9.(iS Mr Johnston's Remarks on Mr Potter's Paper 



have spoken of others so disparagingly, nor committed himself 

 so hroadly. 



Mr Potter thus commences his observations on the law ex- 

 pressed above by the formula SW = C. 



" However, the law as enunciated by the French philoso- 

 phers, that the atoms of all simple bodies have the same capa- 

 city for heat, when applied to the class of metals, holds I find 

 with singular fidelity in common temperatures, excepting in 

 one very remarkable case, that of silver. Whilst the excep- 

 tion remains, the law cannot be said to be established, much 

 less can MM. Dulong and Petit in the least be said to have 

 proved their proposition, as they have taken for the atomic weights 

 numbers completely arbitrary, without referring either to the 

 opinions of chemists, or the results of analyses. Whence they 

 have obtained their numbers I shall now show. They appear 

 to have appropriated such portions of Berzelius's numbers as 

 suited their experiments. Thus, for bismuth they have taken 

 three-fourths of this number, for lead one-half, for gold one-half, 

 for tin one-half, for silver one-fourth, for zinc one-half, for tel- 

 lurium one-half, for copper one-half, for nickel one-half, for 

 iron one-half, and for cobalt one-third of this number, &c. &c." 



Now, after this sweeping condemnation of their mode of 

 proving the law, and of the atomic weights they employed, let 

 us compare their numbers with those of Dalton adopted by Mr 

 Potter, and with those given by Dr Thomson in his lately pub- 

 lished History of Chemistry. 



A single glance at these three columns will show whether the 

 numbers employed by Mr Potter or by the French philosophers 

 in verifying the law of equal specific heats, are the nearer to the 



