C. must have been acid or
alkaline; it could not have been neutral.
We have, therefore, this alternative: In the lilac blue substance 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 explanation, 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 acting with a ferrous salt on tartaric acid,
obtained a neutral colloid substance having the constitution of
arabin, C6 H10 O6.
To appreciate the difficulty of arriving at a correct conclusion, it
must be remembered that the silver precipitate is obtained 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 solutions, under the influence of which the
substance itself slowly but continually changes. Next, the saline
solution used for washing must be removed by alcohol.
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