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LORD RAYLEIGH AND MRS. H. SIDGWICK OX THE 
§ 18. Table I. (p. 437) gives the results of a laborious series of determinations 
made with solutions containing more or less acetate. It will be seen that up to 
August 16 the numbers in the final column are fairly concordant, and they rather 
narrowly escaped being accepted as satisfactory. In the month of November, however, 
the experiments were continued with a fresh stock of depositing solution (probably 
containing less acetate), when a systematic change became apparent in the direction of 
smaller deposits. From the first we had taken, as we thought, full precautions to 
secure adequate washing out of the silver salt, and special experiments had proved 
that the weights were not appreciably changed by further washing with pure water, 
or by resoaking in the depositing solution with a second washing and drying conducted 
like the first. Nevertheless the appearance of the deposits under the microscope was 
such as to suggest a doubt whether a complete elimination of the salt from its pores 
was possible with any amount of washing, and the evidence of the analyses was felt 
not to be decisive, inasmuch as the deficiency to be found in this way would correspond 
to only about one-third of the weight of salt actually present. According to this view 
the diminution in the weight of the deposits after August 16 was due to a more 
complete washing out of the salt, rendered possible by the more open texture of the 
deposits, and we proceeded to test the behaviour of pure nitrate solutions. The result 
was a further small, but distinct, diminution in the weights, as shown in Table II., and 
we were now convinced that the use of acetate had been a great mistake, costing us 
six months’ almost fruitless labour. When the deposits are taken upon the concave 
surface of a bowl they are coherent enough for convenient manipulation without the 
aid of acetate. The danger of retention of salt or other impurity is far greater than 
of loss of metal, and this danger is aggravated by the acetate. Indeed it would be 
scarcely too much to say that the danger is converted into a certainty, for from the 
fine pores of these deposits it seems almost impossible to remove the salt effectually. 
It is evident that, in spite of the retention of a small quantity of salt, a satisfactory 
conclusion might be reached were there anv means of estimating its amount. Theo- 
retically the analysis for silver, as many times effected, is adequate to this purpose, 
since the difference of the total weight of the (impure) deposit, and of the metal found 
on analysis, would represent the N0 3 of the salt. But the circumstances are so dis¬ 
advantageous that no satisfactory result could be looked for without an extraordinary, 
and perhaps impossible, perfection of manipulation. A direct test for nitric acid is not 
applicable ; but at a sufficiently high temperature the silver nitrate would be decom¬ 
posed, so that the loss of weight incurred on heating to redness (after previous 
thorough drying at, say, 160° C.) would represent the N0 3 . Unfortunately this method 
is difficult to carry out thoroughly without injury to the platinum basins, inasmuch as 
silver and platinum begin to alloy at a red heat- But an exposure for five minutes to 
a heat just short of redness does not seriously damage the basins, and appears to be 
nearly, if not quite, sufficient to drive off the last traces of N0 3 . With a pure nitrate 
depositing solution, and with the treatment for elimination of the salt presently to be 
