SULPHURIC ACID AND WATER. 277 
At the time he read his paper, he distributed 
among the persons present, printed copies of 
this and the other tables in his paper. 
In a few days after the paper was read, Dr. 
Dalton (having been, as wellas others who heard 
the paper read, surprised at the announcement 
of results conveying notions so very opposite to 
those generally entertained,) told me that on 
carefully looking over Dr. Thomson’s printed 
table of specific gravities, he perceived that the 
author had been working according to an incor- 
rect theorem in forming his calculation column: 
and such is evidently the fact. The correct 
rule whereby to find the mean of two specific 
gravities, is to divide the sum of the weights by 
the sum of the volumes; but this is not what 
Dr. Thomson has adopted :—his calculation re- 
sults have been obtained by multiplying the 
weights severally by their specific gravities, ad- 
ding the products together, and dividing the sum 
of those products by the sum of the weights ; 
the quotient in which case he gives as the mean 
specific gravity; thus, for example, in the case 
of one atom acid+two atoms water, 
