180 M. Gaudin on the Production of Crystals 



common chloride of calcium, apparently free from barytes and 

 strontia, has afforded, on the same glass-rod, (which I here- 

 with transmit), on the one side, the form almost exclusively of 

 carbonate of barytes, and on the other, the form of Iceland 

 spar, a result which I did not at all expect, since the glass-rod 

 in question was marked with raicrometric divisions, on which 

 I expected to see rhomboids which would be capable of doub- 

 ling the traces made by the diamond. 



The carbonate of barytes supplies crystals which are alto- 

 gether peculiar ; they assume various fantastic forms and ar- 

 rangements, and look more like a- plant than a mineral salt. 



I have for some time been engaged in preparing symmetri- 

 cal crystals, that is to say, such as have no attachment or ad- 

 herence, and formed, consequently, in a liquid or gaseous me- 

 dium. With this object I have carefully studied the pheno- 

 mena presented by the snow during the last winter. Since 

 that time I have obtained crystals of sulphate of tin, similar to 

 those we find presented by snow, and which were formed in a 

 whirlwind of the vapour of sulphur. I have also recognised 

 in the white marble of the Pyrennees microscopic crystals of 

 silica. These two kinds of crystals, of which I send specimens, 

 are, as will be seen, beautifully perfect, and those of the silica 

 will, I trust, one day help us to elucidate the origin of primi- 

 tive limestones. I also transmit crystals of carbonate of lime 

 and carbonate of barytes, which are likewise symmetrical : these 

 I obtained by pouring into a boiling solution of carbonate of 

 ammonia a solution of a salt of lime, or of barytes, and then 

 agitating the mixture. 



I now produce all these crystallizations by pouring into a 

 tube the length' of the finger a saline solution, and placing in 

 the upper part of it, before corking, a small portion of cotton 

 moistened with the substance intended to form the artificial at- 

 mosphere. Finally, to render these and all other microscopic 

 observations less fatiguing, I have thought of adapting a lens 

 of a short focus to a stopper pierced with a hole, which receives 

 the tube, and in which it is made to glide at pleasure. Diame- 

 trically opposed to the lens is a hollow conical mirror, which, 

 by its upper portion, reflects the light upon that part of the tube 

 we desire to examine. In this way we may, with the elbows on 



