the Precipitation of Muriate of Platinum by Lime-xvater. 59 



addition of lime, and which has been well cleared by filtration, 

 is mixed with lime-water, in the dark, no precipitation to any 

 considerable extent takes place, — for a long while indeed, none 

 whatever ; though after very long standing, a slight flocky 

 sediment is formed, after which the action is arrested entirely. 

 But if the mixture, either freshly made, or when cleared by 

 subsidence of this sediment, is exposed to sunshine, it instantly 

 becomes milky, and a copious formation of a white precipitate 

 (or a pale yellow one if the platinic solution be in excess,) takes 

 place, which subsides quickly, and is easily collected. The 

 same takes place more slowly in cloudy daylight. 



This remarkable action is confined to the violet end of 

 the spectrum. I have exposed tubes of the mixed liquids im- 

 mersed in the sulphuric tincture of red rose-leaves, to strong 

 sunshine for whole days, and (after the first slight deposit al- 

 ready mentioned, which ceases in the first hour,) the remainder 

 is altogether insensible to red light; but the moment it is taken 

 out of the red liquor and held in free sunshine, the usual pre- 

 cipitation takes place as copiously as if it had been all the time 

 kept in total darkness. Even yellow liquids suffice to defend it. 

 The precipitate itself is a remarkable one, being a com- 

 bination of the oxide of platinum with lime, in which the 

 oxide seems to perform the part of an acid (a property of 

 this oxide which I believe has been before remarked ; though 

 at this distance from my books I cannot say by whom). 

 Muriatic acid dissolves it readily without effecting any decom- 

 ))Osition, even when added in too small quantity to take up 

 the whole. Nitric acid also dissolves it; (when newly formed 

 and moist, entirely ; when dried, with some residue of oxide). 

 The nitric solution is precipitated by nitrate of silver, and the 

 precipitate, which is of a high orange colour, and which is 

 a true jj/fl'/«?a/^ of silver, is easily distinguished from muriate 

 of silver, not only by its colour, but by its insolubility in the 

 liquid hyposulphites. 



The above facts were observed by me nearly two years 

 ago, and have been shown by me to a great many individuals 

 at various times in the interval ; among whom 1 may mention 

 the Bishop of Cloyne a)id Dr. Somerville, in June last (if I 

 recollect right) ; Sir D. Brewster, Mr. Babbage, Mr. Talbot, 

 antl others, in London, last summer, and more recently to 

 yourself; and have been distinctly described to many of my 

 scientific friends in conversation, among whom I will only 

 particularize Mr. Ritchie. I mention these circumstances 

 merely as ascertaining tny early and independent observation 

 of a liict which, at the lime of its discovery, I considered to 

 be sui generis, and whicii I caiuiul regartl us of sligiil iu)porl- 



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