1899 - 1900 .] Dr Marshall on the Action of Silver Salts. 
167 
at varying intervals of time. A more systematic series of experi- 
ments, under various conditions, may be expected to yield inter- 
esting results. The results expressed by the above curve are 
nevertheless such as to show that, for moderate concentrations, 
the quantity of salt decomposed in a given time is practically pro- 
portional to the quantity of persulphate present. As the reaction 
is not a unimolecular one, this would seem to indicate that one 
of the intermediate stages takes place much more slowly than the 
others. 
The spontaneous decomposition of an aqueous solution of am- 
monium persulphate takes place at a far slower rate than the 
above-noted one. After the lapse of four weeks, under the same 
conditions of concentration and temperature, a pure solution of the 
salt had decomposed to such an extent that 5 c.c. required only 
0'5 c.c. of alkali solution. In this case the bleaching of the methyl 
orange indicator was also much less rapid, and caused no practical 
inconvenience. 
By employing solutions of greater concentration (as regards both 
persulphate and silver) and a higher temperature, considerable 
quantities of nitric acid may be produced in this way. If the 
temperature is kept very high there is a fair amount of other 
decomposition, oxygen mixed with ozone being evolved in con- 
siderable quantity if the liquid is boiled. 
There are probably many other reactions which may be either 
brought about or accelerated by the catalytic action of silver 
compounds in presence of persulphate. We have such a case 
in the oxidation of methyl orange, already noted. A similar one 
is presented by the oxidation of indigo. If a solution of am- 
monium persulphate is coloured by means of indigo, then divided 
into two portions, and a drop of silver nitrate solution added to 
one of them, that one will be found to be decolorised much faster 
than the other. 
A still more remarkable example is provided by the oxidation of 
a chromic salt to chromic acid in acid solution. If solution of, say, 
chrome alum is heated with pure ammonium persulphate no change 
is observable beyond the usual one from purple colour to green. 
If, however, a drop of silver nitrate solution is also added, and 
the mixture gently warmed, the colour changes to green and then 
