498 Mr. M. Carey Lea on 



is obtained with the aid of dextrine dissolves most freely in 

 the strongly alkaline liquid in which it is produced, and when 

 dilute nitric or sulphuric acid is added, the silver is preci- 

 pitated. But with acetic acid the precipitation is very in- 

 complete : the solution retains a brown colour, and contains 

 silver. Even the addition of a large excess of strong acetic 

 acid fails to throw down any more silver. It follows, there- 

 fore, that whilst this form of silver is most freely soluble in a 

 strongly alkaline liquid it is also soluble, to some extent, in 

 one that is either neutral or acid. 



The precipitate, when once formed, appears to be almost 

 insoluble. A small portion of it stirred up with distilled water 

 gives no indication of solution. But if a quantity is thrown 

 on a filter and washed, as soon as the mother-water is washed 

 out, the liquid runs through of a muddy red ; and if this 

 filtrate be allowed to stand, it deposits an insoluble portion, 

 and then has a fine rose-red colour and perfect transparency. 

 Notwithstanding the beautiful colour, it contains a trace of 

 silver only, so great is the colouring-power of the metal. 

 Sometimes, if the alkaline solution stands for a month or two, 

 the silver becomes spontaneously insoluble ; most of it falls 

 to the bottom as a deep-red substance, but part remains in 

 suspension, with a bright brick-red colour. The difference 

 between this and the true solution as originally formed is 

 extremely well marked. 



Dextrine is a very variable substance, and different speci- 

 mens act very differently. Common brown dextrine seems 

 to do better than the purified forms. 



Convenient proportions are as follows : — In 2 litres of water 

 40 grammes of sodium hydroxide may be dissolved and an 

 equal quantity of dextrine, filtering if necessary. 28 grammes 

 of silver nitrate are to be dissolved- in a small quantity of 

 water, and added by degrees at intervals. Complete solution 

 readily takes place. Although the liquid contains less than 

 1 per cent, of metallic silver, it appears absolutely black, 

 when diluted red, by great dilution yellowish. With some 

 specimens of dextrine the solution remains clear ; with others 

 it soon becomes a little turbid. 



Perhaps the most interesting reaction which this solution 

 shows is that with disodic phosphate. A little phosphate is 

 sufficient to throw down the whole of the silver, although both 

 solutions are alkaline. When a gramme of phosphate in solu- 

 tion is added to 100 cub. centim. of silver solution, the colour 

 becomes bright red, sometimes scarlet, and the whole of the 

 silver is presently precipitated. This precipitate on the filter 

 has a colour like that of ruby copper, which colour it retains 

 during the first washing ; but after a few hours' washing with 



