208 Mr. Horner and Sir David Brewster on 



than one of these characters. But tliough the new substance 

 resembles the crystals above mentioned in giving dissimilar 

 pencils of doubly refracted light, it stands unique among all 

 bodies with which I am acquainted in possessing the extra- 

 ordinary system of composite crystallization, in which an in- 

 finite number of crystals are disseminated equally in every 

 possible azimuth through a larger crystalline plate, having 

 their axes all inclined at the same angle to that of the larger 

 plate, and producing similar phaenomena in every direction, 

 and through every portion of the plate; or we may describe 

 this remarkable structure by saying that the minute elementary 

 crystals form the surfaces of an infinite number of cones whose 

 axes pass perpendicularly through every point of the larger 

 plate*. 



The iridescent phsenomena exhibited by the new substance 

 are extremely interesting, and I have been at much pains to 

 examine them in a great number of specimens. The plates 

 into which the substance is divisible have been formed in suc- 

 cession, and certain intervals of time have elapsed between their 

 formation. In general every two contiguous laminae are sepa- 

 rated by a thin iridescent film, varying from the three to the 

 fifty millionth part of an inch in thickness, and producing all 

 the various colours of thin plates which correspond to inter- 

 mediate thicknesses. Between some of the laminae no such film 

 exists, probably in consequence of the interval of time between 

 their formation being too short; and between others the film 

 has been formed of unequal thickness, as happens in the oxi- 

 dations upon steel when they are formed upon or around 

 hard parts of the metal caWed pins by the workmen. 



There can be no doubt that these iridescent films are 

 formed when the dash-wheel is at rest during the night, and 

 that when no film exists between two laminae, an interval too 

 short for its formation (arising perhaps from the stopping of 

 the work during the day,) has elapsed during the drying or 

 induration of the one lamina and the deposition of the other." 



That these iridescent films are not thin films of the sub- 

 stance itself, may be inferred from the fact that light is reflected 

 from their surfaces when they firmly adhere to the laminae 

 which inclose them. If, for example, we remove or raise up 

 from a piece of mica a thin film which gives a bright green 

 tint, and press it again into optical contact with the surface 

 from which it was separated, it will then cease to exhibit any 



* A rude idea of this structure is given by the beautiful cones, or rather 

 pyramids of microscopic crystals of titanium which I have somewhere de- 

 scribed as existing within the pyramids of many crystals of amethyst from 

 the Brazils. 



