188 Wollaston's Camera Lucida, 



and certain manipulation^ dependent upon a standard of light 

 of constant intensity, and easily reproduced in all situations. 

 — Idem, 



20. hduret of Potassium. — This substance being now em- 

 ployed in medicine, the following method of preparing it, 

 as stated by Caillot, Pharmacein, of Paris, may be of some 

 importance. " I take four parts of iodine, two of iron filings 

 free from rust, and about twenty of water ; I put the three 

 substances in a capsule of glass or porcelain,commencing with 

 the iodine and water. I keep stirring until the liquor, which 

 soon acquires a deep brown colour, becomes colourless ; 

 then I place the capsule over the fire, and when the liquid 

 boils, I pour in by degrees, stirring it each time, a solution 

 of pure subcarbonate of potash, until there is no longer a 

 precipitate ; or rather I add a slight excess of carbonate of 

 potash, which I saturate with hydriodic acid, after filtration. 

 I decant upon a filter, and I wash the residuum as long as the 

 water which passes through it occasions a precipitate, with 

 the deuto-chloruret of mercury : 1 then reunite all the 

 liquors and evaporate to a pellicle. 



The same procedure may be applied to the preparation 

 of the lodurets of sodium, magnesium, calcium, barium, 

 and strontium,— that is to say, in boiling the ioduret of iron 

 with magnesia, lime, barytes, strontian, or the subcarbon- 

 ates of these bases. 



We may likewise prepare the ioduret of mercury, by de- 

 composing the proto-nitrate of mercury, and the deuto- 

 chloruret by the ioduret of iron, which, as has just been 

 shown, may be extemporaneously prepared. —.^n. de Chi- 

 mie,Feb, 1823. 



21. The Camera Lucida of Dr. Wollaston has been so 

 modified by J. B. Amici, a professor of Modena, as to re- 

 move the difficulty experienced by most persons in their 

 first attempts to employ this ingenious and useful instru- 

 ment- All the varieties of the Camera Lucida may be ob- 

 tained of Lerebours, optician to the Bureau of Longitude, 

 place de Pont-Neuf, a Paris.— /Jem. 



