TRANSACTIONS OF THE SECTIONS. 29 
The extension of these principles to mineral chemistry had been commenced in the 
cases of the commonest acids and bases, but their general introduction met with 
difficulties, and something seemed wanting to their complete success. 
I must now travel southward for a short time, and ask you to accompany me to that 
sunny land of glorious memories, and to its southern dependency, the Island of Sicily, 
It was reserved for Professor Cannizzaro, of the University of Palermo, to show us 
how the remainder of the knot could be untied. He argued, upon physical as well 
as chemical grounds, that the atomic weights of many metals ought to be doubled, 
as well as those of oxygen, sulphur, and carbon. His conclusion is confirmed by 
the constitution of those organo-metallic bodies which I mentioned just now, and it 
certainly does seem to supply what was still wanting for the extension of our system 
of classification from the non-metallic elements to the heavy metals themselves. 
The elements are now arranged into two principal groups :—I1st, those of which 
each atom combines with an uneven number of atoms of chlorine or hydrogen; 
2nd, those of which each atom combines with an even number of atoms of chlorine 
or hydrogen, Like every classification founded upon nature, this one draws no 
absolute line, as some elements belong to both classes. The first group includes 
the monatomic elements of the chlorine family, the triatomic elements of the 
nitrogen family, hydrogen and the alkali metals, silver and gold, in all about 
eighteen elements. The usual atomic weights of these are retained. The usual 
atomic weights of all the other elements, biatomic, tetratomic, &c., are doubled. 
This second group includes the oxygen family, carbon, silicon, and the alkaline 
earths, the metals, zinc, iron, copper, lead, &c. 
Every step in our theoretical development of chemistry has served to consolidate 
and extend the atomic theory, but it is interesting to observe that the retention of 
that theory has involved the necessity of depriving it of the absolute character 
which it at first possessed. Organic compounds were long ago discovered con- 
taining atoms of carbon, hydrogen, and oxygen in proportions far from simple ; and 
the atomic theory must have been abandoned but for the discovery that the atomic 
or, rather, molecular weights of these compounds correspond invariably to entire 
numbers of the elementary atoms. We now use the term ‘molecule’ for those 
groups which hold together during a variety of transformations, but which can be 
resolved into simple constituents; whilst we receive the word ‘atom’ for those 
particles which we cannot break up, and which there is no reason for believing that 
we ever shall break up. 
Amongst the most brilliant extensions of our means of observation have been the 
researches in spectrum analysis. The application of these beautiful methods to the 
investigation of minerals has already led to the discovery of three volatile metals 
which had previously escaped observation, whilst its extension to the investigation 
of the light which reaches our planet from the heavenly bodies has led to the 
recognition, in several of them, of elements identical, in this respect at least, with 
some of our elements in this earth. An eminent French chemist has recently taken 
occasion, in reporting the results of some researches on the new metal “ thallium,” to 
yolunteer insinuations against Mr. Crookes’s claim to that discovery. M. Dumas 
considers it corroborative of his views that Mr. Crookes did not refer the considera- 
tion of his claims, on the first opportunity, to a jury of gentlemen, formed for exa- 
mining products of manufacturing industry at the National Exhibition of 1862. I 
lave felt it my duty to allude publicly to this proceeding, because it occurred in 
a report of a commission of the French Academy, published by order of that 
distinguished body. All chemists have, however, adopted the name “ thallium ” 
which Mr. Crookes gave to the metal when he first discovered it. 
Before proceeding from the scientific and intellectual progress of chemistry, I 
must bee leave to refer briefly to the educational effects of that progress. Little, 
indeed, would our conquests over nature avail us if they were only known to the 
systematic cultivators of science, and only used by them; and unless the popular 
dissemination of knowledge keeps pace with its extension, the chief fruits of that 
extension will be lost. It would be unjust to deny that some important steps have 
been taken of late years, by various governing bodies in this country, towards giving 
to experimental science a position in national education; but these steps are only 
the beginning of a reform in education, which must go much farther in order to be 
