Dr Davy's Miscellaneous Chemical Observations. 129 



be more liable to change, from exposure to the sun's rays, than 

 a solution of tartar emetic : But it is the reverse. After a year's 

 exposure, a portion of this wine, according to the London Phar- 

 inacopceia, had undergone no change ; and two different samples 

 of antimonial wine, one made with a sweet wine like Malaga, 

 and another with a dry wine, like cherry, which had been kept 

 many years in the Mediterranean, exposed to dull light, were 

 both as good as when first prepared. There was a very mi- 

 nute sediment of extractive matter in the first, and a more con- 

 siderable one in the second, which appeared to be composed 

 chiefly of tartrate of lime and vegetable colouring, and extrac- 

 tive matter, without oxide of antimony. 



The application of these results to pharmaceutical purposes 

 are so obvious, that it is useless to point them out. 



2. Effects of Light 071 Solution of Superacetate of Lead. 



Superacetate of lead, dissolved in water, is more readily de- 

 composed than the preceding salt by tha sun's rays. Carbonate 

 of lead is precipitated. The minute changes attending this ef- 

 fect I have not investigated. 



3. On Nitrate of Silver as a Test for detecting the presence of 

 minute quantities of Vegetable and Animal Matters in 

 Water. 



Nitrate of silver, dissolved in pure water, is not altered by 

 the sun's rays. If the minutest quantity of vegetable or ani- 

 mal matter is present, the solution is discoloured ; and, with 

 common distilled water, the discoloration is strong. To prove 

 that the cause of the change of colour is the one assigned, it is 

 sufficient to allow the coloured matter to subside, decant the co- 

 lourless solution, and expose it again to sunshine. However 

 powerful the sun's rays are, no farther efiect is produced ; but 

 add more common distilled water, and the phenomenon will in- 

 stantly reappear. 



I believe, nitrate of silver, thus used, is pne of the best tests 

 of the presence in water of very minute portions of vegetable and 

 animal matter ; of course, any chloride of silver that may be 

 formed, in consequence of the presence of any muriates, should 



OCTOBER DECEMBER 1828. I 



