318 PRINCIPLES OF CHEMISTRY 



The number of substances taken might be considerably increased,, 

 but the result would be the same that is, the molecules of the com- 

 pounds of oxygen would never be found to contain less than 16 parts 

 by weight of this element, but always n!6, where n is a whole number. 

 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 the 

 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 case be- 

 fore Gerhardt. In similar manner the atomic weights now accepted for 

 the elements oxygen, nitrogen, carbon, chlorine, sulphur, etc., 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 never- 

 theless the law of Avogadro-Gerhardt is here also ultimately re- 

 sorted 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 r 

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

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

 of 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*. 



21 If Be = 9, and beryllium chloride be BeCL>, then for every 9 parts of beryllium 

 there are 71 parts of chlorine, and the molecular weight of BeCl 3 = 80 ; hence the vapour 

 density should be 40 or w40. 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 (50 or nGO. The 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, what seem to be 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, O = 8, C = 6, Si = 14, &c. The Gerhardt equivalents are also satisfied by 



