118 
THE YOUNG SCIENTIST. 
the process of purification is complete, and 
that the nitrate of silver is dissolved. While 
this is going on, it is advisable to stir the 
mixture occasionally. 
By the direct application of heat, the 
same object can be accomplished. The 
liquid is poured into a porcelain or glass 
vessel, gradually heated, and kept boiling 
for from 10 to 20 minutes. In order to 
avoid a reduction of the given quantity of 
the silver solution by evaporation, the 
nitrate of silver should be dissolved in 14 
ounces of water. In the latter operation 
the impurities will float on the water. As 
soon as the liquid, if it has been boiled, is 
cool, or if treated by the first method, is 
transparent like crystal, two grains of iodide 
of potassium, dissolved in a drachm of 
water, are added, and by frequent stirring 
thoroughly incorporated into the silver 
solution, which is then left undisturbed for 
half an hour. At the expiration of this 
time the solution is filtered through double 
filtering paper into another clean bottle, 
where even the most careful inspection 
should be unable to discover the slightest 
impurities. The addition of one drop of 
nitric acid will insure clear negatives and 
prevent fogginess. Kepeated stirring, and 
a rest of half an hour, will be found also 
indispensable to effect a thorough union of 
the acid with the silver solution. 
The silver bath is now ready for use, and 
will keep in good order for a longer or 
shorter time, according to the degree of 
cleanliness and care with which it has been 
treated. 
Sometimes the addition of another drop 
of nitric acid will be found necessary to 
prevent fogginess, but generally only in 
cases where the water used for the nitrate 
solution is strongly alkaline. 
The silver solution should be slightly 
acid. Litmus paper, which can be had in 
every drug store, should, in contact with 
the bath, turn only slightly red. 
The silver solution should be completely 
saturated with iodide. This second point 
can be tested by the addition of some weak 
solution of iodide of potassium in water, 
which, in case of previous saturation, will 
render the liquid turbid. The silver solu- 
tion should be free from every odor or 
smell. 
With proper care 12 ounces of bath will 
sensitize 50 to 80 quarter plates. Small 
baths generally require a second or third 
addition of iodide after a number of plates 
have been dipped, an inconvenience to 
which larger baths are not liable. 
THE DEVELOPER. 
In the wet collodion process, there are 
two developing solutions, the protosulphate 
of iron developer, and pyrogallic acid de- 
veloper. 
Formulse for iron developer: 1. Water, 
16 ounces; protosulphate of iron, 1 ounce; 
acetic acid, ^ to 1 ounce. 
2. Water, 16 ounces; protosulphate of iron, 
1 ounce; pure sulphuric acid, 1 to 2 drops. 
The iron salt is pulverized and intimately 
mixed with the given quantity of rain 
water. Add now the acid, when, after fil- 
tering, the solution is complete. If the 
developer does not flow well, half an ounce 
of alcohol should be added. 
The quantity of acid that must be used 
depends upon several circumstances. In 
summer, when the temperature is high, it 
has to be increased, while in cold weather 
it may be diminished. The action of the 
iron salt without acid is too energetic and 
unmanageable. As a general rule, long ex- 
posure requires a weak developer, while 
short exposure, especially instantaneous 
pictures, demand a very small quantity of 
acid. Too large a quantity of acid will pro- 
duce fogginess. The presence of acid ex- 
ercises also a certain influence upon the 
character of the whole deposit: the more 
acid the finer the deposit, and conversely 
the less acid the grosser the granulation. 
Diminishing the iron salt in the same quan- 
tity of water also weakens the reducing pro- 
perties of the developer, though a certain 
quantity of acid is always necessary to pre- 
vent fogginess. 
Formula for pyrogallic acid developer: 
6 grains pyrogallic acid; 6 grains citrie 
