On certain Double Compounds of Ammonia and Chromium. 191 



Again, let F = be the equation of the covariant conic which 

 passes through the points of contact of common tangents to U 

 and V (see my ' Conies/ pp. 268, .288), then the coefficient of s^ 

 in its equation will be 



— 4rn(l— A). 



Hence it can be seen that the coefficient of z'^ vanishes identically 

 in the equation 



16v"vU-4{4Ov'-O'2}v'F+{4ov'-O'n'V=0, 

 which is therefore the equation of the locus required. Its form 

 shows that it represents a conic touching the four tangents com- 

 mon to U and Y. 



If 40y'— O'2=0, the envelope reduces to V, and the locus 

 to U, in conformity to Mr. Cayley's theorem. It does not seem 

 impracticable to obtain the equation of the locus by the same 

 method when the two sides touch different conies. 



Trinity College, Dublin, 

 February 16, 1857. 



XXXII. On certain Double Compotinds of Ammonia and Chromium. 

 By Charles W. Vincent, Assist. Lib. Royal Institution*^. 



THE solubility of freshly precipitated hydrate of sesquioxide 

 of chromium in ammonia has long been known. In en- 

 deavouring to discover the amount of this solubility and the 

 circumstances which modify it, I met with many difficulties. 

 Operating with solutions of sesquioxide of chromium, obtained 

 by deoxidizing bichromate of potassa by treatment with alcohol, 

 on addition of ammonia I obtained results which did not at all 

 coincide with one another; in some cases the precipitated chromic 

 oxide was readily dissolved, in others it was quite insoluble. The 

 tints of the solutions, when produced, also oftentimes differed very 

 much; and altogether I met with such apparently contradictory 

 effects, that I was led to make the experiments which form the 

 subject of the present note with a view to their reconciliation. 



In order to exclude all other compounds from taking any part 

 in whatever action might occur between the chrome and ammonia, 

 I prepared a precipitate of the green hydrate of chromic oxide 

 from a solution of the sesquichloride of chromium, washed this 

 thoroughly with distilled water, digested it for half an hour with 

 concentrated ammonia, and then filtered it. The ammonia passed 

 through the filter perfectly clear and colourless, without a trace 

 of the red tint which denotes the solution of the chrome. This 

 experiment I repeated many times, with every precaution, and 



* Communicated by the Author, 



