Cuar. VIL, § 1.] 
fessor Stokes has obtained a result truly astonishing. 
In the case of the light derived from a voltaic are, 
produced by the battery of the Royal Institution 
with metallic poles, the visible spectrum formed upon 
uranium-glass extended no less than siz or eight times 
the length of the ordinary spectrum. If, by way of 
contrast, a porcelain tablet be used as a screen, the 
spectrum terminates at the usual point.' 
(581.) {I have already (art. 471) expressed my regret 
Be bee’ that the limits of this Essay prevent me from devot- 
a, ate a section to the physiological part of Optics ; 
HEAT.—BLACK. 
925 
naming Mr Wheatstone’s beautiful invention of the 
Stereoscope, as by far the most interesting contribu- 
tion recently made to the theory of vision, regarded 
in a point of view not strictly anatomical, Although 
Mr Wheatstone’s paper was published in the Philo- 
sophical Transactions for 1838, and the Stereoscope 
became at that time known to men of science, it by 
no means attracted, for a good many years, the at- 
tention which it deserves. It is only since it received 
a convenient alteration of form (due, I believe, to Sir 
David Brewster), by the substitution of lenses for 
mirrors, that it has become the popular instrument 
which we now see it, but it is not more suggestive 
than it always was of the wonderful adaptations of 
the sense of sight. | 
CHAPTER VI. 
HEAT, INCLUDING SOME TOPICS OF CHEMICAL PHILOSOPHY. 
§ 1. Buack.—Latent and Specific Heat.—Irvine.—Hutton.—Doctrines of Heat applied to some 
Natural Phenomena. 
Stereo- but I cannot close the chapter without at least 
scope. 
noe) . Down to the close of the 18th century, the science 
dered in Of Heat was studied and advanced mainly by che- 
the 18th mists, and it was in all respects treated as a branch 
century as of Chemistry ; a position of which we still find traces 
oleae coer in the introduction of the doctrines of heat (even of 
try. radiant heat) into most of our approved treatises on 
Chemistry. This circumstance brings us, in this 
chapter, into close contact with the most illustrious 
chemical names of the second half of the last century 
and of the first years of the present. Such were 
Black, Cavendish, Lavoisier, and Dalton. Of the last 
and two first it may be doubted whether they were not 
as prominent discoverers in Physics as in Chemistry. 
Davy occupies a similar position. It was not, in- 
deed, until the 19th century had made some pro- 
gress that Chemistry assumed a strongly distinctive 
position of its own, and began to attain that large 
development and complex character of detail which 
render it a science now hardly accessible to those 
who do not devote to it their almost undivided at- 
tention. In the days of Black and Cavendish it was 
otherwise ; and in the first section of the present 
chapter I shall attempt to give an outline of the 
characters of the very remarkable men who then 
advanced simultaneously the doctrines of Physics 
and Chemistry ; referring, of course, chiefly to the 
former, but not entirely to the exclusion of the lat- 
ter portion of their researches, particularly with 
respect to the atomic and gaseous theories of Dal- 
ton, which have astrongly physical aspect. I have 
elsewhere noticed the barrenness of the greater part 
of the 18th century in contributions to the experi- 
mental sciences; the temptation is therefore the 
greater to dwell a little even on the personal history 
of men so celebrated and influential as Black, Caven- 
dish, and Dalton, 
JosepH Biack was born at or near Bordeaux in (583.) 
France, in 1728. His biography, little eventful and — 
almost exclusively academic, has been recorded in as a che- 
some detail by his companions and. friends Adam mist. 
Ferguson and John Robison (the former of whom 
was a relation), in the preface to the posthumous 
publication of his Lectures on Chemistry, It is suffi- 
cient for me to state that he entered the University 
of Glasgow as a student in 1746. Being destined 
for the medical profession, he removed in 1750 or 
1751 to Edinburgh, where he benefited especially 
by the lectures of Cullen, a most eminent physician, 
and the author of a beautiful experiment on the cold 
produced during evaporation. Before Black gradu- 
ated (in 1754) he had entered upon a course of chemi- 
cal experiments connected with the causticity of many 
earthy bodies, which ended in his first (and perhaps 
most famous) discovery of the existence of fixed air or 
carbonic acid gas as an essential constituent of marble 
and other solids, together with a train of important 
consequences. Fewinaugural dissertations have been 
so interesting to science as thaton Magnesia, printed at 
Edinburghin 1754, which contained these results. But 
on this purely chemical question we will not enlarge. 
1 On Mr Stokes’s experiments, see Phil. frans., 1862-53 ; and Proceedings of the Royal Institution. 
