The Silvering of Glass 123 



(on no account should more be used), and the result is a 

 clean, white and permanent label, impervious to the action 

 of all reagents, and all liquids, save ether and benzene. 

 The labels are removed when required by scraping with a 

 sharp knife. 



The above process was recommended by Professors Clowes 

 and Coleman, and has proved the most satisfactory during 

 twenty years' experience of all kinds of labelling. 



II. THE SILVERING OF GLASS. 



Several excellent methods for silvering glass are available, 

 making easy the construction of hygrometers, galvanometer 

 mirrors, convex and concave mirrors, reflectors, and other useful 

 apparatus. There are two difficulties 



1. The silver does not always come down bright. 



2. The silver frequently prefers to deposit upon the sides 



of the containing vessel rather than upon the desired 

 object. 



The first is a chemical, the second a mechanical trouble, 

 each to be solved in its own way. 



i. The choice of a formula depends much upon circum- 

 stances, but in the main an ammoniacal solution of a salt of 

 silver is reduced by some organic reducing agent. The 

 following method works very well provided the solution of 

 tin chloride is freshly made, clean, and dilute, otherwise the 

 silvered surface is likely to be of a dark colour 



A. Dissolve 2*5 grams silver nitrate in 100 c.c. of distilled 

 water ; reserve about 25 c.c. of this solution, and add ammo- 

 nium hydroxide solution to the larger bulk until the precipitate 

 at first formed almost redissolves. The precipitate disappears 

 rather suddenly at the end, hence the advisability of reserving 

 a portion of the original solution. The 25 c.c. reserved is 

 then treated similarly and added to the rest, the whole being 

 filtered and made up to 250 c.c. 



B. Dissolve 0*5 gram silver nitrate in distilled water, and 

 pour into 250 c.c. boiling distilled water. Add o'4 gram 

 Rochelle salt, and boil the mixture till the precipitate is grey. 



