i 823 .] Matter in Cavities found in certain Crystals. 49 



taining an expansible but difficultly volatile substance, may be 

 considered as highly favourable to the theory of the igneous 

 origin of crystals : the other experiment is of a nature entirely 

 different, though its result may be explained in the same suppo- 

 sition. 



In examining a crystal in the collection of the Royal Institu- 

 tion, and which from its characters I believe to be from Capao 

 d'Olanda, Province of Minas Geraes, Brazil, I observed that the 

 quantity of aeriform matter was unusually small in proportion to 

 the quantity of fluid, in two or three cavities not occupying one- 

 tenth or one-twelfth of the space ; and from the peculiarity of 

 its motion, it appeared to be more likely to be compressed than 

 rarefied elastic fluid ; and in piercing the sides of the cavities, I 

 found that this was the case ; it enlarged in volume from ten to 

 twelve times ; the fluid was water, but the gas was too minute 

 in quantity to be examined. 



It will be interesting to ascertain under what circumstances, 

 and in what situations, crystals of this kind are found. If they 

 be supposed of igneous origin, they must have been formed 

 under an immense weight of atmosphere or fluid, sufficient to 

 produce a compression much more than adequate to compensate 

 for the expansive effects of heat, a supposition which, in conse- 

 quence of Mr. Perkins's experiments, already alluded to, may be 

 easily formed. 



Article V. 



On Grey Whin. By N. J. Winch, Esq. 



(To the Editor of the Annals of Philosophy.) 



DEAR SIR, Newcastle-upon-Tyne, Dec. 18, ?822. 



Though 1 believe it to be generally known to such of your 

 readers as have paid attention to the geology of Britain, that the 

 workmen engaged in the collieries and quarries of the north of 

 England and Scotland apply the term whin not only to basalt or 

 greenstone, but to most other species of hard stone, yet I am not 

 aware that a peculiar rock called by them grey whih has ever 

 attracted particular notice. The rock constituting the stratum, 

 or possibly strata in question, consists of minute grains of hair- 

 brown compact felspar or clinkstone, with a few spangles of 

 silvery mica scattered through it. From the close cohesion of 

 its small particles, it might be mistaken at first sight for clink- 

 stone, strongly resembling the well-known rock of Lamlash, but 

 by a lens, the granular texture is detected, and the difference 

 between them easily proved. Owing to this compact texture, 



New Series, vol. v, l 



