196 On the Fentllat'ion of Coal-Mines. [March, 



4. The coloured rings formed by this salt, and already noticed 

 tinder a former head, form another optical ianomaly equally sur- 

 prising. 



The carbonate of potash also gives a bright and an imperfect 

 image, the least of which has a very low refractive power. Its 

 indices of refraction are m = 1*482, m' = \•2>^^. 



VIII. Optical Properties of Carbonate of Barytes. 



This mineral resembles the agate in its optical properties. It 

 forms two images ; one of which is distinct, and the other indis- 

 tinct, or nebulous. The following phenomena were exhibited by 

 ditFerent prisms cut out of the same specimen. 



1. One of the prisms formed four images, all of which were 

 indistinct, and consisted of circular arches of nebulous light. The 

 two middle ones were the largest and the brightest, and the other 

 two were probably formed by reflection. All the images were 

 polarised ; but each of the two outer images was polarised in the 

 same manner as the bright image farthest from it. 



2. In another prism the most refracted image is of a brown hue, 

 and so extremely faint, when compared with the least refracted 

 image, that I at first imagined that I had discovered a polarising 

 crj'stal that gave only one image. 



3. In a third prism the most refracted image approximates to 

 distinctness, but is never as luminous as the other. 



The preceding phenomena exhibited by nitrate of potash and 

 carbonate of barytes advance us an important step in the theory of 

 double refraction. In the memoir which I have written on the 

 subject 1 have attempted to explain the new views to which they 

 lead. I am, Dear Sir, 



Your most obedient humble Servant, 



Edinburgh, Jan. 12, 1814. DaYID BrEWSTKK. 



' Article VII. 



On the Ventilation, of Coal-Mines. By Mr. John Taylor, ot 

 Holwell- house, near Tavistock, 



♦' Qtialeis espirat scaptesula subtcr odores? 

 " Qiiidve mali fit ut eslialcnt aurata metalla? 

 •« QucOs hominum reddunt facies, qualeisqne colores? 

 *' Nonne vides, audisve perire in trmpore parvo 

 " Quam solcant." 



Lucretiusy lib. fi. 810. 



(To Dr, Thomson.) 



SIR, 



1 UEAD the account of the dreadful accident at the Felling Col- 

 liery, in the firat volume of your AnnnlSf with peculiar interest, 



