28S 
ME. W. CROOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
third, two sets of Aveights (one for the Exchequer, the other for the Royal Mint) 
Avere compared at the same time. To this paper, as well as to the learned author 
himself, I am indebted for a valuable store of information on the subject of Aveights and 
AA'eighings. 
The weights employed Avere of platinum, made expressly for these investigations by 
Messrs. Johnson and Mattiiey. The platinum Avas quite pure ; it Avas fused, cast, and 
then Avell hammered. The Aveights were adjusted by myself during May, June, July, 
and August 1864: they Avere first roughly adjusted, and then the specific gravity of 
each weight Avas taken. The weights Avere heated to redness in a bath of magnesia 
previous to ascertaining their specific gravity. The density of the larger Aveights was 
ascertained to the second place of decimals, and that of the smaller ones to the first 
place. The record of the final adjustment of these weights will be sufficient to show 
the method I adopted. 
In taking the specific gravity of the Aveights, the distilled Avater was contained in a 
glass beaker of about 250 cubic inches capacity. Each Aveight Avas suspended by a fine 
platinum Avire to be attached to the pan of the balance. With this wire affixed the 
Aveight Avas introduced into a small glass vessel filled Avith Avater, and heated over a 
spirit-lamp to the boiling-point. When all the air-bubbles had been expelled by this 
process, the small jar containing the Aveight Avas lowered into the Avater in the beaker, the 
Aveight, on removing the small jar, being perfectly free from any adhering bubbles of 
air. After the temperature had sunk to the proper point, the specific gravity was taken. 
The 1000-grain Aveight was selected as the standard; for in nearly every process in 
Avhich Aveights are used in chemistry, the object is not to ascertain the absolute Aveight 
of a substance in terms of a grain or gramme, but to determine its relative Aveight in 
comparison Avith that which it possessed at some other time before it Avas submitted to 
certain analytical or synthetical operations. If the weighings are performed Avith the 
same Aveights, it does not at all matter whether the Aveights are absolutely of the value 
which they profess to be ; but it is very important that they should bear a knoAvn pro- 
portion to each other. This must be understood as referring only to ordinary chemical 
research in synthesis or analysis. In many physical investigations it is of great import- 
ance that the 1000-grain Aveight should really represent 1000 normal grains, or that 
its deviation from that value should be accurately determined ; but I confess I do not 
knoAV where a standard Aveight suitable for such a comparison is to be met Avith. The 
Aveights at first tried Avere far from accurate among themselves. I accordingly ascertained 
their errors by the method described beloAV, and then adjusted them myself according 
to the corrections thus found necessary. The residual errors in the weights Avere then 
finally taken in the following manner : — 
The balance being brought into equilibrium and the temperature and barometrical 
pressure carefully noted, the 1000-grain Aveight was placed in the left pan, and in the 
right the GOO, the 300, and the 100-grain Aveights. It was uoav found that, to bring 
the balance back to equilibrium, a slight additional Aveight had to be placed on the 
