320 H. MORLAND ON MOUNTING DIATOMACEJ5. 



as it will not do to press down more than necessary to cause 

 adhesion all round for fear of crushing the diatoms between the 

 slip and cover. It is as well not to make the iron block too hot 

 for fear of over-softening the cement ring. With the addition 

 of a label the slide is now, for all practical purposes, complete, 

 but, if it be desired to put any ornamental ring round the cover- 

 glass, no cement must be used the solvent of which will in any 

 way soften or dissolve the original ring under the edges of the 

 cover-glass, as in such case " dewing " w r ould infallibly 

 eventually appear on the underside of cover-glass, as such 

 solvent would gradually w r ork its way inwards through the 

 hard cement and reappear as " dewing," w T hich even in small 

 quantity is a decided blemish in a " dry " mount. 



In mounting " spread " diatoms in balsam or styrax (both 

 of which media I prefer being dissolved or thinned with 

 benzole to any other solvent) I prepare my cover-glasses as 

 for " dry ' mounts, and, placing a sufficient quantity of my 

 medium on the centre of a slip, I invert my slip over the cover- 

 glass, picking it up and immediately turning the slip over again 

 when the medium gradually spreads to the edges of the cover- 

 glass, if not, it is either because the balsam or styrax is too 

 thick or in insufficient quantity. If the former, the minutest 

 drop of benzole at the edge of the cover will rectify matters, 

 but if the latter, more balsam or styrax must be added. A 

 little judgment and practice is necessary to determine how 

 much of the medium is required. If the slide be now placed 

 under the microscope, in all probability a number of minute 

 bubbles will be observed under the cover. These will gradually 

 disappear, and it is as well to let the balsam slides remain as 

 they are for a few hours. The balsam or st}-rax can be 

 hardened off by placing the slides in a cool oven or other warm 

 place for several days, care being taken that the heat is never 

 great enough to cause the reappearance of bubbles under the 

 cover. The difference in mounting with styrax or with balsam 

 is that I use no heat whatever when using the latter, nor do I 

 do so until after all bubbles have spontaneously disappeared ; 

 but with respect to styrax, I do my mounting entirely on the 

 hot block, first driving away all the benzole from the styrax 

 and then, after the cover-glass has been placed in position, 

 placing the slide on the hot block to get rid of any bubbles 



