278 
ME. W. CROOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
I discussed the position of thallium among elementary bodies, and gave a series of 
analytical notes. 
In the pages of the ‘Journal of the Chemical Society’ for April 1, 1864, I collated 
all the information then extant, both from my own researches and from those of others, 
introducing qualitative descriptions of an extended series of the salts of the metal. I 
propose in the present paper to lay before the Royal Society the details and results of 
experiments which have engrossed much of my spare time during the last eight years, 
and which consist of very laborious researches on the atomic weight of thallium. In 
these researches I owe much to the munificence of the Royal Society for having placed 
at my disposal a large sum from the Government Grant. Without this supplement to 
my own resources it would have been difficult for me to have carried out the investi- 
gation with such completeness. 
Section I. — ON THE DETERMINATION OF ATOMIC WEIGHTS. 
In determining accurately the atomic weight of a metal that stands so high in the 
scale as thallium, difficulties and sources of error which are comparatively small with 
elements of low atomic weight are magnified to serious proportions, and require more 
than ordinary care for their elimination. When so large a proportion of the compound 
under analysis or synthesis consists of the body itself whose atomic weight is the one 
unknown quantity, it is evident that the almost unavoidable errors occasioned by impu- 
rity in the materials employed, the losses incident to imperfect manipulation, or the 
inaccuracies arising during the weighing from the omission of the corrections required 
by temperature, pressure, &c., will all find their way into the number which is finally 
considered to represent the atomic weight of the metal. 
Nearly fourteen years ago, on taking the chair of the Chemical Section of the British 
Association at Leeds, the late Sir John Herschel called attention to the necessity which 
there then was for the introduction of greater accuracy into the determination of atomic 
weights. Speaking of the numerical relations which appear to exist between certain 
groups of elements, he considered that all these speculations took for granted a principle 
with which chemists had allowed themselves to be far too easily satisfied, viz. that all 
the atomic numbers are multiples of that of hydrogen, “ Not until these numbers,” he 
continues, “ 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 of a per cent., and in the presence of which such errors as are at present 
regarded tolerable in the atomic numbers of even the best determined elements shall be 
considered utterly inadmissible — I think can this question be settled ; and when such 
gigantic consequences — so entire a system of nature — are to be based on a principle, 
nothing short of such evidence ought, I think, to be held conclusive, however seductive 
the theory may appear. I do not think such precision unattainable ; and I think I per- 
ceive a way in which it might be attained, but one that would involve an expenditure of 
time, labour, and money, such as no private individual could bestow on it.” Before this 
