Having prepared the acid mixture of a definite strength by the above formula, the paper must be immersed according to directions given at [page 191].
b. Process for mixing Nitro-Sulphuric Acid, the strength of the two acids not having been previously determined.—Take a strong sample of Nitric Acid (the yellow Nitrous acid, so called, succeeds well), and mix it with Oil of Vitriol as follows:—
| Sulphuric Acid | 10 | fluid drachms, |
| Nitric Acid | 10 | " |
Now immerse a thermometer and note the temperature;[32] it should be from 130° Fahr. to 150°. If it sinks below 120°, place the mixture in a capsule, and float upon boiling water for a few minutes.
[32] In the preparation of soluble cotton, and indeed in all Photographic manipulations, a thermometer is almost indispensable. Instruments of sufficient delicacy for common purposes are sold in Hatton Garden and elsewhere, at a low price. The bulb should be uncovered, to admit of being dipped in acids, etc., without injury to the scale.
A preliminary experiment with a small tuft of Cotton Wool (cotton shows it better than paper) will then indicate the actual strength of the Nitro-Sulphuric Acid. Stir the tuft in the mixture for five minutes. Remove with a glass rod, and wash with water for a short time, until no acid taste can be perceived. If the Wool becomes matted, and gelatinizes slightly on its first immersion in the acid, or if, in the subsequent washing, the fibres appear to adhere and to be disintegrated by the action of the water, the Nitro-Sulphuric Acid is too weak. In that case add to the acid mixture.
Oil of Vitriol, 3 drachms.
If the cotton was actually dissolved in the first trial, an addition of half of a fluid ounce of Oil of Vitriol may be required.
Supposing the cotton not to be gelatinized and to wash well, then wring it out very dry, pull out the fibres, and treat it in a test-tube with rectified Ether,[33] to which a few drops of Alcohol have been added. If it be insoluble, dry it by a gentle heat and apply a flame: a brisk explosion indicates that the Nitro-Sulphuric Acid employed is too strong. In that case, add to the twenty drachms of mixed acids, one drachm of water, and test again, repeating the process until a soluble product is obtained.