Sealing 



FLUID WHOLEMOUNTS IN NONAQUEOUS MEDIA 



33 



somewhat thicker than is customary 

 with other cements. A number of slides 

 may be prepared at the same time and 

 left in a light place for an hour or two 

 until the gelatin has become insolubilized. 

 The object, together with a drop of glyc- 

 erol, is placed in the middle of the cell and 

 the coverslip lowered vertically as shown 

 in Fig. 24 (Chapter 6). The covershp is 

 then held firmly in place, either with the 

 finger or with one of the chps shown in 

 Fig. 25, while all traces of exuded glycerol 

 are removed with the aid of a rag moist- 

 ened in alcohol. The shde is then placed 

 on the turntable and a ring of molten 

 dichromate gelatin turned over the edges 

 of the covershp. This ring of cement is 

 cooled — it is not necessary to dry it — 

 and the whole slide then thoroughly 

 cleaned in 95% alcohol, either apphed 

 from a rag, or by waving the slide back- 

 ward and forward in the fingerbowl of the 

 reagent. Great care is necessary at this 

 stage to avoid displacing the coverslip. 

 The purpose of the ring of gelatin, in 

 fact, is not so much to cement the cover- 

 slip in place as to provide a temporary 

 seal which will hold the cover sufficiently 

 long to permit the removal of exuded 

 glycerol. As soon as the slide is dry, and 

 glycerol-free, several coats of gold size 

 are added, allowing ample time for each 

 to dry, and then a final coat of asphalt 

 varnish is turned on top. Slides prepared 

 by this method have a very pleasing 

 appearance but they require a great 

 expenditure of time compared to the use 

 of a thermoplastic resin cement. 



Sealing Glycerol Mounts with Thermo- 

 plastic Resin Mixtures 



The medium most usually recom- 

 mended for heat-sealing glycerol mounts 

 is Noyer (Chapter 28, V 12.2 Noyer 

 1918), a simple mixture of rosin and 

 lanolin. The writer prefers the formula of 

 Fant (V 12.2 Fant 1932), containing a 

 quantity of dried Canada balsam, which 

 appears to make it both easier to handle 

 and more adhesive. Whichever medium is 

 employed, the object in glycerol is 

 placed under the coverslip and, after 

 crudeh' wiping away the excess fluid, a 

 layer of molten cement is applied to the 



edge. For making large quantities of these 

 preparations a most ingenious mechanism 

 has been described by Banard (113G0, 

 54:29), but it is proposed here only to 

 deal with the method of handling indi- 

 vidual slides. 



This method is shown in Fig. 20 where 

 the objects are being mounted under a 

 square coverslip. It is the author's 

 opinion that no satisfactory seal can be 

 made by this method on round coverslips. 

 The dish in the left foreground contains 

 the objects in pure glycerol and, immedi- 

 ately behind it in the center of the picture, 

 there is a tin can containing the cement 

 selected. The author always prepares the 

 cement in such quantities as will just 

 fill an empty boot-polish can, which is 

 admirably adapted to the purpose. The 

 tool being used is the same rather heavy 

 brass tool which is shown being employed 

 in the mounting of paraffin blocks in 

 Fig. 65 (Chapter 12). An ordinary section 

 lifter, sometimes recommended, is too 

 thin and does not hold enough cement. 

 In the illustration, it is presumed that 

 the object has been placed under the 

 coverslip, the coverslip lowered in place, 

 the glycerol roughly wiped away, and 

 the metal tool heated to about 150° to 

 200°C. This tool is now dipped into the 

 can of cement, so that the edge accumu- 

 lates molten cement along it, and then 

 touched down on the edge of the cover- 

 shp. It will be noticed that the edge to- 

 ward the front of the illustrations has 

 already been finished and that the second 

 edge is being apphed. Before this was 

 done, a minute drop of the cement was 

 placed at one corner of the covershp to 

 hold it in position. Having finished two 

 sides in this manner, it is easy to apply 

 cement to the third side, but the whole 

 trick of a successful mount lies in the 

 method in which cement is applied to the 

 fourth side. It will be obvious that this 

 very hot cement, when it is applied to 

 the coverslip, will cause an instantaneous 

 expansion of the fluid. This does not 

 matter as long as one side remains open. 

 The last side, however, cannot be sealed 

 in one piece, and it is necessary to apply 

 the cement in such a manner that about 

 a one-milhmeter gap is left at a corner 



