MODES OF ORIGIN COMPARED 45 



crystallises from water or hydrochloric acid in regular octahedrons, 

 but from alkaline solutions in trimetric prisms." Taking some 

 other specific instances, we find that — " If a solution of carbonate 

 of calcium in water containing carbonic acid be left to evaporate 

 at the ordinary temperature, nothing is obtained but calcspar, in 

 microscopical, and, for the most part, truncated primitive rhombo- 

 hedrons ; if, on the contrary, the solution be evaporated over the 

 water-bath, arragonite is obtained in small six-sided prisms — mixed 

 with a few crystals of calc-spar, because the temperature of the 

 solution is lower at first than it afterwards becomes." 



In other examples we may find not only a striking difference in 

 physical form, but also a notable change in colour, occasioned by 

 the molecular rearrangements which the change of temperature 

 seems to necessitate. Thus I have before me now a sheet of paper 

 having a large circular area smeared over with a scarlet patch 

 composed of the double iodides of mercury and copper ; if I hold 

 it over a lamp, almost immediately the scarlet patch assumes a 

 brownish-black colour ; if the exposure has been only momentary, 

 the patch almost immediately, on withdrawal, again assumes the 

 scarlet colour ; if I allow it to remain exposed to the heat for five 

 seconds it takes about fifteen seconds for the brownish-black 

 colour to disappear and for the scarlet colour to be restored. 

 These changes will recur over and over again invariably, and just 

 as they did more than thirty years ago when the specimen first 

 came into my possession. 



It has also been found that the whole nature of a crystal already 

 in existence may be changed by the action of causes which seem 

 the most trivial : a slight elevation of temperature, or even the most 

 deUcate touch, in some cases, is capable of initiating changes which 

 spread through their entire substance, or throughout a whole 

 aggregate of cohering crystals. Thus in the same article on 

 ' Dimorphism ' to which we have already referred, we find the 

 following statement : — " Crystals formed at one particular tempera- 

 ture, and then exposed to that temperature at which crystals of a 

 different kind are produced, often lose their transparency, and 

 without alteration of external form, become changed into an aggregate 

 of small crystals of the latter kind: examples of this alteration of 

 structure are afforded by sulphur, carbonate of calcium, mercuric 

 iodide, and many other bodies." Again : — "Mercuric iodide separates 

 from solution, and Ukewise sublimes at a very gentle heat, in 

 scarlet tables belonging to the dimetric system ; but when subUmed 



