EULOGY ON GAY-LUSSAC. 155 



great hesitation, the independent and sober judgment of Gay-Lussac 

 would have placed him beyond influences which would not have been 

 called into play except under cover of eminent merit or fertility and of 

 imagination. His publications in the three volumes of the memoirs of 

 the Society of Arcueil deserve in every respect, from their variety, their 

 novelty, and also their exactness, to occupy the most distinguished place 

 in an impartial history of the sciences. 



The first volume of the collection, published by the Society of Arcueil, 

 begins by a memoir in which Gay-Lussac has combined the results of 

 all the magnetic observations made in conjunction with M. tie Hum- 

 boldt, during the journey through France, Italy, and Germany, of which 

 we have already spoken at length. This branch of the science has for 

 some years been making very considerable progress, and yet we can confi- 

 dently recommend to physicists those pages on which Gay-Lussac has-, 

 examined all the causes of error which may affect the measurements of 

 inclination and intensity, and the precautions to be taken to avoid them. 

 We know now that the horizontal force which directs the magnetic 

 needle is subject to a diurnal variation which depends in part, but only 

 in part, upon a corresponding variation in the inclination. We have 

 likewise learned that in a given place and at a given time the duration 

 of the oscillations of a needle depends upon its temperature. It would 

 therefore be now necessary, if a magnetic voyage were undertaken, to 

 take into account all of these disturbing causes ; but, and we can say 

 it without flattery, at the period when it was published, the work of 

 Messrs. de Humboldt and Gay-Lassac was a model. 



If we cast our eyes over the second volume of the Memoirs of Arcueil, 

 we will find there, among other clever works of interest — a Mcmoire snr 

 la Combinaison des Substances Gazeuses entre dies, " Memoir on the com- 

 bination of gaseous bodies with each other." This memoir contains re- 

 sults so remarkable, so important, that they are habitually called the 

 laws of Gay-Lussac. It would now be very difficult for me to give a 

 detailed and perfectly accurate account of the atomic theory. This 

 sketch should, I think, go back as far as Higgins, an Irish chemist, 

 whose work, published in 1789, is only known to me through very 

 short quotations by Humphry Davy. Then come the researches of 

 Dalton in 1803. It is a matter of certainty that the law of volumes 

 was demonstrated experimentally by our associate in 1808, without any 

 knowledge on his part of the first more or less systematic investigations 

 of his predecessors. 



The laws to which we have alluded may be announced in these terms : 



Gases, in acting upon each other, combine in volume in the simplest 

 ratios ; such as 1 to 1, 1 to 2, or 2 to 3. Kot only do they only unite in 

 these proportions, but again, the apparent contraction of volume which 

 sometimes occurs by the combination bears also a simple ratio to the 

 volume of one of the combined gases. Gay-Lussac, later, had the bold- 

 ness to deduce from his laws the density of the vapors of several solid 



