26 



Devonshire and Cornwall. It may be fused by the blow- 

 pipe into a transparent glass *. Its refraction is single. 

 The powder projected on a hot poker gives a phosphorescent 

 light, of a bright and glowing purplish or lilac colour. The 

 Rev. Mr. J. Dal ton favoured me with some from Cumber- 

 land, greenish within, and of a dull pale crimson on the 

 outside, which gives this glow in great perfection, in rather 

 large pieces, without cracking or dispersing so soon as usual $ 

 and if not too much heated, the same pieces will do again. 

 In this it agrees with the chlorophane of Siberia, which 

 much resembles it in external appearance, but gives a ver- 

 diter green glow on exposure to heat without falling to 

 pieces. 



The fluoric acid was discovered by Scheele. It may be 

 disengaged from the lime by means of dilute sulphuric 

 acid, and has been used for etching on glass. One of the 

 methods may be acceptable to my general readers. Having 

 a plate or piece of glass thinly covered with wax, etch, or 

 draw, by cutting through the wax with a point or needla 

 whatever may be desired, placing the glass horizontally, so 

 as to retain the fluid, (it may be best perhaps to surround 

 the plate with a wall of wax, for the greater security) ; then 

 having some fluor pounded to a fine dust, sift or spread it 

 over the whole within the waxen wall. Mix one part of 

 sulphuric acid to two or three of water, and pour it on 

 gently. The strength of the strokes will depend on the 

 quantity of dust of fluor, and the strength of the acid 



* It is apt to crack and disperse ; which may be prevented by powder- 

 ins it. 



