360 Sir D. Brewster's Observations on the Absorption of Specific 



entirely deprived of its fine blue colour by a very slight ad- 

 mixture of water. 



Muriate of copper is described in most books of chemistry 

 as a liquid of a bright green colour. But how imperfect an 

 account of it this is, will be seen from the following experi- 

 ment: — If sulphate of copper and muriate of lime (dry, or only 

 slightly damp) be pulverized together in a mortar, muriate 

 of copper is formed, of a dark yellow colour, or more frequently 

 of a yellowish brown. If a few drops of water are now added, 

 the yellow speedily changes to a bright green. If more water 

 is added, the mixture becomes greenish blue, sky-blue, and 

 finally colourless. If the water is evaporated by heat, the same 

 colours reappear, in the reverse order*. 



In Turner's Elements of Chemistry, it is said that nitric 

 acid when containing a small portion of the orange nitrous 

 gas, acquires a green tint ; upon which he takes occasion to 

 make the following remark (p. 193.) : — 



" It is difficult to perceive how an orange-coloured liquid 

 should give different shades of green and blue merely by be- 

 ing diluted." 



Now the above-mentioned property of muriate of copper 

 seems to furnish an instance that is very analogous. 



The mere application of heat often produces great change 

 of colour, which disappears again when the substance becomes 

 cold. Red lead, vermilion, and white oxide of zinc are in- 

 stances well known, and never accounted for. Another curious 

 example is furnished by the sulphate of molybdenum, which 

 when warmed changes its fine blue tint to a pale yellow, again 

 reverting to the blue when cold. 



LX. Observations on the Absorption of Specific Rays, in re- 

 ference to the Undulatory Theory of Light. By Sir David 

 Brewster, LL.D. F.R.S. 8?c. 



\ S Mr. Potter has referred in the last Number of this Journal 

 **> to some opinions of mine respecting the absorption of 

 light, I am anxious to state the views which I have taken of 

 this class of phasnomena, in reference to the undulatory theory. 

 I have long been an admirer of the singular power of this 

 theory to explain some of the most perplexing phaenomena of 



* The yellow state of the muriate of copper is best exhibited by warming 

 a sheet of paper on which letters have been written with it. The writing 

 disappears again on cooling, because it absorbs the atmospheric moisture. 

 The use of this salt as a sympathetic ink has been already mentioned by 

 chemical writers. 



