THE YOUNG SCIENTIST. 
91 
paper should be used. It is first freed from 
all resinous and fatty matter by boiling it 
in a two per cent solution of carbonate of 
potassa for a few hours. The cotton is then 
taken out, washed in several clean waters, 
:and finally left to soak in clean water for 
twenty hours. All superfluous water is 
pressed out, and the cotton is dried either 
by artificial heat or in the sun. 
Any vessel of glass or porcelain, large 
•enough for the quantity of gun-cotton to be 
prepared, can be used. For half an ounce 
of cotton take 12 ounces commercial sul- 
phuric acid, which is first poured into the 
vessel; next add gradually, constantly stir- 
ring with a glass rod, three ounces water, 
and finally pour in four ounces commercial 
nitric acid. The mixture is well stirred 
before the cotton is introduced in small 
portions, and left in the acid for from eight 
to ten minutes. The cotton is moved about 
in the fluid with a glass rod to prevent un- 
equal action of the acid. The acids should, 
by means of a water bath, be kept at a tem- 
perature of 150° Fahrenheit. At the expir- 
ation of the prescribed time the cotton is 
removed, with glass rods, out of the acid, 
and then washed in several clean waters, 
till it shows no acid reaction on litmus 
paper. It can be dried by hot air below 
120°, and has to be preserved in an air-tight 
box, well protected against light and moist- 
ure. Cotton is thus transformed into 
pyroxyline, filtering paper into papyroxy- 
line. 
The best solvents of pyroxyline are sul- 
phuric ether and alcohol, generally taken in 
equal proportions. The ether should be 
<jhemically pure, and the alcohol at least 95 
per cent. A want of ether in the collodion 
•sometimes causes the film to peel from the 
glass after it has been taken from the silver 
'bath. Excess of ether will sometimes result 
in a species of unevenness called curtains. 
When the collodion contains too much alco- 
hol, the fact is sometimes betrayed by its 
-containing "nobs," that is, small lumps of 
undissolved collodion. Poor alcohol will 
produce crossnetting in the film, namely, a 
Mnd oi network extending over the whole 
surface. A little ether added to the col- 
lodion sometimes cures the evil. Such 
plain collodion is sensitized by the iodides 
and bromides of different metals. Iodides 
produce vigor and brilliancy in the nega- 
tive, while bromides soften and harmonize 
great contrasts, and show greater sensitive- 
ness for middle tints. In the preparation 
of collodion for special purposes, these 
characteristics of the sensitizer should de- 
cide the selection of the formula. 
The iodides of cadmium giutinize col- 
lodion, whereas an alkaline iodide liquefies 
it. The iodides and bromides most gener- 
ally employed are those of lithium, potas- 
sium, sodium, ammonium and cadmium. 
We here give some of the best formulae 
for bromo-iodized collodion: 
1. For Negatives — Alcohol, 95 per cent, 4 
ounces; ether, concentrated, 4 ounces; 
iodide ammonium, 16 grains; iodide cad- 
mium, 8 grains; bromide cadmium, 16 
grains; pyroxyline, 48 grains. 
2. For Positives — Alcohol, 4 ounces; 
ether, 4 ounces; pyroxyline, 32 grains; 
iodide ammonium, 24 grains; bromide cad- 
mium, 16 grains; 4 drops of tincture of 
iodine. 
The preparation of bromo-iodized collo- 
dion is conducted in the following manner: 
Measure off the alcohol and dissolve in it 
the iodides and bromides, and let this solu- 
tion stand at least one hour, the longer the 
better. Add now the cotton in small flocks, 
and finally pour in the ether. The bottle 
containing all the ingredients should now 
be thoroughly shaken, and is then set aside 
for two or more days to ripen and settle. 
The clear portion can be decanted into an- 
other clean bottle, and is ready for use. If 
a test with litmus paper should prove the 
collodion to be alkaline, a few drops of the 
tincture of iodine can be added. 
The bromo-iodized collodion is preserved 
in full bottles, well corked, in a dark place 
and cool temperature. It will keep for 
about a year. Collodion prepared by the 
above formulae will ripen in one or two 
days, and keep eight or twelve months. 
Collodion prepared only with cadmium can 
