374 Royal Society : — 



as required, and put together very compactly. The rauge of its 

 indications can also be made longer by increasing the size of the 

 bulb and the diameter of the tube EFG; but in the same pro- 

 portion the cylinder would have to be made larger. 



The chief novelty of this instrument consists in the means 

 adopted for the correction of temperature ; and this, I believe, 

 can advantageously be employed for other purposes. 



LVI. Proceedings of Learned 'Societies, 



ROYAL SOCIETY. 



[Continued from p. 312.] 



Jan. 13, 1870. — Warren De La Rue, Vice-President, in the Chair. 



THE following communications were read : — 

 " On Fluoride of Silver.— Part I." By George Gore, F.R.S. 



This communication treats of the formation, preparation, analysis, 

 composition, common physical properties, and chemical behaviour of 

 fluoride of silver. 



The salt was prepared by treating pure silver carbonate with an 

 excess of pure aqueous hydrofluoric acid in a platinum dish, and 

 evaporating to dryness, with certain precautions. The salt thus ob- 

 tained invariably contains a small amount of free metallic silver, 

 and generally also traces of water and of hydrofluoric acid, unless 

 special precautions mentioned are observed. It was analyzed by 

 various methods : the best method of determining the amount of 

 fluorine in it consisted in evaporating to dryness a mixture of a 

 known weight of the salt dissolved in water, with a slight excess of 

 pure and perfectly caustic lime in a platinum bottle, and gently ig- 

 niting the residue at an incipient red heat until it ceased to lose 

 weight. By taking proper care, the results obtained are accurate. 

 The reaction in this method of analysis takes place according to the 

 following equation, 2AgF + CaO-=CaF 2 +2Ag-fO. Sixteen parts 

 of oxygen expelled thirty-eight equal parts of fluorine present. One 

 of the methods employed for determining the amount of silver con- 

 sisted in passing dry ammonia over the salt in a platinum boat and 

 tube at a low red heat. The results obtained in the various ana- 

 lyses establish the fact that pure fluoride of silver consists of 19 

 parts of fluorine and 108 of silver. 



Argentic fluoride is usually in the form of yellowish brown earthy 

 fragments ; but when rendered perfectly anhydrous by fusion, it is 

 a black horny mass, with a superficial satin lustre, due to particles of 

 free silver. It is extremely deliquescent and soluble in water ; one 

 part of the salt dissolves in '55 part by weight of water at 15° # 5 C; 

 it evolves heat in dissolving, and forms a strongly alkaline solution. 

 It is nearly insoluble in absolute alcohol. The specific gravity of 

 the earthy-brown salt is 5*852 at 15°'5 C. ; the specific gravity of 

 its aqueous solution, at 15°'5 C, saturated at that temperature, is 

 2*61. By chilling the saturated solution, it exhibited the pheno- 

 menon of supersaturation, and suddenly solidified, with evolution of 



