ELECTRO-CHEMICAL EQUIVALENT OF SILVER. 
431 
described, there was sometimes no loss on heating (Table II.), but perhaps more often 
the balance indicated a loss of one or two-tenths of a milligram. With respect to the 
interpretation of this, it is difficult to say whether or not it ought to be regarded as 
due to traces of salt retained in spite of all the washings. If so, the true weight of 
deposit is smaller still by nearly twice the apparent loss; but it is very possible that 
there may be traces of grease liable to be burnt off at a red heat, so that the loss in 
question cannot with confidence be attributed to nitrate. On this account the real 
amount of the deposit remains somewhat uncertain to nearly half a milligram. 
With respect to the procedure best adapted to eliminate the salt from the pores of 
the deposit, it is evident that the difficulty is to cause any displacement of the liquid 
in the interior. It was thought that this object might to some extent be attained by 
rapid alternations of temperature, and for this purpose the basins were (after thorough 
rinsing) passed backwards and forwards between cold and boiling distilled water. 
Recourse was had also to soaking in alcohol, somewhat diluted. Still wet with the 
alcohol, the basins were plunged into boiling water with the idea of promoting dis¬ 
turbance inside the cavities of the deposit. After a course of treatment of this kind 
the basins were filled and allowed to stand over night so as to give free play to diffusion. 
They were then rinsed a few times, and placed in the air closet to be dried at 160° C. 
§ 19. In order to meet the difficulty of the alloying of silver and platinum at a 
temperature high enough to decompose with certainty the last traces of silver nitrate, 
we made, at the suggestion of Professor Dewar, several attempts to replace platinum 
by silver bowls. One evident objection to the silver is the impossibility of removing 
the deposit with nitric acid, so as to restore the original condition of the bowl. But a 
more serious difficulty arises from the want of constancy in the weight of a silver 
bowl (without deposit) when strongly heated. On more than one decision a gain of a 
milligram or two was observed after heating in a porcelain basin over an alcohol flame. 
We have reason to believe that this effect depends upon the presence of traces of 
copper. In order to test the question we carefully cleaned and dried at 160° a piece 
of the highest quality of silver, such as was used latterly for the anodes. The weight 
was now 28'1628, and after heating to redness for a quarter of an hour over a naked 
alcohol flame fell to 28H619, On another occasion a loss of 2 milligrms. was observed 
under similar circumstances. On the other hand, a parallel experiment with a less 
pure sample of silver, known to contain a small quantity of copper, gave after the 
first heating to redness a gain of 3 milligrms., followed by a further gain of 
2 milligrms. after a second heating. 
These changes are, however, insignificant compared to that observed by Mr. Scott, 
who heated one of our large silver basins in a porcelain bowl for a long time over a 
Bunsen gas flame. After two nights treatment the weight had risen from 57'3008 
to 57'4521. Mr. Scott traced the increase in his case to the formation of silver 
sulphate, but it does not appear possible that this can be the explanation of the 
changes observed by us. The matter appears worthy of the further attention of 
