Crystals for the Micro-Polariscope. 443 



given can be crystallized either upon the slides or upon the 

 circular cover-glasses by immersion in the fluid. This plan 

 has peculiar advantages in some cases. In mounting crystals, 

 for example, as objects for the oxy-hydrogen microscope, I 

 know of no other method by which even surfaces, of sufficiently 

 large size for use with low powers, can be obtained. 



I must not omit to mention, that very beautiful flower-like 

 forms may often be obtained by making starting points for the 

 crystallization. This is done by touching the slide with a 

 needle just as crystals begin to appear, at the points from 

 which it is desired that they should proceed. With solution of 

 the double sulphate of copper and magnesia, solution of bichro- 

 mate of potash with gelatine, and the chloroform solution of 

 santonin, exceedingly pretty fern-like crystals may be thus 

 produced. 



Having described the best methods of obtaining polari- 

 scope crystals, it now only remains for me to say a few words 

 on the process of mounting. Crystals which are not altered 

 in the air — which are not either deliquescent, efflorescent, or 

 volatile — are best mounted dry, as the only fluid which is appli- 

 cable to the great majority of crystals, namely, castor-oil, is not 

 very manageable and has a great tendency to leak from even 

 the most carefully-made cells. For dry mounting, I have 

 found nothing nearly so convenient and so generally applicable 

 as small india-rubber rings. These may be readily obtained of 

 any required size and thickness, but it is best to fix upon one 

 diameter, and keep to the use of that size alone. The cover 

 glasses must be of such diameter that they will, when placed 

 on the rings, reach not quite to their external edges. The 

 best cements are black japan and gilder's gold-size. The 

 former has the advantage, that being quite opaque, it conceals 

 altogether the india-rubber ring. 



We will suppose now that we are about to finish off the 

 slide of chlorate of potash, of which I have described the pre- 

 paration. The first step is to place it upon a Shadbolt turning 

 table, and, with a stiff, mounted needle, describe upon the 

 crystalline surface a circle very little less in diameter than the 

 inside of the caoutchouc ring. Next, with a small piece of wet 

 sponge, clear away all the salt which lies outside the circle, 

 and then carefully dry the slide. Now, replacing it upon the 

 turn-table, make a ring of gold- size all round the crystalline 

 disc, and lay the slide aside until this has become — not dry, 

 but " tacky ." Then give one of the surfaces of an india-rub- 

 ber ring a thin coating of gold-size, and having laid it evenly 

 upon the varnish circle on the slide, press it with another slide 

 until perfect contact is obtained at all points. The cover glass 

 must now be applied, being cemented on with gold-size, and 



