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as an indispensable tool in the hands of the pioneer who seeks to extend the 
boundaries of knowledge. The ‘Times,’ in the leading article already quoted, 
says that, as with a Shakespeare, ‘it is the same with imaginative discoverers in 
science. .. . But the faculty is not merely a fairy gift that can be exercised 
without pains. As the sense of right is trained by right action, so the sense 
of truth is trained by right thinking and by all the labour which it involves. 
That is as true of the artist as of the man of science; and one of the greatest 
achievements of science has been to prove this fact and so to justify the imagina- 
tion and distinguish it from fancy.’ 
Again, let it not be forgotten that chemistry im its highest sense—that is, 
in its most general and useful sense—is purely a world of the imagination, is 
purely conceptual. And in addition to this, moreover, it is based, like all 
science, on the underlying assumption of the uniformity of Nature, an assump- 
tion incapable of proof. If we think of the science as a body of abstract general 
theory, and exclude for the moment from our purview its innumerable practical 
applications, and also all special individual facts not yet known to be related to 
general theory, then what remains are the more or less general facts or laws. 
These it is which give the power of prediction in newly arising cases of a 
similar character; the power of foresight by which the claim of chemistry to 
its position as a science is justified. Chemistry, as such, is a complicated ideal 
structure of the imagination, a gigantic fairy palace, and, be it noted, can only 
continue to exist so long as there are minds capable of reproducing it. Think 
of all the speculation—and speculation too of the highest utility when trans- 
lated into concrete applications—about the internal structure of molecules. 
I venture to say that the most magnificent creations of the world’s greatest archi- 
tects are not more elaborate or more beautiful or more fairylike than the chemist’s 
conception of intramolecular structure and the magical transformations of which 
molecules are capable; and yet no one has had direct sensuous experience of 
any molecule or atom, or possibly ever will. It is well from time to time to 
recall these truths and realise where we are. But although the conceptual 
nature of science is unquestionable, it certainly contains truth in some form as 
tested by deductive concrete realisation, by correctness of prediction, and during 
the last century or two has undoubtedly given to man a mastery over Nature 
never before dreamt of. 
The foundations of chemistry, as we now know it, were laid under the 
influence, the guidance, of three great theories : first, the theory of the alchemists 
of the transmutation of metals by means of the philosopher’s stone; 
A brief his- second, the theory of phlogiston, connected so much with the names 
torical retro- of Becher and Stahl, which held sway for some two centuries; third, 
spect. the theory of combustion, the quantitative period of chemistry, 
inaugurated by the great Scottish chemist Black by his introduction 
of the balance. How this led to a veritable renascence of chemistry in the 
hands of Lavoisier and the other giants of that stirring period—the close of the 
eighteenth and commencement of the nineteenth centuries—is well known. 
Looking back at the warfare which was waged about these older theories, for 
and against them, one realises now that there were elements of truth on both 
sides; for have we not in the work of Sir William Ramsay and others the 
revival of transmutation, and does not the essential truth of phlogiston survive 
in the modern doctrine of heat? In one of Dr. Johnson’s letters to Boswell 
there is a curious reference to transmutation. He says that a learned Russian 
had at last succeeded, but, fearing the consequences to society, he had died 
without revealing the secret. 
After the discovery of oxygen and the beginnings of quantitative chemistry, 
the science was ready for Dalton’s great discoveries respecting combination 
by weight; the corresponding discoveries by Gay-Lussac on combination of 
gases by volume, and, through the latter, for Avogadro’s famous hypothesis. 
Dalton had indeed, by reviving an old Greek suggestion, proposed to explain 
his discoveries by his atomic theory, but neither this nor our molecular theory, 
though the latter was inherent in the laws of gaseous combination of Gay-Liussac 
and in Avogadro’s hypothesis, was finally put upon its present basis until 
Cannizzaro took up the subject half a century later. Meanwhile Dulong and 
Petit had completed their studies of atomic heat, and Mitscherlich had pointed 
