EULOGY ON GAY-LUSSAC. 153 



not unknown to his countrymen, occupied in the small town of Kendal 

 the very humble and somewhat unprofitable position of private tutor of 

 mathematics, and had only at his command for his experiments imper- 

 fect instruments. There would then have been no impropriety in sub- 

 jecting his results to careful verifications. Gay-Lussac was not ac- 

 quainted with the works of the illustrious English physicist, as there 

 was no mention of them in the full and instructive account of the 

 experiments made by the physicists who had preceded him. Dalton 

 had found that air expands 0.392 in the interval between 0° and 100° of 

 the centigrade thermometer. Already, previously, as I had ascertained 

 from a printed document, Volta had given for this expansion 0.38. 

 Finally, in 1807, Gay-Lussac found it to be 0.S75. This number was 

 generally adopted up to a recent period, and employed by all the 

 physicists of Europe. 



According to the late determinations of Rudberg, and Messrs. Magnus 

 and Regnault, there was an error of about 1 Jj in the value of the dila- 

 tation of air given by Gay-Lussac ; our colleague never objected to 

 the number 0.3665 substituted by our fellow-laborer, M. Regnault, for 

 the number 0.375 which he had given. But what could be the real 

 cause of this difference ? Gay-Lussac has never given any public ex- 

 planation of this disagreement. Not anticipating the catastrophe which 

 so suddenly removed him from us, I was guilty of the fault of not inter- 

 rogating him directly upon this subject. 



It would not be uninterestiug, however, to investigate how so careful 

 a physicist could allow himself to be drawn into such an error. 



A German professor, celebrated"for the importance of his discoveries, 

 M. Chladni, visited Paris some years since. Smarting under the diffi- 

 culties which he had encountered in all his investigations, he said in an 

 impressive tone and petulant manner, never to be forgotten, for in their 

 exaggeration they almost bordered upon the ridiculous : " When you de- 

 sire to lift the smallest corner of the veil which envelopes nature, she 

 invariably exclaims *' No ! i^To! No !" M. Chladni might have added that 

 at the moment when it seems about to yield, it surrounds the observer 

 with snares into which the most skillful fall without suspecting it. 



What could be the causes of error in the experiments of Volta, Dalton, 

 and Gay-Lussac that these illustrious physicists had not perceived 1 I 

 have heard it said that the drop of mercury designed to intercept com- 

 munication between the vessel in which the air was expanded and the 

 external atmosphere, leaving a slight space and giving passage to a 

 portion of the dilated air, was not displaced as much as it would have 

 been without that ; but this cause would evidently have given too small 

 a co-efficient, and it was in the opposite direction, according to the recent 

 observations, that the number upon which Gay-Lussac had decided was 

 in fault. It was much more probable that the interior of the sides of 

 the vessel in which the celebrated academician operated were not suffi- 

 ciently dry J that the hygrometric vapor, adhering to the glass at low tem- 



