202 ON SIMILARITY OF FORM IN SNOW CRYSTALS. 



by analogy we might be able to throw some light on the 

 subject. 



The well-known substance, camphor, I find, fulfils all these 

 conditions, and possesses some peculiar properties that make 

 it an interesting subject of study. Some of these peculiarities 

 have been long known ; and I believe that the old-fashioned 

 instrument or philosophical toy, called the weather-glass, was 

 a tube hermetically sealed, and containing a solution of 

 camphor. Camphor, crystallized slowly, does not usually 

 assume the form of hexagonal crystals, but like snow or ice 

 takes the arborescent form, very similar to the fronds of 

 Ferns : this may often be observed in the case of ice in the 

 beautiful forms assumed on the surface of windows in winter. 

 Camphor usually takes the same form under like conditions, 

 but requires a rapid crystallization to produce hexagonal 

 crystals. The most convenient way to repeat these experi- 

 ments on camphor is to make a solution of this substance in 

 alcohol or spirits of wine, and add thereto some water of 

 ammonia — the precise quantities are not of much moment, 

 provided there be an excess of camphor undissolved. I 

 usually add a small quantity of water to the clear solution 

 drop by drop, till the precipitated camphor ceases to be re- 

 dissolved. The simplest form assumed by crystallized 

 camphor is a flattened disc : very frequently two discs are 

 united together by a smaller one in the centre, giving rise as 

 the process of crystallization goes on to twin crystals, super- 

 posed one on the other precisely like snow crystals. These 

 discs frequently possess what appears to be a nucleus, which 

 is coloured more or less; the disc and nucleus differing 

 in colour, the colours being most frequently complementary 

 to one another. I had proposed to myself to mount some of 

 these hexagonal crystals of camphor to exhibit to the Society, 

 but have found a difficulty in obtaining a suitable medium 

 which would not possess a solvent action upon them : they 

 are so readily procured, however, by a rapid evaporation of 

 the solution of camphor under the microscope, that tliis will 

 hardly be an objection. It must not be expected that we 

 should meet with that infinite variety of form observed in 

 snow crystals in the case of hexagonal crystals of camphor, 

 since we can only produce them on a very small scale, com- 

 pared with the millions we can select from in the case of 

 snow. My object, therefore, has been, in introducing the 

 subject to the notice of the members of this Society, to draw 

 their attention to it as an interesting subject for experiment 

 during leisure moments in the course of the winter, when 

 most probably they will have an opportunity of comparing 



