398 



Mr. C. Tomlinson on some Effects of small 



oxide. Badly coloured carnelians are converted by the action 

 of heat into a nniform red. 



The influence of small quantities of foreign matter, in 

 modifying structure, may also be noticed in some cases of 

 saline compounds. A striking experiment may be shown 

 on a large scale with a supersaturated solution of potash 

 alum. If a nucleus be added, octohedral crystals slowly 

 descend, until the whole of the solution becomes solid. But 

 if to the boiling solution a pinch of carbonate of potash 

 be added, there is a brisk momentary effervescence on the 

 surface, and when the solution is cold it will deposit, not 

 octohedral, but cubic crystals of alum. So also sal-ammoniac 

 crystals are modified by the presence of small doses of urea or 

 of boracic acid. Common salt is another case, when under 

 the influence of a small quantity of corrosive sublimate. 

 Pasteur's discovery of two unsymmetrical varieties of tartaric 

 acid* are also well known, but his curious experiments on 

 bimalate of ammonia will bear repetition t- This salt crys- 

 talizes at ordinary temperatures in two forms : first, in that 

 of a right rhombic prism in which the edges are bevelled ; 

 secondly, one in which the form is the same, but the top and 

 bottom edges have a double bevel, as at R L, fig. 2. The 



bevels are also striated, but the striae are not found on the 

 side truncations M, M. If one or more of the solid angles be 

 rubbed or filed down, as at a, b, c, fig. 3, and placed in a pure 

 solution of the salt, the damage becomes restored in a few 

 hours. But if the solution be contaminated with some of the 

 products of decomposition of the salt, occasioned by the heat, 

 not only are the angles restored, but the upper and lower 

 edges become bevelled, as in fig. 2. The presence of these 

 secondary faces seems to be due to impurities in the solution, 

 and to exert some action on the growth of the crystal in its 



* Ann. de. Chem. et de Phys. s<5r. 3, xxiv. et xxvii. 

 t Ibid. xlix. 



