476 Royal Society : — 



chantiteG=3 , 87to3*9), and hardness sufficiently distinguish it from 

 that mineral. The mountain-green streak offers an available means 

 of contrasting Waringtonite and Brochantite with Atacamite, the 

 streak of which is of a characteristic apple-green. 



M. Pisani has published analyses of the two above-described mi- 

 nerals. In the former (possibly from having driven off part of the 

 water in the preliminary desiccation of the mineral) he has found 

 less water than I consider it really to contain, and he has conse- 

 quently given to Langite the formula of Waringtonite. 



The green mineral which he has analyzed and described as Bro- 

 chantite seems, from his analysis, to have contained a slight admix- 

 ture of the ferruginous matrix, and also differs from mine in the esti- 

 mate of the water. 



I confined my preliminary desiccation to a careful treatment of the 

 bruised mineral with dried and warm blotting-paper, as many hydrated 

 minerals of this class yield up part of their water when long exposed 

 to a perfectly dry air, or to a temperature of 100° C. 



" Preliminary Note on the Radiation from a Revolving Disk." By 

 Balfour Stewart, M.A., F.R.S., and P. G. Tait, M.A. 



The authors having been led by perfectly distinct trains of reason- 

 ing to identical views bearing on the dissipation of energy, have had 

 preliminary experiments made on the increase of radiation from a 

 wooden disk on account of its velocity of rotation, both in the open 

 air and in vacuo. 



These experiments were made with a very delicate thermo-electric 

 pile and galvanometer. In the experiments in the open air the disk 

 was of wood ; its diameter was 9 inches, and it was made to rotate 

 with a velocity somewhat less than 100 revolutions in one second. 



A sensible effect was produced upon the indicating galvanometer 

 when the disk was made to rotate, and this effect appeared to be due 

 to radiation, and not to currents of air impinging against the pile. In 

 amount it was found to be nearly the same as if the disk had increased 

 in temperature o, 75 Fahr, 



In the experiments in vacuo the diameter of the wooden disk was 

 over 12 inches ; its velocity of rotation was about 100 revolutions in 

 one second, and the pile was nearer it than when in air. Under 

 these circumstances, with a vacuum of 0-6 in., an effect apparently 

 due to radiant heat was obtained, amounting to nearly the same as 

 if the disk had increased in temperature 1°*5 Fahr. 



Bearing in mind the increased diameter of the disk, the effect is 

 probably equivalent to that obtained in air, and these preliminary 

 experiments would tend to show that when a wooden disk is made 

 to revolve rapidly at the surface of the earth, its radiation is increased 

 to an extent depending on the velocity ; and it would appear that 

 this effect is not materially less in a vacuum of 0*6 in. than in the 

 open air. 



The authors intend to work out this and allied questions experi- 

 mentally, and hope, if successful, to communicate the result to this 

 Society. 



