113 On the Optical Properties 



able defect, of depth of pit, (in situations where such are neces- 

 sary,) cannot be substantiated. 



Mr. Buddie has stated, that some pits not long opened, are 

 subject to very destructive explosions ; What then becomes of 

 our Editor's alleged accumulation of gas throng h oges, in the 

 works of the ])it ? Surely he must have blundered on the no- 

 tion, that the deepest pits were also the oldest ? though the 

 reverse is the case. 



The geognostical sketch furnishes us with this paragraph, 

 viz. "both the sand-slove and slate-clay form the roof and, ^oor 

 of coal-beds, but the Latter much more frequently than the for- 

 mer." Are we to understand from this too loose description, 

 that sand-stove, properly so called, was ever observed by the 

 writer as the immediate J?oor of a coal bed ? If so, he will confer 

 ■MX obligation on myself and many others, who have never seen 

 the like, if he will mention the particular pit and coal bed, in as 

 many instances as he may Ije able. 



Before 1 conclude, let me remark, that however clearly the 

 learned Editor may have himself understood, and may have ac- 

 quiesced in, the positions contained in the first twelve pages of 

 his last number, and two plates, as to " rents ;" to men of my 

 standard of intellect, the language appears not less unintelligible, 

 than the cases that I guess the writer meant to describe, are 

 contrary to the real appearances of faults in coal mines. 

 I am, yours, &;c. 



Feb. 3, ir.ir.. " An Engineer. 



XXII. O?} the Optical Properties of Sulphuret of Carl ov, Car- 

 lonate of Bnnjtes, and Nitrate of Potash, with Inftrcnces 

 respect 171 g the Structure of douhhi rfractiiig Crystals. By 

 David Brewsthr, LL.D. F.R.S. Edin. and F.A.S.E.'^ — 

 Corinnunicaled by the Author. 



Xn examining the changes which light undergoes during its 

 passage through transparent bodies, we not only receive infor- 

 mation respecting the properties of that mysterious agent ; but 

 we are in some measure made acquainted with the composition 

 of the substances themselves, and with the manner in which their 

 ingredients are combined. The optical pha'nomena, therefore, 

 which bodies exhibit in their action upon light, are so many 

 tests, to Avhich the pliilosopher may have recourse, either in sup- 

 plying the place of chemical analysis, or in correcting and mo- 

 difying its results. A difference in the optical properties of two 

 bodies, is generally an infallible indication of a difference in their 



* From the Trausuctiuiii of ihcj Ilojal Society of Ediuburgli for 1814. 



elementary 



