82 HOOKHAM ON MICROSCOPIC CRYSTALS. 



metlaod of crystallisation, a method whicli the writer ventures to 

 believe represents a very important brancli of the study of crystal- 

 lography — perhaps the most important. The exception, as we all 

 know, proves the rule— the scientific (not the popular) interpreta- 

 tion, which is that an examination of an apparently exceptional 

 phenomenon points to the general law for the whole class. Here 

 we force matter to crystalHse under exceptional and artificial condi- 

 tions. The resulting forms assumed, however, are not irregular, 

 but most regular. Surely they should have exceptional signifi- 

 cance ; rightly interpreted, they must point to a true theory of 

 crystallisation. 



At last a successful experiment indicated the right method. This 

 good fortune happened to the writer at Mentone, in the south of 

 France, at a time when the hygrometer indicated extraordinary 

 dryness even in that dry atmosphere : and it is not at all impossible 

 that his success was in the first place owing to this circumstance. 

 Feeling now, no doubt that he was on the right track, he first applied 

 his new method to the crystal which Mr. Thomas had invested with 

 an especial interest. In an experiment with sulphate of copper, a 

 crystal was obtained at so low a temperature as 80° in which the 

 spiral was mathematically perfect, and the lines not merely pure 

 lines, but so finely ruled as to give to the naked eye the most 

 briUiant effects of the interference of light the writer ever remem- 

 bered to have seen, by whatever means produced. In some cases 

 he has measured 6000 lines to the inch : and' they often extend over 

 an area of l^in. by fin. So that when held about half way 

 between the eye and a candle about 5ft. distance in an otherwise 

 dark room, the effects of interference are almost dazzling ; while 

 the lines themselves are so perfectly regular that it has occurred to 

 him that in its larger sweeps, where the lines becomes nearly 

 straight, or at least the curve is inappreciable, this crystal might 

 even be used as a micrometer. 



The spirals, as Mr. Thomas observed, have sometimes a right, 

 sometimes a left-handed twist. The result of this is often some- 

 what unfortunate, for, as a rule, both twists occur in the same 

 spiral. Radii proceed from the centre, marking the divisions 

 between the two sets of curved lines, and thus cutting up the spiral 

 into sections. There are almost always, however, on some parts of 

 the crystal sweeps of lines sufficiently unbroken to give the spiral 

 form in a very marked manner. Perfect spirals cannot at present 



