156 EULOGY ON GAY-LUSSAC. 



bodies, such as carbon, mercury, and iodine, integral parts of certain 

 gaseous combinations. This boldness, as proved by subsequent exper- 

 iments, was crowned with perfect success. 



Eecently it was thought possible to deduce from the unequal dilata- 

 tion of different gases by heat the proof that the law of volumes is not 

 mathematically exact. Let us suppose, the learned critics say implicitly, 

 that two gases combine in equal volume and at a fixed temperature — 

 for example, that of 20<^ centigrade — and that the combination is made 

 molecule by molecule let us carry to 40° the temperature of the two 

 gases. If at 20'^ equal volumes contain the same number of elementary 

 particles, such would not be the case at 40o. There will then be unequal 

 volumes which will enter into combination, supposing that the union 

 must always be effected molecule by molecule. It will be seen that the 

 criticism implies the absolute truth of the atomic theory of combination, 

 which, by the way, may seem not so firmly established as the law of 

 Gay-Lussac. Besides, would not that have been a very singular coin- 

 cidence which should have led our colleague to operate precisely at the 

 temperatures at which this law should be rigidly exact? 



Let us remark, in point of fact, that in the study of nature it has 

 rarely happened that experiment has led, through some light devia- 

 tions, to simple laws, unless these laws have become the definite regu- 

 lators of the phenomena. The system of the earth offers a striking 

 exaqiple of this truth. The laws of the elliptical movement of the 

 planets are only exact by disregarding the irregularities known under 

 the name of perturbations, and which place each planet sometimes in 

 advance, sometimes behind the position assigned to it by the immortal 

 laws of Kepler. 



If it is ever established by direct experiments that the principles laid 

 down by Gay-Lussac are not confirmed when the temperatures come to 

 vary, it will be the time to investigate whether there be not a natural 

 cause to which these perturbations may be attributed. 



In the limited compass assigned me I could only present simple doubts 

 on the nice question I have ventured to broach ; at all events, the assimi- 

 lation which they have suggested to me seems of a nature to satisfy the 

 most enthusiastic partisans of the scientific glory of Gay-Lussac. When 

 Laplace, looking at capillary phenomena in a new light, desired to com- 

 pare the results of his skillful calculations with those of observation, 

 and when he wished the subject to have the final seal of experiment, he 

 applied to Gay-Lussac. The latter fully responded to the confidence of 

 the immortal geometer. I should add that the instrument which he 

 invented is of small dimensions — the same, under the name of cathe- 

 tometer, now so generally in use among physicists. I leave to those 

 who consider they have the right, the responsibility of laying claim to 

 priority in the use of the word cathetometer now generally adopted ; 

 but the instrument in principle and even in form will not the less remain 

 one of the valuable inventions with which our colleague has endowed 

 science. 



