330 
ME. W. CBOOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
(40-21), lanthanum (94-13), strontium (87-25), analogous opposed evidence, as in the 
case of the weight found for thallium. 
In the preceding pages I have given the method by which I have arrived at the atomic 
weight of thallium, whilst I have endeavoured to show that experimental chemistry is 
strictly a science of precision. Accordingly the determination of the atomic weights 
of other of the elements (gold, chromium, platinum, rhodium, ruthenium, palladium, 
iridium, lithium, glucinum, cerium, and boron) is not a matter of supererogatory 
labour undertaken for the sake of verification merely, but is necessary to the progress 
of chemistry immediately as a science and technologically. For, until these elements 
shall have assigned to them more accurately their combining numbers, it will not 
be safe to enter into theoretical speculations respecting the interrelations between 
elementary bodies, nor possible to estimate them quantitatively without some error, 
which, small as it may be, yet remains an inaccuracy inadmissible to true science. 
One of the sources of error (that of neglecting the variation of atmospheric pressure 
during the weighings), it will have been seen, frequently introduces as much defect from 
the truth as impurity of the chemicals employed — a fact that has hitherto been greatly 
overlooked. Undoubtedly larger errors are sometimes admitted to general chemical 
manipulation from deficiency or excess in the weights employed than would obtain from 
the use of materials of ordinary purity, although in the latter case to notice effects of 
variable pressure, or the loss by weight of air displaced, would be labour wasted. 
Bearing these teachings of experience in my mind, I have striven to eliminate all 
erroneous influence in the number I submit to the Koyal Society as the Atomic W eight 
of Thallium ; and I shall be amply rewarded for my long labour if I can know that the 
determination has secured to researches of this character a nearer approach to the 
standard of truth. 
