Mr. Hen wood on certain Meteorological Phenomena. 103 



modified in the details, has been lately recommended to the no- 

 tice of the medical profession by Mr. Laming. In the original 

 process a small portion of bitartrate of potash always remained 

 in solution, so that the acid was always distilled when required 

 for chemical use. The alcohol employed by Mr. Laming to 

 precipitate the bitartrate of potash renders the acid even more 

 unfit for chemical purposes, although it is highly useful to add 

 a small portion in the preparation of the medicinal acid. 



However, pure hydrocyanic acid may be obtained at once 

 by the decomposition of a solution of the cyanuret of calcium 

 by oxalic acid. Heat is developed during the decomposition. 

 The oxalate of lime is completely precipitated in a few hours, 

 leaving the supernatant acid quite pure; but if slight traces 

 of oxalate of lime are held in suspension, one filtration is suffi- 

 cient, whilst in the process of Proust two filtrations are re- 

 quired. The liquid is quite pure, as the oxalate of lime is in- 

 soluble in hydrocyanic acid. In this process, as well as in 

 Mr. Clarke's, the use of a mineral acid is avoided. The chief 

 objection to its adoption for the preparation of hydrocyanic 

 acid for chemical purposes, is the difficulty of procuring the 

 cyanuret of calcium; but it is probable that an easy mode 

 of obtaining that cyanuret will soon be discovered. The pro- 

 portions required are one equivalent of oxalic acid and one 

 equivalent of the cyanuret of calcium. 



XX. On some Meteorological Phenomena observed in the 

 Mount's Bay, Cornwall. By W. J. Henwood, F.G S. Loud, 

 and Paris, Hon. M. Y.P.S. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 f\^ the 18th of November, at about 4 h p.m., as I was vvalk- 

 ^-* ing on the beach between Penzance and Newlyn, my 

 attention was drawn to a very dense black cloud (a) hanging 

 over the plateau of Paul Hill {b). Towards the sea it shaded 

 off, and immediately over the beach in that direction it ter- 

 minated, a few small clouds appearing at intervals in the 

 otherwise clear sky. 



On continuing to observe it for some time, I found that 

 although the mass of cloud was in motion before a some- 

 what brisk breeze from the south-east, yet, as one portion re- 

 ceded, another in taking its place became equally dark. 



In fact, as successive portions of air, which, whilst over the 

 sea, were transparent, approached the land, small light spots 

 of cloud began to be formed; and when the scane mass of air 



