190 Prof. Kigaud on a remarkable Deposition of Ice. hQ 



hydrogen being least resisted in passing through the plug. The 

 experiments on die permeability of the stucco by gases under 

 pressure, above detailed, were projected with a view to settle 

 this point among others ; and they are evidently incompatible 

 with such an application of the theory, for hydrogen passes 

 2*4? times more swiftly, and not 3*8 times, as in the diffusion 

 experiments. Carbonic acid, too, permeates the plug, under 

 pressure, as rapidly as air does, or even somewhat more ra- 

 pidly, for our results inclined to this side rather than to the 

 other; whereas carbonic acid diffuses through the plug more 

 slowly than air does, or is replaced by more than an equal 

 volume of air, as will presently appear. 



Those experiments, previously narrated, are perhaps suffi- 

 cient to establish the law in regard to hydrogen, particularly 

 when we find it hold in the case of other gases. 



As hydrogen is a very light gas, I was anxious to establish 

 the law also in regard to a heavy gas, such as carbonic acid. 

 [To be continued.] 



XXVIII. Notice of the Occurrence on a Stone Wall of a re- 

 markable Deposition of Ice, similar to that described in the 

 preceding Number of the Philosophical Magazine. By Pro- 

 fessor Rigaud. 



To Sir David Brewster. 

 Dear Sir, 

 Tj^ VERY thing connected with an unusual fact is interesting 

 -" in itself, and variations may assist in leading to the ex- 

 planation of it — no further apology seems requisite for offering 

 the following notice to your consideration. 



The account in the last Number of the London and Edin- 

 burgh Philosophical Magazine, of a remarkable deposition of 

 ice, immediately recalled to my recollection that I had once ob- 

 served the same phenomenon, though not on vegetables. The 

 description there given answers precisely to what I saw : the 

 engraving, indeed, gives me the idea more of snowy white- 

 ness than of the semi-pellucid icy appearance which occurred 

 in the instance which I witnessed ; but this may have arisen 

 from accidental circumstances*. 



On looking back to a memorandum which was made at the 

 time, I find that in 1821, between 11 and 12 at noon on the 

 18th of February, I observed the fact on a stone wall, with an 

 eastern aspect, in a lane of this place. It occurred in many 

 parts, which were from three to six or seven feet from the 



* The engraving represents the appearance imperfectly. It was, as Mr. 

 Rigaud describes it, scini-pellucid. — 



