48 M. Carey Lea — Allotrojric forms of Silver. 



Pure water was then poured on in successive portions until 

 more than half the substance was dissolved. The residue, evi- 

 dently quite unchanged, was of course tolerably free from 

 mother water. It was found that by evaporating it to dryness 

 over a water bath, most of the silver separated out as bright 

 white normal silver ; by adding water and evaporating a 

 second time, the separation was complete and water added dis- 

 solved no silver. The solution thus obtained was neutral. It 

 must have been acid had any citric acid been combined origi- 

 nally with the silver. This experiment, repeated with every 

 precaution, seems conclusive. The ferrous solution, used for 

 reducing the silver citrate had been brought to exact neutrality 

 with sodium hydroxide. After the reduction had been effected 

 the mother water over the lilac-blue precipitate was neutral 

 or faintly acid. 



A corroborating indication is the following. The portions 

 of the lilac-blue substance which were dissolved on the filter 

 (see above) were received into a dilute solution of magnesium 

 sulphate, which throws down insoluble allotropic silver of the 

 form I have called B, (see previous paper.) This form has 

 already been shown to be nearly pure silver. The magnesia 

 solution, neutral before use was also neutral after it had effected 

 the precipitation, indicating that no citric acid had been set 

 free in the precipitation of the silver. 



It seems therefore clear that the lilac-blue substance contains 

 no combined citric acid. Had the solubility of the silver been 

 due to combination with either acid or alkali, the liquid from 

 which it was separated by digestion at or below 100° C. must 

 have been acid or alkaline ; it could not have been neutral. 



We have therefore this alternative. In the lilac-blue sub- 

 stance we have, either pure silver in a soluble form, or else a 

 compound of silver with a perfectly neutral substance generated 

 from citric acid in the reaction which leads to the formation of 

 the lilac-blue substance. If this last should prove the true ex- 

 planation, then we have to do with a combination of silver of 

 a quite different nature from any silver compounds hitherto 

 known. A neutral substance generated from citric acid must 

 have one or more atoms of hydrogen replaced by silver. This 

 possibility recalls the recent observations of Ballo, who by act- 

 ing with a ferrous salt on tartaric acid, obtained a neutral colloid 

 substance having the constitution of arabin, C 6 H 10 O 5 . 



To appreciate the difficulty of arriving at a correct conclu- 

 sion, it must be remembered that the silver precipitate is ob- 

 tained saturated with strong solutions of ferric and ferrous 

 citrate, sodium citrate, sulphate, etc. These cannot be removed 

 by washing with pure water, in which the substance itself is 

 very soluble, but must be got rid of by washing with saline 



