112 H. MORLAND ON MOUNTING MEDIA FOR DIATOMS. 



(belonging to a friend of miner), and as something was men- 

 tioned about crystallization 1 decided to leave it alone. Since 

 then there have been complaints made about this objectionable 

 property. On the other hand some microscopists continue to 

 use it, and seem satisfied with it. 



Gum Dammar has also been used as a mounting medium ; 

 it requires dissolving in some solvent, such as benzole ; its 

 advantages are that it is colourless and dries quickly ; its dis- 

 advantage is that when quite hard it is about as friable as 

 chalk. I consider its proper place is in the preparation of 

 " ringing" cements to which it gives a nice gloss. 



A number of other resins have also been tried, but I don t 



know that any of them have shown any advantages over Canada 



balsam or styrax, and I am unable to report upon any of them. 



Although slides of diatoms mounted in any of the resins will 



stand fair treatment, they will not bear any amount of rough 



handling. It must not be forgotten that diatom valves are 



composed of silex, a brittle substance ; if, therefore, the balsam, 



styrax, or other medium be tough, and the cover glass be 



rubbed or wiped somewhat roughly, the medium will yield to a 



slight extent, but not so the diatom valves, which consequently 



will get cracked, though the fragments will continue to be held 



in position by the medium in whicli they are mounted. Balsam 



slides may be perfectly hard at the edges of the covering glass, 



but only tough in the centre part ; if such slides be left to 



themselves for a few days in a cool place, the density of the 



balsam will become a little more homogeneous in consequence of 



the diffusion of the residual solvent in the centre part throughout 



the mass, though the outer portion will always remain a little 



harder than in the centre, as the solvent gets dried up almost 



as soon as it reaches the circumference. 



Chemicals. — The sole object of using chemicals as mounting 

 media, is to obtain a higher refractive index than is afforded by 

 the resins ; unless such refractive index can be obtained they 

 have no raison ftetre. So far as my experience of them goes, 

 they are either difficult to manipulate or they cannot be relied 

 upon for permanency, with, perhaps, the single exception of 

 monobromide of naphthaline, which, however, I do not consider 

 to be any advance upon styrax, as diatoms mounted in this 

 medium do not exhibit anything like the brilliancy given them 



