378 Prof. Miller 07i the Specific Gravity of Sulphur of Nickel. 



P. S. It has been pointed out to me, that in my memoir on 

 Dispersion, the calculations are affected with an error, in that 

 I have neglected to shorten X for the interior of the substances. 

 The rectification of this error will change all the values of q. 

 But as the formulaa are of necessiti/ capable of fulfilling the 

 conditions required of them, it does not appear necessary to 

 re-calculate them. 



Edinburgh, March 4, 1842. 



LV. On the Specific Gravity of Sulphuret of Nickel. By 

 W. H. Miller, M.A., F.R.S., Professor of Mineralogy in 

 the Universit// of Cambridge *. 



IN a description of the form of sulphuret of nickel (Haarkies) 

 which appeared in the London and Edinburgh Philoso- 

 phical Magazine for February 1835, I stated that the specific 

 gravity of the mineral, obtained by weighing not quite two 

 orains and a half of it in air and in water at 15° C, accord- 

 ing to one observation was 5*28, according to another 5'262. 

 Professor Breithaupt, at my request, repeated the observation 

 with crystals from Johann-Georgenstadt and Kamsdorf, and 

 in both cases obtained a specific gravity of 5*00 (Poggendorff's 

 Annalen, Band li. S. 511). The difference between this and 

 the former determination induced me to repeat the observa- 

 tions with the utmost care. About three grains and a half 

 of the crystals, the whole quantity at my disposal fit for such 

 a purpose, were inclosed in a light silver tube having small 

 openings in it to permit the escape of the air. The tube was 

 suspended in a vessel of distilled water which was kept boiling 

 lor some time. When the temperature had descended to 15° 

 C. the apparent weight of the tube and crystals was observed. 

 The crystals were then taken out and the apparent weight 

 of the tube determined, using the same precaution as before. 

 The balance employed was made by the late Mr. Robinson. 

 With this instrument, the greatest error of any weighing does 

 not amount to l-2000th grain in air, or 1 -200th grain in water. 

 Three different observations gave for the specific gravity, 



5-2774, 5-295, 5-277, at 15° Centigrade. 

 The mean of the five observations is 5-278. The very great 

 difference between this result and 5*00, that obtained by Pro- 

 fessor Breithaupt, renders it not improbable that the crystals 

 he examined do not belong to the same species as those in my 

 possession. The latter cleave very readily in three directions, 

 normals to which make with each other angles of 35° 52', 



* Communicated by the Author. 



