128 



THE AMERICAN MONTHLY 



[July, 



the very finest specimens. Do not 

 keep any others, because in making 

 such slides they rapidly accumulate, 

 and soon one has too many. 



It will be found that great variety 

 in the crystals can be obtained by 

 heating the slides, previous to placing 

 the solutions upon them, or by heating 

 them during the process of crystal- 

 lizing. Another way of controlling 

 the crystallization, which often gives 

 rise to the beautiful radiating groups 

 of crystals found in the shops, is to 

 add to the solution some gum arable, 

 or some " soluble glass." As a rule, 

 the more rapid the crystallization the 

 smaller will the crystals be. 



Many crystals may be mounted dry, 

 but if they will bear covering with Can- 

 ada balsam, they are all the better 

 when mounted in that medium, espe- 

 cially for polariscope effects. 



Very beautiful crystals may be ob- 

 tained by the sublimation of white 

 arsenic. We have long contemplated 

 writing an article on this subject, but 

 it must be still deferred. However, 

 the process in outline is simply this : 

 place a small quantity of arsenic in a 

 watch-glass, or on a slide, and gently 

 heat it, holding another slide above it 

 so as to condense the fumes. If the 

 operation is properly conducted, most 

 beautiful and perfect crystals will be 

 found upon the upper slide, when it 

 is examined with a microscope. In 

 this way very minute quantities of 

 arsenic may be detected. 



Microscopic Objecfs at the Fish- 

 eries Exhibition. 



The great International Fisheries 

 Exhibition now open in London at 

 South Kensington, is far more com- 

 prehensive and complete than any 

 previously held. Being devoted ex- 

 clusively to fisheries, there is com- 

 paratively little to be seen of the 

 minute forms of life which especially 

 interest microscopists. Such as we 

 have been able to find will be here 

 briefly described. 



In the United States department, 

 under the charge of Commissioner G. 

 Brown Goode, there is more micro- 

 scopic material than in any other. 

 Taking up first the fresh-water organ- 

 isms, there is a fine collection of 

 slides by Mr. Edward Potts, of Phila- 

 delphia, illustrating the various spe- 

 cies of sponges found in fresh-water 

 in America. In most cases each spe- 

 cies is represented by three slides, 

 one showing the dry sponge as an 

 opaque object, the other a section or 

 a thin portion mounted in Balsam, 

 and finally a slide of the spicules. 

 Many of the genera might be readily 

 identified by the spicules alone. It 

 may be of interest to know what spe- 

 cies and varieties Mr. Potts has dis- 

 tinguished, so we will enumerate them 

 in the order they occur in the collec- 

 tion. The first is the Spongilla pau- 

 percula of Bowerbank, which has 

 smooth, curved spicules tapering to- 

 ward both ends. Sp. aspi?iflsa has sim- 

 ilar spicules. Then comes Sp. lacus- 

 tris, with large spicules still of the 

 same kind, mingled with numerous 

 smaller ones covered with spines or 

 minute projections. It may be put 

 down as a rule that when two kinds 

 of spicules are found on a slide, the 

 smaller ones belong to the gemmules 

 of the sponge. Several varieties of 

 lacustris are represented, viz., media, 

 miitica, Dawsofti, Alontana, and mul- 

 tiforis. In the next species, Sp. fra- 

 gilis, Leidy, the spinous spicules are 

 larger and rather more straight. The 

 varieties of this species are Ottawa- 

 ensis, minnta, minutissivia, irregularis 

 and Lordii. 



The genus Meyenia is represented 

 by some very fine specimens. M. 

 Leidii, of Bowerbank is a beautiful 

 sponge, the dried specimen appearing 

 like a mass of minute, incrusting 

 cones, formed of glassy spicules ar- 

 ranged around a central aperture in 

 the apex. The spicules of the gem- 

 mules are very curious, resembling 

 two thin, parallel plates, slightly con- 

 cave outward, connected by a short 

 thick axis. These are very minute. 



