706 REPORT—1899. 
Atomic Weights. By Professor W. A. TiLpEn, 7.2.8, 
The question of atomic weights has two aspects, the theoretical and the prac- 
tical. or the purposes of theory we require to know the relative values of the 
atomic weights of all the elements with the utmost possible accuracy, with the 
object chiefly of explaining observed relations and discovering new ones. The true 
significance of the periodic scheme of arrangement will never be discovered till the 
atomic weights of a much larger number of elements are known more correctly than 
at present. And the employment of numbers which only roughly approximate to 
the true values for the atomic weights has led in the past to a large amount of 
speculation and discussion, of which nearly the whole is fruitless, because of 
necessity successive hypotheses have had to be put aside as knowledge of the sub- 
ject has advanced, and numbers less inaccurate have been gradually substituted. 
This is true not only of such crude hypotheses as that of Prout, but of ideas pro- 
posed in more recent times concerning the relations of the several series in 
Mendeléef’s table, 
Other questions have arisen, such as the possibility of the variation of the 
atomic weights within certain limits, but they only serve to illustrate the extreme 
difficulty of the subject in its present position; for while the facts are about 
equally well known to all chemists who have studied it, they have led some to 
consider variation possible, while others upon the same evidence are convinced 
that it is impossible. Unfortunately the settlement of such questions is still far 
off, for the complete series of determinations of all the elements made with a 
degree of accuracy comparable with that which has made the work of Stas 
famous is not to be expected in the present generation. 
Another subject which has been reopened by the action of the distinguished 
Committee of the German Chemical Society relates to the unit to be adopted. 
The practice which has prevailed universally since the time of Berzelius, that is, 
for nearly three-quarters of a century, of expressing the atomic weights in terms 
of hydrogen (H =1) is now abandoned by the Committee in favour of the new 
scale, in which oxygen is the standard and O=16. The considerations which 
have influenced the several members of the Committee are chiefly three, namely, 
first, the uncertainty still supposed to attach to the ratio H:0O, though this is 
now as accurately known as it is likely to be ; secondly, the fact that the atomic 
weights of many elements may be deduced directly from the composition of their 
compounds with oxygen, but less frequently from compounds with hydrogen ; 
thirdly and chiefly, because the oxygen scale brings many atomic weights very 
near to whole numbers, It is evident that this consideration is one which con- 
cerns alike the analyst, the student, and the investigator in every analytical 
operation, and in all circumstances which do not involve the discussion of the 
numerical interrelations of the atomic weights. For that purpose the H=1 scale 
will always be preferable, until an element is discovered having a smaller atomic 
weight than hydrogen, and of that there is at present no indication so far as 
terrestrial chemistry is concerned. 
On the whole the proposal of the German Chemical Society is probably the 
best solution of the difficulty. The scale in which O=16, however, implies the 
value 1:01, approximately, for hydrogen; and though it is true that for common 
analytical use the neglect to recognise this value will not lead to very serious con- 
sequences, it must be remembered that an appreciable error will be involved in 
expressing the composition of compounds which are comparatively rich in hydro- 
gen, such as the chief hydrocarbons and their derivatives. For C,H,,, for 
example, the percentage of hydrogen is 16°66 or 16°80, according to the value 
assigned to the atomic weight of hydrogen. 
But supposing the O=16 scale to be generally adopted, it is still highly 
desirable that there should be an understanding among the several Chemical 
Societies, and if possible among the members of these societies, as to the numbers 
to be chosen for ordinary use. Are we to use 27:1 for Al, 137-4 for Ba, 208°5 for 
Bi, 79:96 for Br, 35:45 for Cl, 52:1 for Or, 126°85 for I, 2069 for Ph, 24°36 for Mg, 
200'3 for Hg, 14:04 for N, 1948 for Pt, 39°15 for K, 23:05 for Na, 32:06 for S &c., 
a 
