344 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



measuring the density of a gas or vapour it is often conve- 

 nient to calculate the density relatively to some selected gas 

 as standard, rather than to give the actual density in grams 

 per litre. Thus whilst the actual densities of air, oxygen, 

 hydrogen and nitrogen may be compared at oC. and 760 

 mm. pressure, vapours such as steam cannot be included 

 in the comparison because they would condense to liquids 

 under the conditions selected. But as all gases expand and 

 contract in much the same way (see Section A of this 

 chapter), their relative densities are almost independent of 

 temperature and pressure. A table of relative densities may 

 therefore include vapours as well as gases ; e.g. steam may 

 be compared with air at 130, whilst hydrogen is com- 

 pared with air at 20 or at o. Vapour densities, and 

 gas densities, are often given relatively to air ; but hydrogen, 

 as being the lightest known gas, is usually preferred as a 

 standard, since the relative densities of all other gases can 

 then be represented by numbers greater than unity. 



In selecting a standard of molecular weights, the hydrogen 

 molecule would provide a convenient unit, since the rela- 

 tive weights of gaseous molecules could then be represented 

 by the same number as their relative densities. But 

 since the hydrogen molecule is known to contain two atoms, 

 its selection as a standard would necessitate the use of a 

 fraction 0*5 to represent the hydrogen atom ; Cannizzaro 

 therefore selected the half-molecule or atom of hydrogen as 

 a unit both of molecular and of atomic weights. Cannizzaro 

 writes (A.C.R. XVIII. 6-7): 



" I prefer to take as common unit for the weights of the 

 molecules and for their fractions, the weight of a half and 

 not of a whole molecule of hydrogen, I therefore refer the 

 densities of the various gaseous bodies to that of hydrogen = 2. 

 If the densities are referred to air = i, it is sufficient to 

 multiply by 14*438 to change them to those referred to that 

 of hydrogen = i ; and by 28*87 to refer them to the density 

 of hydrogen = 2. 



