37-1 Inquiries inlo I he Laws of Dilalation of Solids, Liquids, 



who took up the subject, supposed that the capacity of water does 

 not vary between () and 100"; so that, by mixing two equal 

 masses of this fluid at different temperatures, he took for that 

 of the mixture the mean of the temperatures of the separated 

 masses. 



Mr. Dalton thinks that it is not equal masses which must be 

 mixed, but rather equal volun)es ; because he supposes that the 

 capacity of the same hkiss increases with the temperature in 

 proportion to the volume which it acquires ; or, in other words, 

 he supposes that the capacity of bodies referred to their vokune 

 will remain constant. It is very ea'^y to prove that tlie specific 

 heat of the gases, and even of the liquids, undergoes variations 

 in the ratio indicated by Mr. Dalton ; but we do not find in the 

 work of this eminent philosopher any experiment in support of 

 the law which he seeks to establish. 



We see that the researches hitherto set on foot on tlie true 

 gradatious of a thermometer are reduced to very little, and we 

 may even add that no jjerson iias undertaken direct experiments 

 to ascertain them beyond 100°. 



After having compared with each otiier the different expcri^ 

 mental methods which may be employed in order to attain tlie 

 solution of this important question, we have thought that the 

 determination of the quantities of heat, particularly at high 

 temperatures, was not susceptible of sufficient precision. We 

 thought it preferable to comp;ire, in the first place, with the mer- 

 curial thermometer the progress of the dilatation of bodies the 

 most homogeneous, and of such a nature that the causes which 

 visibly affect the uniformity of the dilatation should have no 

 influence over them. These bodies ought evidently to be taken 

 from among the gases, or from among the solids endowed with a 

 great infusibility. The experiments of M. Gay Lussac having 

 taught us that all the gases are dilated exactly in the same way when 

 they are placed in the same circumstances, it is natural to con- 

 clude that the dilatability of one and the same gas ougiit to be 

 constant, and that consequently, at increments equal in volume 

 or in elastic force, they ought to answer to equal increments of 

 temperature. 



It was by setting out from this principle that M. Gay Lussac 

 ascertained that the march of the mercurial thermometer was 

 regular between the freezing and boiling ))oints. 



Experimentalists are pretty generally agreed in regarding the 

 dilatation of solids which are very difficult to melt as being uni- 

 form. The experiments of Messrs. Laplace and Lavoisier on 

 the dilatation of most of the metals between and lOO"" come 

 in aid of this opinion. 



Now, if bodies so different as the metals and the gases fol- 

 lowed 



