258 Mr. Darnell's Reply to X. [Oct. 



in measuring heights, is much more affected by the alteration in 

 the specific gravity of the air by the existing temperature, than 

 by that of the metal. He proceeds, " So that whether the tube 

 expand or contract, or were it possible, do neither, whatever the 

 material of which it is made, whatever its sectional form, 

 equality or inequality of calibre, still the absolute dilatation and 

 not the apparent must regulate the correction for difference of 

 temperature." 



It is evident that X. here speaks of the change in specific 

 gravity as if it were to be ascertained by weight, and not by 

 measure. He forgets that in the barometric experiment the 

 only way in which the result is affected is by alteration of 

 volume ; which alteration of volume is ascertained by measure. 

 Ts T ow as this measure cannot be taken upon any scale which is 

 not itself liable to expansion and contraction from changes of 

 temperature, it is clear that the alterations of the latter must be 

 taken into account; so that if the expansion of the mercury be 

 measured upon brass, the absolute dilatation per degree of the 

 former must be taken minus that of the latter, or if it be com- 

 pared with glass, minus that of the glass. 



I shall leave MM. Dulong and Petit, whom your correspondent 

 asserts are so " egregiously wrong," to defend themselves, 

 should they think it worth while, being perfectly assured, in my 

 own mind, that they are fully competent to the task ; and I have 

 not much doubt that even M. Biot would be able to rescue him- 

 self from the imputation of having made "a false conclusion 

 from his own premises." 



With regard to the filtration and distillation of mercury, I 

 must beg to assure X. that notwithstanding his knowledge of 

 that metal " derived from a peculiar application of it," he may 

 acquire much further information by inquiry of any competent 

 workman. 



If I shall have succeeded in making myself intelligible to X. 

 I may, perhaps, be permitted to hope, that he will see the pro- 

 priety of hereafter excluding from scientific controversy such 

 expressions as that of " mere drivelling." 



I remain, Gentlemen, with great esteem, 



Yours faithfully, 



J. F. Daniell. 



