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LIX. Remarks on Chemical Changes of Colour. By H. F. 

 Talbot, Esq. M.P.F.R.S* 



"1TERY little is known with certainty concerning the cause 

 * of those striking changes of colour which we so frequently 

 witness in chemical experiments. No theory has yet been pro- 

 posed which will by any means account for the whole of them, 

 and we are often at a loss even for a plausible explanation. 

 And yet these phaenomena have probably some very close con- 

 nexion with the ultimate constitution of bodies, and are there- 

 fore well worthy of our particular attention. I will therefore 

 mention a few facts relating to this subject, hoping to engage 

 others to pursue such inquiries further. 



Water, being a colourless substance, ought, one would ima- 

 gine, when mixed with other substances possessing no decid- 

 ed colour, to produce a colourless compound. Nevertheless 

 it is to water only that the common vitriol or sulphate of cop- 

 per owes its extremely vivid blueness; as is plainly evinced 

 by a simple experiment. For if we calcine the vitriol at a 

 low red heat, and pulverize it, we shall obtain a powder of a 

 dull and dirty white appearance. Now pour a little water 

 upon this, and a slight hissing noise is heard, accompanied by 

 an evolution of heat, very similar to what happens in the sla- 

 king of quick-lime. At the same moment the blue colour sud- 

 denly reappears. Mr. Faraday, to whom I showed this ex- 

 periment, informed me it was new to him. I therefore pre- 

 sume it is a fact little, if at all known, and may interest your 

 chemical readers. 



Under the microscope this is a very pretty experiment, for 

 the instant a drop of water is placed in contact with the vi- 

 triol, the amorphous powder is seen to shoot into blue prisms. 



Are we then to infer that water has a tendency to commu- 

 nicate a blue rather than any other colour, to bodies in gene- 

 ral? By no means ; for in other instances its operation is ex- 

 actly the reverse, and it is a destroyer instead of a promoter of 

 blueness. For instance : sulphate of molybdenum is a liquid 

 of a very rich dark blue, when sufficiently concentrated : but 

 a very small portion of water suffices entirely to annihilate 

 this blue tint, and to produce a mixture which is perfectly 

 colourless. Thus the action of water upon the two metals 

 copper and molybdenum is of an entirely opposite character. 

 Instead of water if ammonia be used, the same contrast is seen 

 still more strikingly. Another remarkable and well-known 

 instance analogous to this is the muriate of cobalt, which is 



* Communicated by the Author. 



