H. M0RLAND ON MOUNTING MEDIA FOR DIATOMS. HI 



fine sediment in the bottle some considerable time after filtra- 

 tion, which I consider to be due to the presence of moisture 

 contracted during the process of obtaining these resins, the pre- 

 pared media being what I believe is called in the varnish trade 

 " chilled." If only in small quantity the heat applied during 

 the process of mounting will drive it off again, but it is better 

 either to pour off the upper clear portion into another bottle or 

 to use a " capped balsam bottle ',' with a suspended glass rod for 

 mounting, and which is never allowed to touch the bottom and 

 stir up the sediment. Whilst on the subject of this sediment I 

 may here state that I once had a small quantity of the ordinary 

 styrax in a pot, which I placed in a cool oven and left there for 

 some considerable time until it became perfectly hard. I then 

 dissolved it with difficulty in alcohol, filtered it, evaporated 

 nearly the whole of the alcohol off again, and thinned it down 

 to a proper consistency with chloroform. Although very dark 

 this styrax is as clear now as it was when first prepared, now 

 over two years ago. 



The ordinary styrax is somewhat difficult to harden. The 

 best way of doing so is by a long continued heat, a considerable 

 amount of which it will bear, but usually this styrax is not 

 finished off hard, but merely tough, and I therefore always 

 strengthen my mounts by the addition of a ring of cement to 

 the covers. If the styrax be over-heated it becomes charred, 

 and small specks of carbon can be seen disseminated through it 

 when viewed under the microscope. Styrax is a very useful 

 medium for the finer kinds of diatoms, which it shows up much 

 clearer and brighter than balsam. Some forms, almost invisible 

 in balsam, are shown up with great distinctness in styrax. For 

 the coarser kinds of diatoms this medium is not really needed. 



American styrax, unlike the other, hardens just like balsam, 

 but when heated it becomes extremely fluid. Having a high 

 refractive index it is particularly useful for the finer diatoms. 

 On account of its fluidity when heated I should be inclined to 

 confine the use of this medium to the smaller diatoms, lest the 

 cover should, during the process of mounting, sink down on the 

 larger forms and crush them. 



Balsam of Tolu. — I have never had anything to do with this 

 medium. I did once think of taking it in hand, but before 

 doing so I referred to Watts' " Dictionary of Chemistry " 



