ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1103 



These advantages, if indeed they be such, in all except dry mounts, 

 are more than counterbalanced by a radical defect, which all such 

 hastily prepared mounts must have, whether the cement be zinc 

 white or Brunswick black, or the mounting medium be glycerin or 

 balsam. And in this defect lies the secret of most of the failures and 

 disappointments which produce the bitter complaints against this or 

 that cement or mounting medium in the technical journals. 



All the cements described in the preceding chapters, with the 

 exception of gold size, consist of some solid material or materials dis- 

 solved or held in suspension in a medium more or less volatile, the 

 evaporation of which again leaves a solid mass. The exception, gold 

 size, hardens partly, though very slightly, by evaporation, its solidi- 

 fication depending princij^ally upon oxidation. In the process of 

 hardening or setting, the bulk or mass of the cement is very materially 

 altered, a decrease in volume occurring which is i)roportionate to the 

 amount of volatile matter lost in drying. The cement shrinks. 



Now, when a cell is properly finished it must be entirely filled 

 with the mounting medium. If it is not so filled we are bound to 

 have air-bubbles, the bete noir of microscopists, which are not only 

 unsightly, but will, in process of time, ruin the mount. If the cell 

 walls were not entirely dry when the cell was closed it is plain that 

 the process of shrinkage had not yet been completed, and that it is yet 

 to occur to a greater or less extent. What is the inevitable result ? 

 The fluid within the cell is practically incompressible, yet jiressure 

 is brought ujjon it. It has no space within its container into which it 

 can retreat, and consequently it must force its way out of it. This it 

 does slowly and gradually. It may be some time before it is noticed, 

 but it is bound to come. The cement gives way at its weakest point, 

 and the fluid exudes — ' creei)S ' out. It is discovered, washed off, 

 and a fresh ring of cement applied. This puts off" the evil day a while, 

 but in a few months the process has to be rci^eated. Meanwhile the 

 pressure is continuously exerted, and minute quantities of the mount- 

 ing medium gradually infiltrate the walls at fresh points ; the cement 

 disintegrates, scales and splits off." 



It should theref<jre be an axiom " never to use a cell until the 

 cement walls are thoroughly dry and hard." 



Coloured Crayons for Marking Preparations— Finder. * — Prof. 

 E. Strasburgcr rec(jnimeiids Faber's coloured crayons for writing on 

 glass or porcelain for marking preparations provisionally. The 

 yellow crayons arc most suitable for this object. 



In order to find given places in a specimen, circles sliould bo 

 made with some sharp instrument on both sides of the aperture in the 

 stage of the Microscope, similar circles being drawn with the crayon 

 in corresponding positions on the slide. 



Filtering Minute Quantities.f — The ordinary method of filtering 

 by moans of paper funnels is not practicable for quantities less than 



* rv.t. Central})!., xxiv. (1885) pp. l.'iO-?. 



t IlfiuMlKjfcr, K., ' Mikro.ikopi.idio Roactionen,' vii. ami 102 pp., 137 figs., 8vo, 

 liiauniiclnvtig, 188.'). 



