MOLECULES AND ATOMS 325 



The molecular weights of the above compounds are found either directly 

 from the density of their vapour or gas, or from their reactions. Thus, 

 the vapour density of nitric acid (as a substance which easily decom- 

 poses above its boiling point) cannot be accurately determined, but tho 

 fact of its containing one part by weight of hydrogen, and all its pro- 

 perties and reactions, indicate the above molecular composition and no 

 other. In this manner it is very easy to find the atomic weight of all 

 the elements, knowing the molecular weight and composition of their 

 compounds. It may, for instance, be easily proved that less than nl2 

 parts of carbon never enters into the molecules of carbon compounds, 

 and therefore C must be'taken as 12, and not as 6 which was the number 

 in use before Gerhardt. In a similar manner the atomic weights now 

 accepted for the elements bxygen, nitrogen, carbon, chlorine, sulphur, &c., 

 were found and indubitably established, and they are even now termed 

 the Gerhardt atomic weights. As regards the metals, many of which do 

 not give a single volatile compound, we shall afterwards see that there 

 are also methods by which their atomic weights may be established, but 

 nevertheless the law of Avogadro-Gerhardt is here also ultimately 

 resorted to, in order to remove any-doubt which may be encountered. 

 Thus, for instance, although much that was known concerning the 

 compounds of beryllium necessitated its atomic weight being taken as 

 Be = 9 that is, the oxide as BeO and the chloride BeCl 2 still certain 

 analogies gave reason for considering its atomic weight to be Be =13*5, 

 in which case its oxide would be expressed by the composition Be 2 3 , 

 and the chloride by BeCl 3 . 21 It was then found that the vapour density 

 x>f beryllium chloride was approximately 40, when it became quite clear 

 that its molecular weight was 80, and as this satisfies the formula 

 BeCl 2 , but does not suit the formula BeCl 3 , it therefore became neces- 

 sary to regard the atomic weight of Be as 9 and not as 13^. 



Z1 If Be = 0, and beryllium chloride be BeCl 2 , then for every 9 parts of beryllium 

 there are 71 parts of chlorine, and the molecular weight of BeCl 2 = 80 ; hence the vapour 

 density should be 40 or n40. If Be = 18'5, and beryllium chloride be BeCl 3 , then to 13'5 

 of beryllium there are 106'5 of chlorine ; hence the molecular weight would be 120, and 

 the vapour density, 60 or 60. JThe composition is evidently the same in both cases, 

 because 9 ! 71 " 13'5 : 106'5. Thus, if the symbol of an element designate different 

 Atomic weights, apparently very different formulae may equally well express both the 

 'percentage composition of compounds, and those properties which are required by the 

 laws of multiple proportions and equivalents. The chemists of former days accurately 

 expressed the composition of substances, and accurately applied Dalton's laws, by 

 taking H = l, = 8, C = 6, Si = 14, &c. The Gerhardt equivalents are also satisfied by 

 'them, because O = 16, C = 12, Si = 28, &c., are multiples of them. The choice of one or 

 the other multiple quantity for the atomic weight is impossible without a firm and con- 

 crete conception of the molecule and atom, and this is only obtained as a consequence of 

 the law of Avogadro-Gerhardt, and hence the modern atomic weights are the results 

 Of this law (see Note 28). 



