376 Royal Society : — 



By passing anhydrous hydrofluoric acid vapour over perfectly an- 

 hydrous and previously fused fluoride of silver, at about 60° Fahr., 

 distinct evidence of the existence of an acid salt was obtained. This 

 acid salt is decomposed by a slight elevation of temperature. 



Numerous experiments were made to ascertain the behaviour of 

 argentic fluoride in a state of fusion with chlorine ; and great diffi- 

 culties were encountered in consequence of the extremely corrosive 

 action of the substances when brought together in a heated state. 

 Vessels of glass, platinum, gold, charcoal, gas-carbon, and purified 

 graphite were employed*. By heating the salt in chlorine, contained 

 in closed vessels formed partly of glass and partly of platinum, more 

 or less corrosion of the glass took place, the chlorine united with the 

 platinum and fluoride of silver to form a double salt, and a vacuum 

 was produced. By similarly heating it in vessels composed wholly 

 of platinum, the same disappearance of chlorine, the same double 

 salt, and a similar vacuum resulted. Also, by heating it in vessels 

 composed partly of gold, an analogous double salt, the same ab- 

 sorption of chlorine and production of rarefaction were produced. 

 And by employing vessels partly composed of purified graphite, a 

 new compound of fluorine and carbon was obtained. 



"Approximate Determinations of the Heating-powers of Arcturus 

 and a Lyrse." By E. J. Stone, F.R.S., First Assistant at the Royal 

 Observatory, Greenwich. 



About twelve months ago I began to make observations upon the 

 heating-power of the stars. My first arrangements were simply 

 these : — I made use of a delicate reflecting astatic galvanometer, and a 

 thermo-electric pile of nine elements. The pile was screwed into the 

 tube of a negative eyepiece of the Greenwich Great Equatorial, from 

 which the eye-lenses had been removed. 



I soon convinced myself that the heat, condensed by the object- 

 glass of twelve and three-quarters inches upon my pile, was appreci- 

 able in the case of several of the brighter stars ; but the endless 

 changes in the zero-point of the galvanometer-needle, and the mag- 

 nitude of these changes, compared with those arising from the heating- 

 power of the stars, prevented me from making any attempts to esti- 

 mate the absolute magnitude of the effects produced. Every change 

 in the state of the sky, every formation or dissipation of cloud, com- 

 pletely drove the needle to the stops. 



At the February Meeting of the Royal Astronomical Society I 

 first became aware of what Mr. Huggins had done upon this ques- 

 tion. His arrangements, however, did not appear to me to meet the 

 difficulties which I had encountered. After some trials, I arranged 

 my apparatus as follows, and with its present form I am satisfied. 



a and j3 are two pairs of plates of antimony and bismuth. The 

 areas are about (0'075) 2 inch, and their distance is about 0'25 inch. 



The poles are joined over in opposite directions to the terminals of 

 the pile and galvanometer. The whole pile is screwed into a tube 



* In the next communication will be described the results obtained with 

 vessels formed of other materials. 



