160 



Irregular 

 dilatation 

 of glass. 



(723.) 

 Dulong 

 and M. 

 Kegnault 

 on the elas 

 ticity of 

 steam. 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 





(724.) 

 M. Keg- 

 nault on 

 latent heat. 



(725.) 



the great difficulties not only of these enquiries, but 

 generally, in constructing comparable thermometers. 

 It varies so much with the composition of the glass 

 as to leave a serious amount of uncertainty in the 

 measure of temperatures above that of boiling water. 

 M. Regnault has had the perseverance himself to 

 graduate the thermometers which he uses. 



These enquiries were in some measure introductory 

 to the determination of the elasticities of steam at 

 different temperatures, which had been already ascer- 

 tained with so much care by Dulong (nominally in 

 conjunction with Arago and other academicians), as 

 to leave little need for their repetition, except on the 

 ground of the uncertainty of the indications at high 

 temperatures of the thermometers which they used. 

 All M.Regnault's results are most carefully expressed 

 in terms of temperatures measured on an air thermo- 

 meter corrected for the expansion of glass. They agree 

 well with those previously ascertained by Dulong. 



In connection with this subject, the important law 

 of the latent heat of steam at different temperatures 

 has been correctly ascertained for the first time by M. 

 Kegnault. Watt maintained that the sum of the 

 sensible and latent heats of steam is constant at 

 all temperatures : Southern and others, on the con- 

 trary, believed that the latent heat has a constant 

 value, whatever be the temperature of vaporization. 

 M. Regnault shows that the true law is intermediate. 

 The latent heat diminishes as the sensible heat of steam 

 increases, but in a, slower proportion. 



Our author also followed the steps of his prede- 



cessor Dulong in verifying the law of Mariotte and Law of 

 Boyle on the compressibility of the gases. He found Mari tte 

 it to be indeed very approximately true, yet not ab- a " 

 solutely so for any gas. For atmospheric air and for 

 most other gases the compression increases rather 

 faster than the strict proportionality to the pressures 

 would assign. In hydrogen gas the contrary is the 

 case. This result, together with that on the dilatation 

 of different gases, shows (if fully confirmed hereafter) 

 that mere simplicity or uniformity of result is not by 

 any means a sure ground of induction as to a law of 

 nature. That simplicity often appears to be predi- 

 cable only of some abstract condition of matter which 

 we may assign or imagine, but which we rarely if 

 ever find realized amongst the bodies around us. 



The preceding investigations, the result of an 

 amount of minute and assiduous labour almost fear- 

 ful to contemplate, are to be found collected in the 

 21st volume of the Memoirs of the French Academy 

 of Sciences, which they entirely occupy. 



M. Regnault has elsewhere published analogous 

 researches on the Specific Heat of a great variety of M - Re S' 

 substances, and on the theory and practice of Hygro- gpecifi^ 

 metry, both of which are highly in4eresting and im- Heat and 

 portant, but which I have not here space to analyze. Hygrome- 

 He is also favourably known as the author of an try ' 

 excellent treatise on chemistry, and in fact sits in the 

 Institute as a member of the Chemical Section. He 

 now directs the manufactory of porcelain at Sevres ; 

 and being still in the prime of life, much may yet be 

 hoped from his devotion to science. 



(726.) 



(727.) 



CHAPTER VII. 



(728.) 

 Galvani. 



(729.) 

 His posi- 

 tion as a 

 discoverer 

 difficult to 

 estimate. 



ELECTRICITY MAGNETISM ELECTRO-MAGrNETISM. 



1. GALVANI. Discovery of Galvanism ; Proper Animal Electricity. -~The subject revived by 

 Nobili. MM. Matteucci and Du Bois Reymond. 



THERE are few discoverers in science whose posi- 

 tion it is more difficult to assign with accuracy than 

 Galvani. Attaining at first, by a curious observa- 

 tion patiently reflected on and carefully repeated in 

 detail, to the rank of the founder of a new science, 

 he was so far outstripped in its applications, that his 

 merit was soon in a measure overlooked, and his un- 

 questionable discoveries ascribed to capricious acci- 

 dent. 



We find that a concurrence of circumstances con- 

 tributed to this result. Galvani was advanced in 

 years, and, it would appear, somewhat exhausted in 

 constitution, when he made his famous observation 

 on muscular contractions. He was an anatomist, 

 far more than either a chemist or physicist; no 

 blame, surely, is to be attributed to him on that 

 account ! His discovery was chiefly interesting in 

 his eyes, as illustrating the laws of sensation and 



the source of nervous irritability. It was calculated 

 to throw great light on these most abstruse enquiries. 

 Groping to find the thread which should reveal to 

 him that labyrinth, is it surprising that he devoted 

 himself exclusively to those effects which gave him a 

 real promise of success ? a promise held out still by 

 the same facts still the envied goal of physiologists 

 yet how little realized by the unremitting labours 

 of two subsequent generations ! 



Again, the almost irresistible temptation of con- (730.) 

 verting successful philosophers into heroes, at the Contrast o 

 expense of their contemporaries, added to a less par- an Yvolta. 

 donable wish to relieve the tedium of scientific dis- 

 cussion, by the introduction of a lively, though ques- 

 tionable anecdote, has induced the eulogists of Volta 

 to exalt his unqestionable claims by the deprecia- 

 tion of those of his less widely known and less for- 

 tunate countryman, Galvani. Their contrasts in cha- 



