406 Mr. Talbot's Facts relating to Optical Science. 



the microscope, and it will be seen, not only that the colour 

 is changed, but that all trace of the white capillary crystals 

 has vanished, and instead of them the field of view of the 

 microscope is covered with an assemblage of transparent yel- 

 low crystals which are in shape thinjlat regidar hexagons. 



But after a few minutes, as the plates of glass grow cool, 

 the white colour returns as before, and the microscope now 

 shows again a multitude of white capillary crystals, the hexa- 

 gonal ones having in their turn entirely disappeared. 



The singularity of this change, which may be repeated 

 several times, — the remai'kable fact of being able to view 

 the same substance, alternately of two different colours, and 

 with different yo;7H5 belonging to those colours, induced me 

 to endeavour to see in what manner such a singular meta- 

 morphosis took place. I therefore took the plates of glass 

 when cold and adjusted the microscope upon one of the ca- 

 pillary crystals contained in them. It looked, when much 

 magnified, like a cylindrical thread of glass, of a clear white 

 colour and transparent. I then, without deranging the ad- 

 justment, placed a small spirit-lamp beneath the glass, at a 

 moderate distance, and watched the effects of the heat. Af- 

 ter a short time I observed the cylindrical thi'ead shrink in 

 diameter, and at the same moment the axis of the cylinder split 

 open, and a yellow crystalline plate protruded itself through 

 the opening, increasing in size every moment, while the re- 

 mainder of the white crystal quickly dissolved and disappeared. 

 This happened at several points of the axis of the cylinder, 

 so that when the change was complete, the yellow hexagons 

 were not unfrequently found arranged in a row or straight 

 line indicating the position of the former crystal. When the 

 heat is more suddenly applied, the dissolution of the white 

 crystal is proportionably more rapid, and the yellow hexagons 

 start into existence before the observer's eye with a sudden- 

 ness which is very surprising, and increase so rapidly as to 

 triple or quadruple their diameter in a second of time, pre- 

 serving all the time the exact figure of the regular hexagon. 

 Most of them are of a full yellow tint, but some are of a 

 greenish yellow, and some of a peculiar light brown, which 

 variety of tint appears a circumstance worthy of remark, but 

 I do not know upon what cause it can depend. 



There is something in this experiment which is very pe- 

 culiar. We are accustomed to see salts dissolved or melted 

 by heat; or if they are of an insoluble nature, at any rate 

 they remain inert and passive when heated. 



But here we have a salt which crystallizes when heated, and 

 the more rapidly the greater the heat. I have described the 



