386 Dr. Heddle on the " Davidsonite " of Thomson. 



tcDiperature, — the method, in fact, which we have employed in 

 onr experiments referred to by the celebrated German physicist. 

 At the commencement of the memoir* containing the first 

 results of these experiments, we remark that Mayer^s hypothesis, 

 which has been adopted by lloltzmann, Clausius, and other phi- 

 losophers, had been verified approximately for air at ordinary 

 temperatures by my experiments, and by Prof. Thomson's theo- 

 retical investigation, founded on a conclusion of Carnot's which 

 requires no modification in the dynamical theory of heat. I may 

 add, that in Prof. Thomson's account of Carnot's theory, com- 

 municated to the Royal Society of Edinburgh, April 1849, the 

 formula is demonstrated which gives Carnot's function in terms 

 of the temperature, and the ratio of the work spent to the heat 

 evolved in compressing air kept at the same temperature. The 

 results we have arrived at in our joint investigation have tended 

 certainly to develope our views, and to give a more definite know- 

 ledge of the constitution of elastic fluids, but they do not con- 

 tradict our original statements, pubhshed before the appearance 

 of Prof. Clausius's papers. 



Oakiield, Moss Side, 

 ■■! Oct. 20, 1856. J 



' i '<< n II ' li . w . 



XLIX. Note on the " Davidsonite " of Thomson. 

 By Dr. Heddle f. 



SOME time ago I obtained from the Rev. Dr. Fleming a por- 

 tion of a crystal of " Davidsonite," which he requested me 

 to analyse. The Doctor informed me that he had obtained from 

 it fflucina, and believed it to be merely beryl, and not a distinct 

 species, a conclusion at which I had myself arrived from a simple 

 mineralogical examination of the substance. The crystal on ana- 

 lysis afforded 12*52 per cent, of glucina. I do not give the full 

 analysis, as the specimen was contaminated with plates of mica 

 and grains of quartz, which 1 was unable entirely to exclude ; from 

 this cause the proportion of glucina must be somewhat greater 

 than that stated. Setting aside the above mechanical impurities, 

 the analysis in all respects agreed with that of beryl. 



Thomson considered the substance to be oblique, as his crystal 

 had an oblique termination ; this was the result of one of the 

 terminal planes truncating so deeply as to carry off all the others. 

 All the crystals are rough and coarse. 



The localities at Aberdeen are Tory, opposite the mouth of the 

 harbour, and the Rubieslaw quarry ; at both it occurs in coarse- 

 grained granitic veins, which traverse the ordinary compact gra- 

 nite of the district. At Rubieslaw the crystals of felspar in these 

 veins are twins of very large dimensions. 



* Phil. Mag. vol. iv. p. 481. 

 t Communicated by the Author. 



