78 REPORTS ON THE STATE OF SCIENCE. 
centre. To prevent the disintegrated silver which is formed on the anode 
from falling upon the kathode the anode should be inserted into a cup of 
filter paper separately supported. 
The liquid should consist of a neutral solution of pure silver nitrate, 
containing about fifteen parts by weight of the nitrate to eighty-five parts 
of water. 
The resistance of the voltameter changes somewhat as the current 
passes. To prevent these changes having too great an effect on the cur- 
rent some resistance besides that of the voltameter should be inserted in 
the circuit. If the value of the current is desired and the measurement 
is one of high precision, this external resistance should be from 50 to 100 
ohms ; in other cases the resistance should not be less than 10 ohms. 
Method of making a Measurement. 
1. The Solution.—The silver nitrate should be purchased as pure and 
recrystallised twice ; the recrystallisation is preferably done by evaporat- 
ing a saturated solution in a flask over a water-bath. The mother liquor 
should be drained away and the crystals dissolved in pure freshly distilled 
water. Prolonged contact of the crystals or of the solution with impure 
air must be avoided. The solution should be neutral to sensitive litmus- 
aper. 
If the silver nitrate is recovered from much used or contaminated 
solutions, or from an acid solution, the recovered salt should be fused 
(preferably in an electric oven) and afterwards dissolved, and the solution 
filtered before the recrystallisation processes ; otherwise it may be neces- 
sary to recrystallise more than twice. 
During electrolysis in the voltameter herein specified the silver nitrate 
solution does not change in composition as a result of the electrolysis by an 
amount which is detectable by any tried means, but, owing to the presence 
of impurities in the atmosphere, the solution should not be used more than 
once if great accuracy is desired. 
2. The Anode.—The anode should be prepared by cleaning the silver 
late or disc with sandpaper or a scratch-brush. It should be washed 
with distilled water and supported so as to form the kathode of a silver 
voltameter. The anode of this latter should be a silver bow] or a platinum 
bowl coated with silver, and the liquid should be a 15 per cent. solution 
of silver nitrate in water ; this solution need not be specially pure. If 
the anode bow] is of platinum coated with silver and of the dimensions 
already specified, it is convenient to employ about 350 cubic centimetres 
of the solution and support the silver plate or disc horizontally in the 
liquid near the top of the solution. A convenient current for depositing 
silver on the plate is 0°3 ampere. The plate is washed with distilled 
water and dried in an electric oven. 
The cup of filter-paper should be about 5 centimetres deep and of 
a diameter a little greater than that of the silver plate. It is made by 
folding a large filter-paper (such as Schleicher and Schull No. 595, 24 cm. 
diameter) over a glass cylinder (such as a bottle) of appropriate diameter 
and securing the upper portions of the folds of the paper with sealing wax 
or with platinum wire. The cylinder is removed and that portion of the 
paper which is above the seals is cut away. The upper parts of the 
internal folds are also secured with sealing wax or with platinum wire. 
3. The Kathode.—The platinum bowl should be cleaned with a strong 
