1885.] 



Phenomenon of Crystalline Reflection. 



175 



by some internal reflection, or possibly oblique refraction, at the 

 surfaces of the crystalline plates that the light was polarised and 

 analysed, being modified between polarisation and analysation by 

 passage across the crystalline plate, the normal to which I supposed 

 must be sufficiently near to one of the optic axes to allow colours to 

 be shown, which would require no great proximity, as the plates were 

 very thin. To make out precisely how the colours were produced 

 seemed to promise a very troublesome investigation on account of the 

 thinness and smallness of the crystals : and supposing that the issue 

 of the investigation would be merely to show in what precise way the 

 phenomenon was brought about by the operation, of well-known 

 causes, I did not feel disposed to engage in it, and so the matter 

 dropped. 



But more than a year ago Professor E. J. Mills, F.H.S., was so good 

 as to send me a fine collection of splendidly coloured crystals of the salt 

 of considerable size, several of the plates having an area of a square 

 inch or more, and all of them being thick enough to< handle without 

 difficulty. In the course of his letter mentioning the despatch of the 

 crystals, Professor Mills writes r " They (the coloured crystals) are, I 

 am told, very pure chemically, containing at most O'l per cent, foreign 

 matter. They are rarely observed — one or two perhaps now and then, 

 in a large crystallisation . .... I have several times noticed 

 that small potassic chlorate crystals, when rapidly forming from a 

 strong solution, show what I suppose to be interference colours ; but 

 the fully formed crystals do not s-how them," 



Some time later I was put into communication with Mr. Stanford, 

 of the North British Chemical Works, Glasgow, from which establish- 

 ment the crystals sent me by Professor Mills had come. Mr. Stanford 

 obligingly sent me a further supply of these interesting crystals, and 

 was so kind as to offer to try any experiment that I might suggest as 

 to their formation. 



I am informed that at the recent Health Exhibition a stand was 

 exhibited from the chemical works at Widnes, showing a fine collec- 

 tion of brilliantly coloured crystals of chlorate of potash. I did not 

 see it. It would seem that the existence of these coloured crystals is 

 pretty generally known, but I have not seen mention of them in any 

 scientific journal, nor, so far as I know, has the subject been in- 

 vestigated. 



On viewing through a direct- vision spectroscope the colours of the 

 crystals which I had just received from Professor Mills, the first 

 glance at the spectrum showed me that there must be something very 

 strange and unusual about the phenomenon, and determined me to 

 endeavour to make out the cause of the production of these colours. 

 The result of my examination is described in the present paper. 



