Royal Astronomical Society, 71 



per cent, of the whole of the ferrocyanide of potassium origi- 

 nally used in precipitating the silver. 



When cyanide of potassium is added to the ferridcyanide 

 of silver a similar substitution takes place, requiring 6 equiva- 

 lents of cyanide of potassium to get the whole into solution; 

 but the primary action is somewhat different in this salt from 

 any other we have tried. When cyanide of potassium is added 

 to ferrocyanide of silver, the formation of the double salt of 

 silver and potassium, and the ferrocyanide of potassium, are 

 simultaneous; and if there be not 4 equivalents of cyanide of 

 potassium added, the remaining precipitate is undecomposed 

 ferrocyanide of silver. But on adding cyanide of potassium 

 to the ferridcyanide of silver, the first action is the decompo- 

 sition of the whole ferridcyanide of silver, with the formation 

 of cyanide of silver and ferridcyanide of potassium ; 3 equi- 

 valents of cyanide of potassium effect this change, the preci- 

 pitates passing from an orange to a white ; the ferridcyanide 

 of potassium may be decanted from the precipitate without 

 having any silver in solution. Several other interesting re- 

 actions take place with these substances, which have not yet 

 been sufficiently investigated to be brought forward. In con- 

 clusion, however, we may mention a reaction with the ferro- 

 cyanides of the metals, which, so far as we are aware, has 

 not yet been observed in any chemical work ; it is that the fer- 

 rocyanides of the metals are converted into the ferridcyanides 

 by adding an excess of nitric acid to these compounds ; if, for 

 example, we add nitric acid to the ferrocyanide of silver, it is 

 immediately changed from a white to a deep orange ; 1 equi- 

 valent of silver is dissolved, and every 2 equivalents of ferro- 

 cyanide of silver with nitric acid produces one of nitrate silver 

 and one of ferridcyanide of silver. 



When the ferridcyanide of silver is heated in water to about 

 150°, it loses its orange colour and becomes green, which 

 colour is permanent; any acid or nitrate of silver present pre- 

 vents this change until the precipitate is dried and heated. 

 This green precipitate has not yet been examined. It can 

 be obtained either with the ferridcyanide formed by the nitric 

 acid upon ferrocyanide, or by precipitating silver with ferrid- 

 cyanide of potassium. 



X. Proceedings of Learned Societies. 



ROYAL ASTRONOMICAL SOCIETY. 

 [Continued from vol. xxiv. p. 540.] 

 March 8, '^I'^HE following communications were read : — I. Obser- 

 1844. J- rations of the Comet of Faye*. 

 [* Other observations of this comet will be found referred to in vol. xxiv. 

 pp. 519—522.] 



