PRESERVATIVE MOUNTING MEDIA 521 



glass. The alcohol and water soon evaporate, so that the glycerin 

 alone is left ; and another drop of the liquid is then to be added, 

 and a second evaporation permitted, the process being repeated, if 

 necessary, until enough glycerin is left to fill the cell, which is 

 then to be covered and closed in the usual mode. 1 



Canada balsam is one of the most universally employed mounting 

 media ; very old hard balsam should be dissolved in enough pure 

 xylol or chloroform to make a thin solution, which should be care- 

 fully filtered. 



Dammar. Dissolve gum -dammar with heat in a mixture of 

 equal parts of benzole and turpentine, and evaporate to a syrupy 

 consistency. This is pleasant to use, but treacherous. Dammar 

 dissolved in pure xylol in the cold gives a beautiful solution, but 

 on the score of permanency is not so trustworthy as balsam. 



Gum Styrax. This is a resin which must be dissolved in benzole, 

 chloroform, or ether. It should have the consistency of olive oil ; 

 all the benzole must be evaporated before putting the cover on the 

 slip; its refractive index is said to be then 1'583. Its value is in 

 the mounting of diatoms, where a marked difference between the 

 refractive index of the siliceous frustules and the medium in which 

 they are mounted facilitates the discovery of obscure details. There is 

 a marked increase of visibility in proportion as the mounting medium 

 has a refractive index higher than the object (diatom) mounted. 



Now the refractive index of the silex of diatoms is 1'43. But 

 Canada balsam is 1'52 : hence the ' index of visibility' in obscure 

 markings is 9, while styrax by comparison is 15. 



Monobromide of naphihalin is another of the media which 

 may be used with a high refractive index. It is colourless and oil- 

 like, soluble in alcohol and ether. It has a refractive index of 

 1 '658, and therefore a splendid index of visibility above balsam or 

 styrax ; but after a lapse of many months some change takes place 

 which leaves the preparation as apparently perfect as before, but 

 having lost all the benefit of great refractive index. 



The cover-glass should be run round with a ring of wax, then 

 with a ring of Heller's porcelain cement, and be finally closed with 

 shellac. 



But with the exception of some media of very high refractive 

 index not by any means easy to use, devised by Professor H. L. 

 Smith, there is no medium of such high value as that suggested and 

 very successfully employed by Mr. J. W. Stephenson, viz. 



Phosphorus. Its refractive index is 2*1, and its consequent 

 increase of visibility is of immense value in some objects. 



Phosphorus, it need hardly be said, is difficult and somewhat 

 dangerous to handle on account of its spontaneous combustion in 

 air, and the severe nature of the burns it inflicts. But it is with 

 slight practice by no means an unmanageable medium. 



To prepare it, take a 2 -drachm bottle with no contraction for the 



1 See the Rev. W. W. Spicer's Handy-book to the Collection and Preparation 

 of Freshwater and Marine Algce, &c.. pp. 57-59. ' Nothing,' says Mr. Spicer, ' can 

 exceed the beauty of the preparations of .Desw^'ace^ prepared after Herr Hantsch's 

 method, the form of the plant and the colouring of the endochrome having under- 

 gone no change whatever." 



