1878.] 
On Residual Phenonena. 
29 
between whose atomic weights there was no definite relation. 
According to the dodtrine of probabilities, there would be no 
reason why we should expedt any of the ten digits to prepon- 
derate over another ; yet in this instance the decimal place 
in question is occupied nine times out of the eleven by figures 
which are more in favour of the hypothesis than against it, 
whilst in two only — the cases of bromine and chlorine — do 
we find figures which hold an unequivocally half-way position 
between the whole number and the fradtion 0*5. This coin- 
cidence we can scarcely attribute to accident. Some reason 
there must be why the fradtions should be either a little in 
excess of the whole number, a little below it, or else bear a 
similar relation to 0*5. 
If, then, this state of fadts is utterly adverse to our recep- 
tion of Prout’s hypothesis as a complete and accurate law, 
it is, we submit no less incompatible with the antagonistic 
view of its being utterly baseless and imaginary. The case 
is apparently one of a “ residual.” Prout’s hypothesis is 
interfered with and modified by an x, which prevents the 
resulting atomic weights from being either exadlly whole 
numbers or merely fradtions midway between two such 
whole numbers. What is this unknown law no one has yet, 
we believe, made any systematic attempt to discover ? 
The error, if we may so call it, is as we perceive by no 
means constant ; sometimes it is in excess, and sometimes 
in deficiency. It appears also to vary with the class of the 
elementary bodies. It is considerable in the Halogens, 
chlorine showing a deficiency of 0*132, bromine a deficiency 
of 0*250, and iodine an excess of 0*33. These numerical 
peculiarities are not easy to account for, especially in the 
case of bromine. We might have expedted that it would 
have shown a smaller deficiency, as compared with the 
calculated cipher, than does chlorine, forming thus a tran- 
sition to the excess perceived in the atomic weight of iodine. 
It is remarkable that bromine shows in another manner this 
tendency to deficiency. Its atomic weight, as determined 
by M. Stas, is 79*750 ; but as calculated from the weights 
of its congeners, chlorine and iodine, it is— 
3 5-368+ 126- 533 = 80 - 950 , 
thus showing in the adtual weight a deficiency of 1*200 to 
be accounted for. 
The group of the alkaline metals, lithium, sodium, and 
potassium, is less markedly exceptional. Here, as the 
