268 TECHNOLOGY OF DIATOMS. 



a mounted needle. Under the influence of the heat the upper 

 part of the cell softens, adheres to the cover-glass, and fixes it 

 there in a manner absolutely hermetic. The plate, when cooled, 

 is put on the turn-table, with a coat of cement to protect the 

 edges of the cover-glass, and the preparation may be considered 

 as permanent, unless after a certain time the cover-glass should 

 crack and allow damp to enter. 



Mounting in Resinous or Mixed Mediums. 



This mode of mounting is that usually adopted, and, besides, 

 it ensures absolute permanence if the media employed have been 

 well prepared, and also the details of the more delicate of the 

 siliceous carapaces of the Diatoms are most clearly shown, espe- 

 cially if the medium employed is one which possesses a high 

 refractive index. 



The Diatoms are spread on the cover-glasses in the way I have 

 previously indicated. If the gathering or the deposit contains the 

 larger species, it will be well to separate these from the smaller 

 and to make separate preparations ; that is to say, the result of the 

 washing of this gathering should be divided into a heavier and a 

 lighter part. This division, as I have explained in the chapter on 

 " Cleaning," can be effected by repeated decantations, or, better 

 still and in a more regular manner, by the employment of tamis- 

 filters. 



If the materials are the result of washings made with acids, 

 and have been freed from all organic matter, the burning on the 

 platinum plate becomes unnecessary. Only those special kinds 

 which I have already mentioned require this operation. These 

 materials, and also the lighter deposits, can be mounted directly 

 in the balsam without it being necessary to fix the valves on the 

 cover-glasses. It is not the same with the heavier kinds, for then 

 a cover-glass carefully spread with Diatoms of large size becomes 

 unsightly (affreuse), when heat is applied for hardening the resin, 

 which developes bubbles of vapour that crowd the valves one on 

 the other, causing;; them to unite in groups, and often driving them 

 to the edge of the cover. 



This fixation is obtained by placing the Diatoms that it is 

 desired to spread and fix in water containing a very small quantity 



