CHAP. VI., 1.] 



HEAT. BLACK. 



127 



fessor Stokes has obtained a result truly astonishing. 

 In the case of the light derived from a voltaic arc, 

 produced by the battery of the Royal Institution 

 with metallic poles, the visible spectrum formed upon 

 uranium-glass extended no less than six 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. 1 



(581.) 

 r Wheat- 



Stereo- 

 cope. 



[I have already (art. 471) expressed my regret 

 fa e limits of this Essay prevent me from devot- 

 i n a section to the physiological part of Optics ; 

 but I cannot close the chapter without at least 



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. BLACK. Latent and Specific Heat. Irvine. Hutton.- 



Natural Phenomena. 



-Doctrines of Heat applied to some 





) . Down to the close of the 18th century, the science 

 lered in ^ H ea ^ was studied and advanced mainly by che- 

 he 18th mists, and it was in all respects treated as a branch 

 -,entury as o f Chemistry ; a position of which we still find traces 

 - * n tne introduction of the doctrines of heat (even of 

 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 a strongly physical aspect. I have 



(583.) 



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 BLACK was born at or near Bordeaux in 

 France, in 1728. His biography, little eventful and 



* eminence 



almost exclusively academic, has been recorded in as a c h e . 

 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 affixed air or 

 carbonic acid gas as an essential constituent of marble 

 and other solids, together with a train of important 

 consequences. Few inaugural dissertations have been 

 so interesting to science as that on Magnesia, printed at 

 Edinburghin 1754, which contained these results. But 

 on this purely chemical question we will not enlarge. 



On Mr Stokes's experiments, see Phil. Trans., 1852-53 ; and Proceedings of the Royal Institution. 



