PROGRESS or CHEMISTRY. 
certainly docs 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. 'Ldie elements are 
now arranged into two r)rincipal groups:—1st. Tliose 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 mon-atomic 
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 
weifj-hts 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 containing atoms of carbon, hy¬ 
drogen, and oxygen, in proportions far from simple; and the 
atomic theory must have been abandoned but for the dis¬ 
covery 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 variet}^ of transforma¬ 
tions, but which can be resolved into simpler 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 re¬ 
searches in spectrum analysis. The application of this 
beautiful method to the investigation of minerals has alreadv” 
led to the discovery of three volatile metals which had pre¬ 
viously escaped observation, whilst its extension to the 
investigation of the light which reaches our planet from the 
heavenly bodies lias 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 ^volunteer in¬ 
sinuations against Mr. Crookes’ claim to that discovery. 
IM. Dumas considers it corroborative of his views that Mr. 
