523 



light, got from a spirit-lamp with salted wick, and with the 

 same results. 



" 3. Upon substituting for the light of the spirit that 

 transmitted through a window-blind of a yellowish colour, the 

 phenomena of the last experiment were very distinctly repro- 

 duced. 



" 4. In operating, by aid of the apparatus used in the 

 preceding experiments, with heterogeneous or solar light, and 

 making the analyser revolve, a series of tints were produced, 

 much feebler than those exhibited by quartz, but following 

 apparently the same law. 



" From these facts I think I am justified in concluding, 

 that this hyalite exercises the power denominated rotatory 

 polarization. This property, too, it possesses, no matter in 

 what direction it is traversed by the plane polarized ray, a cir- 

 cumstance which would seem to identify it with that exerted 

 by liquids, and distinguish it from the analogous influence of 

 quartz, which is manifested only in the direction of the optic 

 axis. 



" Before concluding this paper I may be perm.itted to 

 give expression to an opinion, which is, I believe, shared by 

 most mineralogists, namely, that hyalite, opal, and calcedony, 

 have all had a similar origin, or were originally silex in the 

 gelatinous condition which it assumes when certain alkaline 

 silicates are dissolved in dilute acid, and their solutions are 

 slowly evaporated. All three include water, the hyalite in 

 greatest, and the calcedony in smallest quantity ; and in some 

 specimens that I have seen, these minerals may be observed 

 to pass by insensible gradations into each other. This is well 

 illustrated by some lumps of the Mexican hyalite which have 

 nodules of calcedony attached to them, the latter mineral dif- 

 fering in no respect from the former, save in being less trans- 

 parent, and containing a smaller quantity of water, — its amount 

 being about 0.53 per cent., according to a single experiment. 

 The water, too, in all, is, I believe, present, not in a state 



VOL. III. 2 z 



