OPTICAL PEOPEKTIES OF CRYSTALS 1 097 



fig. 816. Mr. Slack has shown that a great variety of spiral and 

 curved forms can be obtained by dissolving metallic salts, or saliciii. 

 santonin, <tc., in water containing 3 or 4 per cent, of colloid 

 silica. The nature of the action that takes place may be under- 

 stood by allowing a drop of the silica solution to dry upon a slide ; 

 the result of which will be the production of a complicated series of 

 cracks, many of them curvilinear. When a group of crystals in for- 

 mation tend to radiate from a centre, the contractions of the silica 

 will often give them a tangential pull. Another action of the 

 silica is to introduce a very slight curling with just enough eleva- 

 tion above the slide to exhibit fragments of Newton's rings, when it 

 is illuminated with Powell and Lealand's modification of Professor 



FIG. 815. Radiating crystallisation of santonin. * 



Smith's dark-ground illuminator for high powers, and viewed with 

 a ^th objective. With crystalline substances these actions add to 

 the variety of colours to be obtained with the polariscope, the 

 best slides exhibiting a series of tertiary tints. 1 Yery interesting 

 results may often be obtained from a mixture of two or more salts, 

 and some of the double salts give forms of peculiar beauty. O. Leh- 

 mann has done excellent work in this department ; but reference 

 must be had to his previously mentioned work on ' Molekularphysik ' 

 for a description of the phenomena such mixtures exhibit. The 

 following list specifies the salts and other substances whose crystalline 

 forms are most interesting. When these are viewed with polarised 

 light some of them exhibit a beautiful variety of colours of their 

 own, whilst others require the interposition of the selenite plate for 



1 ' On the Employment of Colloid Silica in the preparation of Crystals for the 

 Polariscope,' in Monthly Microsc. Journ. v. p. 50. 



