Form of the Black Cross in Biaxal Crystals. 305 



slightest appearance of cyanogen, warrants the conclusion that 

 paracyanogen once formed from cyanogen or its elements can- 

 not be rechanged into cyanogen by heat." For it can hardly, 

 we apprehend, be argued that the paracyanogen employed in 

 our experiment, prepared from the crude product resulting 

 from the decomposition of hydrocyanic acid, solution in sul- 

 phuric acid, and precipitation by water, is less pure than that 

 obtained from paracyanogen procured from bicyanide of mer- 

 cury, dissolved in sulphuric acid, and gradually precipitated by 

 the absorption of atmospheric moisture; nor can we see how 

 it can be argued that the failure in effecting the transformation 

 of carbon into silicon, in the experiment just detailed, can be 

 attributed to the employment of an impure paracyanogen, or 

 to a substance used not being paracyanogen, because it was 

 not obtained by the action of heat on bicyanide of mercury. 



As our object in performing the experiments we have de- 

 tailed was simply an inquiry respecting the fact of the state- 

 ment made by Dr. Brown, of the conversion of the carbon of 

 paracyanogen into silicon, we have not entered into a critical 

 examination of the paper, nor do we attempt to offer any ex- 

 planation of the results arrived at by us in every one of our 

 experiments (that the carbon (if paracyanogen is incapable) by 

 such of the processes as we have tried, which are recommended 

 by Dr. Brown, of conversion into silicon), being in every in- 

 stance in complete opposition to those published by that au- 

 thor. We leave those interested in this subject to decide whe- 

 ther Dr. Brown's experiments or ours are correct, and con- 

 clude this examination by stating that the terms " white heat" 

 and "yellow heat," when employed by us, mean respectively 

 temperatures at which malleable iron and manganese fuse in 

 the first case, and at which copper melts in the second; whereas 

 Dr. Brown's white heat is a temperature evidently below our 

 yellow heat, as he speaks of exposing a copper tube to a white 

 heat for upwards of an hour, and does not mention the fusion 

 of the "gypsum" when kept at a white heat for one hour 

 and a half. 



Liverpool, Sept. 17, 1841. 



XLIV. Solution of a Geometrical Problem on the Form of the 

 Black Cross in Biaxal Crystals. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN turning over some old papers, I have fallen upon the fol- 

 lowing very simple method of solving a geometrical pro- 

 blem which occurs in investigating the form of the black cross 

 Phil. Mag. S. 3. Vol. 19. No. 12*. Oct. 1841. X 



