NATURAL PHILOSOPITY. 133 



wave-length, the short waves at one end, and the long waves at 

 the other. Onh' the waves in .the centre oftlie spectrum will be 

 visible to the eye, in the order of violet, indigo, blue, gre(;n, yel- 

 low, orange, and red. Be3'ond tlie vicjlet end, wliere nothing is to 

 be seen b}- the eye, many chemical rays fall, and beyond the red 

 end are raanv other invisible waves, noted for their great heatinjj 

 properties. The chemical ra3's extend from the green or blue 

 di\ isions through the violet and indigo portions, and a considera- 

 ble distance beyond. Thus some of the chemical rays are visible 

 to the eye, but the rest cannot be seen. 



They are called chemical rays, because they act upon and tend 

 to set up decomposition in some chemical compounds, wherein the 

 affinities of the atoms for each other are weak. Chloride of silver 

 is one of these substances, and the chemical ravs will change this 



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white salt into finely divided black metallic silver, or into sub- 

 chloride of silver, a portion of the chlorine being driven olf. The 

 whole art of photography depends upon the action of the chemical 

 rays of light upon unstable chemical compounds. 



There are ccsrtain substances which, when placed in these feebly 

 luminous rays, become apparently self-luminous, and shine out 

 in the partial darkness of the room with strange brilliancy and 

 beauty. When most of the luminous rays of white light are cut 

 ojff by sheets of manganese glass, by cobalt glass, or by a trough 

 filled with ammonia-sulphate of copper, rays scarcely visible to 

 the eye may be made to pass through the room. Then, when 

 slabs of uranium-glass are placed in the patli of these rays, they 

 appear to be self-luminous, and glow with unearthly beauty. 

 When uranium glass is the recipient of the raA's it glows with a 

 yellow color like the moon when illuminated with the blue rays 

 of the spectrum ; but the nearer the color of the incident light ap- 

 proaches to violet, the greener and the more ethereal is the glow 

 of the uranium glass. 



The chemical rays are usually supposed to end in the blue part 

 of the spectrum, but bromide of silver, unlike the other salts of 

 silver, is sensitive also to some of the green nijs. 



THEORY OF DOUBLE REFRACTION. 



At the 1868 meeting of the British Association, Mr. Alfred R. 

 Catton read a paper on this subject. His view was that the 

 double refraction of light is caused by the action of the material 

 molecules on the ethereal medium, and he believed this to be a di- 

 rect and not an indirect action. He adduced a number of physical 

 facts, with the object of showing that the optical properties of 

 crvstals are a function of the arrangement of the material mole- 

 cules in space, and not of the matter of which these molecules were 

 built up. It might be stated as a physical law, notwithstanding 

 apparent difficulties, that a body of definite chemical constitution 

 always crystallized in the same form, under the same phAsical 

 conditions, and therefore he believed tliat the chemical constitu- 

 tion of the molecule did not affect the optical properties directly, 

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