50 Transactions of the 



ll. — On the Employment of Colloid Silica in the Preparation of 

 Crystals for the Polariscope. 



By Henry S. Slack, F.G.S., Sec. E.M.S. 



(Read hefore the Royal Microscopical Society, Jan. 11, 1871.) 



I LATELY called the attention of tlie Society to the nature of the 

 cracked films obtained by slowly, or quickly, evajwrating a dialyzed 

 solution of colloid silica in distilled water. As these films solidify 

 they crack in various directions, and make complicated patterns of 

 straight and curved lines, sometimes appearing hke the plan of a 

 city, and at others assuming curious columnar forms with trans- 

 verse striations. It is also common to find slight changes in the 

 level of difierent parts of the films, sufficient, as explained in a pre- 

 vious paper, to exhibit Xewton's rings in a very beautiful manner. 



It seemed probable that by dissolving crystallizable bodies 

 suitable for exhibition with the polariscope, in a solution of silica 

 instead of in plain water, some interesting modifications of crystalline 

 patterns might be obtained. Using sulphate of copper in this way 

 afforded the following results. First, what may be termed a " pave- 

 ment pattern," in which the sihca cracks divide the film into a 

 great number of small compartments of three, four, five, or more 

 sides, which may be roughly likened to the appearance of vegetable 

 ivory, or a section of pinna shell. In each of these divisions crys- 

 tallization takes place, usually radiating from the centre, and suffi- 

 ciently diversified in thickness or position to produce polariscope 

 patterns that remind me of a richly and variegated pavement, devia- 

 ting from " tesselation," inasmuch as the compartments are not 

 squares. The crystals thus reduced have a tendency to exhibit 

 Maltese crosses of colour, changing with the j)ositions of the polar- 

 izing and analyzing apparatus. 



In these " pavement patterns " the force exerted by the silica 

 films in contracting and cracking, causes the crystalline force to 

 operate in limited compartments without change in the linear 

 direction, common to radiating groups. 



In other cases the contractile force acts tangentially, or in curves, 

 while the crystalline force operates radially, and then we obtain exqui- 

 site spiral and turbinate forms, very similar to those which Mr. Thomas 

 brought before the notice of the Society in 1866, and which pro- 

 bably resulted from an analogous combination of contractile and radial 

 forces operating so as to give curved resultants. If a number of 

 trials are made with different modifications of the temperature and 

 times of drying, all sorts of conflicts l)etween the two forces will be 

 found to occur. One very instructive instance supplied a pat- 

 tern commencing with circles filled with radii, and marked with 



