the Manufacture of Artificial Tourmalines. 355 



the plate carrying the crystal (this being downwards) upon the 

 oil, elaine, &c., press together slightly, and then if too much 

 medium has not been used, the plates are just kept apart by the 

 interposed fluid. The edges are then surrounded or painted 

 with a solution of gum-arabic, slightly toughened by the addition 

 of a little isinglass or gelatine. This is a very rapid mode of 

 mounting, but not cei'tain or safe ; sometimes the edging of gum 

 cracks, air is admitted, the definition of the plate is lost, and its 

 brilliancy for the polariscope much damaged. Oil and elaine 

 appear to dissolve and disintegrate the crystals. 



I therefore infinitely prefer the retherial and iodized Canada 

 balsam. 



Crystals thus mounted keep without change or disintegration j 

 some of the author's plates have been so prepared for months 

 and are still perfect, whilst others mounted very recently in 

 iodized oil, or elaine, or glycerine, have shown symptoms of de- 

 struction in as many weeks or days even. I have some which 

 are quite as good as ever after three years' wear, these were 

 mounted in fluid Canada balsam. When disintegration occurs, 

 it appears to be greatly influenced by imperfection in the wash- 

 ing process, for those plates which have been mounted with ciy- 

 stals of sulphate of quinine beneath them certainly break up and 

 dissect themselves with great rapidity ; it appears to be produced 

 by some unknown molecular action going on, and a rearrange- 

 ment of the atomic constituents. The plates appear to become 

 henpecked by minute parallelogrammatic holes ; these begin at 

 the edges and go onwards through the disc, rendering the plate 

 useless. 



The aetherialized Canada balsam has no action on the plates ; 

 however, it is perfect as a mounting fluid ; it dries very readily 

 too, a few days' exposure to air of 60° or 70^ being alone suffi- 

 cient for this pui*pose. 



As turpentine has no solvent power over Herapathite, even at a 

 boiling temperature, fluid Canada balssm may be employed; 

 - I therefore was in error in ascribing the disintegration to that 

 medium. I have since found that the crystals must be washed to 

 render them secure for mounting in it ; the only objection to 

 this medium is, that it dries so slowly. 



The object of using the iodized compound is to give a black 

 tone to the field; this it certainly accomplishes; the periodized 

 crystals, when examined by daylight, are black as midnight, 

 even if the plates are no thicker than gold-leaf, but an intense 

 lamp or gaslight appears to pass through them with a violet 

 hue, perhaps more inclining to brown-red ; this is an unpolarized 

 ray, and it cannot be obstructed by tourmaline or Nichol's prism ; 

 it requires four thicknesses of Herapathite (all parallel) to stop 



2B2 



