On the Symbol s/ — 1 in Geometry. 171 



The corresponding dingy yellow precipitate of ferridcyanide 

 also contains potassium. On decomposing it by potash, fer- 

 ridcyanide of potassium and oxide of copper are obtained, 

 which proves that it is a true ferridcyanide, and not analogous 

 to Turnbull's blue. 



The reducing action of light shows itself most strikingly in 

 the case of this precipitate. It had, after drying, been kept 

 in large pieces in a glass bottle which stood for some time 

 near a window. All the outsides of the pieces turned towards 

 the light, became of a red-brown colour from the formation 

 of ferrocyanide. 



On treating the ferrocyanide of copper above described 

 with sulphuretted hydrogen, no action at first seemed to take 

 place ; after some time however it commenced ; and on the 

 decomposition being completed a strongly acid solution was 

 formed, which became gradually blue on exposure to the air. 

 It gave a blue precipitate with perchloride of iron, but did 

 not possess the characteristic reaction of hydroferrocyanic 

 acid, for it was not precipitated by aether. On adding mu- 

 riatic acid this reaction however immediately appeared. By 

 standing over sulphuric acid it dried into a blue mass with a 

 coppery lustre, which with water formed a liquid not unlike 

 a solution. 



On analysis it gave to 0*487 peroxide of iron 0*261 sul- 

 phate of potash. The formula, 4Cfy< ir , requires 0*270 

 sulphate of potash to the quantity of iron found. 



XXIX. On the Symbol V — 1 in Geometry. 

 By a Correspondent of Twenty-five Years. 



To the Editors of the Philosophical Magazine and Journal, 



Gentlemen, 



r^URING the progress of the papers of Sir William 

 ■*-* Hamilton, one in your own Magazine and the other in 

 the Cambridge and Dublin Mathematical Journal, it would 

 be unseemly to enter into a discussion of the general question 

 respecting the correctness of the views entertained by this di- 

 stinguished analyst, or even respecting their originality. This 

 must be reserved till we have their full development before us. 

 However, the limited case of which some small use has been 

 attempted in your last Magazine by Mr. Warner, is fairly 

 open to discussion, since the doctrine itself is one of consi- 

 derable standing (almost eighty years) in the mathematical 



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