30 
On Residual Phenomena. 
[January, 
atomic weights rise, we have in lithium an excess of 0*004, 
in sodium a deficiency of 0*020, and in potassium again an 
excess of 0*040. The atomic w r eight of sodium, calculated 
from those of lithium and potassium, — - 
39-040+7-004 = . 022 . 
2 
is in excess of its experimental weight only by 0*042. 
The atomic weights of the alkaline earthy metals are un- 
favourable to the hypothesis of Prout, or at least point to 
the more decided interference of some unknown cause. 
According to Marignac the atomic weight of calcium is 
40*21, and that of strontium 87*25, — numbers of a very in- 
tractable character, and which, lying in an intermediate 
position between a whole number and the fraction would 
seem as favourable to the total rejection of Prout’s law as 
to the prospeCt of its exceptions being explained as residual 
phenomena. 
Lead, as determined by Professor Stas, has for its atomic 
weight 103*136(0 = 15*960), or an excess of 0*136. Thallium, 
according to the careful determination of Mr. Crookes, 
= 203*642. Here, then, the excess is also 0*142, very 
closely approximating to the unexplained margin in the case 
of lead. 
Until, however, the atomic weights of all the elements 
shall have been verified with the precautions and refinements 
employed by M. Stas in the case of silver, nitrogen, the 
halogens, and the alkaline metals, and by Mr. Crookes in 
the case of thallium, — a task which no single chemist can 
possibly complete in the longest life-time, — all speculations 
concerning the relations of their combining weights must 
be regarded as premature. It ma}^ be that when all the re- 
quired numbers are before us the deviations in excess or 
deficiency from what Prout’s hypothesis would in stridtness 
require will be found to display a “ periodic ” character. 
All that can be said at present amounts to very little more 
than the recommendation to suspend judgment till the fadts 
of the case are fully before us — a consummation, we fear, 
scarcely to be hoped for in the life-time of the present gene- 
ration. Seventeen years ago the late Sir John Herschel, in 
an inaugural address as Chairman of the Chemical Sedtion 
of the British Association, remarked — “ Not until these 
numbers are determined with a precision approaching that 
of the elements of the planetary orbits — a precision which 
can leave no possible question of a tenth or a hundredth 
