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Proceedings of the Royal Society 
thirty years later, we maintained a correspondence which, though 
often recurring at long intervals, was not discontinued. By the aid 
of these letters I can trace some particulars of his migrations, 
which might otherwise have escaped me. 
In 1833 and 1834 he appears to have been much engaged in the 
preparation of a treatise on Mineralogy, which had for long occupied 
his thoughts. He spent the winter of 1834-5 in Paris, carrying it 
through the press. This was M. Necker’s most considerable and 
most systematic work. It shows to advantage the combination of 
scientific knowledge which he possessed, — which, as I have already 
intimated, extended over a wide range of subjects, including not 
only the Natural History Sciences, but Physics and Chemistry. 
Such a combination is eminently required by the philosophical 
mineralogist. His science is unfortunately at present cultivated 
by few, and profoundly studied by hardly any. Had this not been 
so, Necker’s fame would have been more widely spread than it is. 
In a very remarkable paper, first published in Jameson’s Edin- 
burgh Philosophical Journal for 1832, he treated of “Mineralogy 
as a Branch of Natural History.” He showed that a well character- 
ized mineral is to be regarded as an individual^ and that such 
individuals are to be grouped under species, genera, orders, and 
classes, as in the classification of the organic creation, by having 
a philosophical regard to the ivhole of the characters and properties 
which belong to the individuals of each species, in the same way 
as was done by Cuvier for animals, and by Decandolle for plants. 
His aim was to conciliate as far as may be the hitherto conflicting 
systems of classification, — that by Chemical properties alone, and 
that from External characters alone. His doctrine was (in brief) 
that those chemical characters are most to be regarded which visibly 
and palpably affect the external features of the mineral indi- 
vidual; that the indications of ultimate chemical analysis are not, 
correctly speaking, mineralogical characteristics at all ; and that, 
where chemical and external indications are in apparent contradic- 
tion (which is rare), the latter are to be preferred. 
Necker applied his principles, which he had derived from a large 
and wide study of Natural History in its most general sense, with 
very great ingenuity to the distinction and classification of minerals. 
His system is dotailed at great length, and with much clearness 
