162 



THE AMERICAN MONTHLY 



[September, 



ever, to use a wax ring. These rings, 

 punched out of sheet wax, of such 

 size as to cover the edge of the thin 

 glass, are put on the mount when it 

 is finished, and, by cautious applica- 

 tion of a small flame, just melted, but 

 not so as to run. If any bubbles form 

 under the ring they may be removed 

 by touching with a hot needle or pin- 

 point before the wax cools. A mount 

 made this way will stand indefinitely, 

 and can, at any time, receive a sup- 

 plemental colored ring of shellac or 

 other varnish for a finish. Amphi- 

 pleura pelhicida is very beautifully 

 shown in this medium, and the vari- 

 ous pleurosigmas — indeed, all dia- 

 toms except the very coarse ones, 

 which appear almost black in the 

 medium. A very little experimenting 

 will enable one to use the medium 

 successfully. 



The vise of the gelatine is only to 

 give such a hold upon the cover as 

 will permit the necessary pressure in 

 cleaning. Many mounts have been 

 made, both by myself and others, ii\ 

 the earlier experiments with this me- 

 dium, without the gelatine, but in all 

 these cases the cover was less firmly 

 attached to the slide. If the protect- 

 ing ring keeps out moisture fro in im- 

 mersion media, or the atmosphere, the 

 mounts will remain unchanged. As 

 the medium dissolves gelatine, albu- 

 men, etc., arranged diatoms must be 

 fastened to the cover by heating the 

 latter, supported on a bit of thin 

 sheet-iron or platinum, nearly to 

 melting or softening point. A larger 

 proportion of the stannous chloride 

 can be dissolved than that mentioned 

 above, even as much as sixty 

 grammes, but then, on heating to 

 harden the mass, crystals will ap- 

 pear ; the crystals never give any 

 trouble when forty grammes are 

 used. 



The second medium is realgar, 

 the transparent sulphide of arsenic, 

 dissolved in bromide of arsenic by 

 aid of heat. Both of these sub- 

 stances should be pure, and the 

 mount should be boiled as long as 



mfffiwfT 



Fig. 25.— Method of 

 mounting with Realgar. 



bubbles are readily given oflf, with 

 considerable heat, and when cold 

 the cover should be more firmly 

 attached than with balsam. These 

 inounts are of a deep lemon-yellow 

 color, and the compound has a re- 

 fractive in^ex of 2.4. Excellent, 

 and even better, mounts, as to per- 

 manence, may be made by using 

 realgar only by sublimation. A bit 

 of the realgar is put on a plate of 

 mica about one inch square, and 

 thick as a penny. This is melted 

 by strong heat of a' spirit-lamp. 

 On this mica plate is placed an- 

 other, with a hole ^ of an inch 

 in diameter, and above this a thin 

 glass plate with a hole slightly less 

 than the glass cover on which the 

 diatoms are ^ 



mounted. 

 In Fig. 25, 

 a and b are 

 the two mi- 

 ca plates, c the glass plate, and d the 

 cover, with the diatoms facing the real- 

 gar. The whole is now supported on a 

 metal ring. A strong heat will volati- 

 lize the i-ealgar without change, and 

 often a clear deposit is made all over 

 the diatoms and underside of the 

 cover, and the latter can now be 

 mounted in balsam ; but, if bubbles 

 are formed in the operation, as prob- 

 ably will be the case, the heat must 

 be continued till these disappear, 

 and, as the deposit will now be 

 thickest at the centre just over the 

 realgar, the mount may be finished 

 by putting the cover, ' realgar side 

 down, on a clean slide, and on top 

 of it, to prevent breaking, a piece 

 of thick glass, and then, grasping 

 tightly with forceps to give pressure, 

 heating strongly over a spirit-lamp. 

 The realgar will soften (it must not 

 be melted, else bubbles will form 

 which cannot be removed) and 

 spread out, more or less, between 

 the cover and slide, making a nice 

 clear mount. The color of the 

 heated realgar is very much deeper 

 than when cold. Instead of the 

 solid realgar a drop of the solution 



