure is 

 always greater than would follow 

 from a linear relation ; if only a 

 small fraction of the number of 

 hydrogen molecules is substituted 

 by an equal number of carbonic acid molecules hardly any change 

 in the pressure takes place. 



Table XII shows clearly that neither the coefficients of pressure 

 undergo any material change, when the number of hydrogen mole- 

 cules substituted by carbonic acid molecules is only a small fraction 

 of the whole number of hydrogen molecules. 



a at tn QS t» o^ O* ly s^ a^ xa 



§ 2. The unit of volume. 



It was desirable to express the volume for each mixture in a 

 special unit of volume, chosen so that one c.m.^ of each of the 

 mixtures contains the same number of molecules when its volume, 

 expressed in this unit, has the same value. We can do this by taking 

 for each mixture as unit of volume: the volume which the quantity 

 of substance used would occupy at 0° C. and 1 atm., if the mixture 

 behaved like a peifect gas. This volume I will call the theoreticalUj 

 normal volume-^ it is obtained by multiplying the volume generally 

 called the normal volume viz. the volume which is actually occupied 

 by the substance at 1° C. and 1 atm., by a factor expressing the 

 deviation from Avogadho's law. 



In connection with Avogadro's law this measure for pure 

 substances has been proposed by Kamerlinoh Onnes in 1881. At 

 the same time he pointed out the desirability of expressing hence- 

 forth the molecular quantities also in absolute measure. In order 

 to determine the deviation of the normal volume of hydrogen from 

 that which is to be used in applying Avogadro's law, he employed 

 VAN DER Waals's law and the value 0,00050, deduced by van der 



