32 M. H. Fizeau on the Expansion of Solids bij Heat, 



a general law which appears to rule these phenomena, having, 

 moreover, lately succeeded, thanks to the able advice of M. Des 

 Cloiseaux, in observing with certainty some more complex crystals 

 belonging to the oblique system, I may now attempt to develope 

 with more detail, and with the sanction of experiment, the theo- 

 retical views laid down in my first memoir*. 



Some remarks may first be made and some principles laid 

 down with the view of removing objections which might be 

 raised relative to the regularity and constancy of the pheno- 

 mena in question. 



All observations agree in showing that the phenomenon of the 

 change of volume of a body by heat, wmether it be an expansion 

 or a contraction, is produced in a continuous manner and with 

 a regularity perfectly similar to that of the changes in tempera- 

 ture — the same volumes corresponding constantly and identically 

 to the same temperatures. 



Any idea of sudden and accidental, and, so to say, capricious 

 variations may then be rejected in this class of phenomena, as 

 well as the existence of slow variations which in the course of 

 time might occur in the numerical value of the coefficients of 

 expansion. 



I may cite, in support of the invariability of these coefficients, 

 two determinations made with great care, at an interval of a year, 

 on the same quartz crystal supported on the same platinum tri- 

 pod, — this latter having during this interval undergone several 

 hundred alternations of temperature between 7° and 80°. These 

 two coefficients correspond to the direction of the axis of the 

 crystal. 



First determination . a = 000000781 18, 

 Second determination a = 0-00000781 17. 



When, in my former memoir, 1 endeavoured to coordinate the 

 phenomena then known regarding the expansion of crystals be- 

 longing to the various crystalline systems, the limited number 

 of the observations might indeed indicate a simple and general 

 law, but did not permit its being established with sufficient 

 precision ; yet, guided by the analogies which were already 

 manifest between the principal characters of these phenomena 

 and those which the phenomena of luminous and calorific pro- 

 pagation present in their relations to the general symmetry of 

 crystals, I have entered upon the path which for some time has 

 been opened to us by FresneFs celebrated researches on the pro- 

 pagation of light in crystals — a path which has been successfully 

 followed by Senarmont in his discoveries of the unequal propa- 

 gation of heat in the same bodies. And just as these two classes 



* May 21 and 28, 1866. 



