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. [33] Observe that the Ether be pure; if it contains too much water and Alcohol, it will not dissolve the Pyroxyline, or will yield an opalescent solution. There is a third condition of Pyroxyline, different from either of the above, which may be puzzling:--the fibres of the Cotton mat together very slightly or not at all on immersion, and the washing proceeds tolerably well; the compound formed is scarcely explosive, and dissolves imperfectly in Ether, leaving little nodules or hard lumps. The ethereal solution yields, on evaporation, a film which is opaque instead of transparent. In this case (presuming the Ether to be good) the acid mixture is slightly too weak, or the temperature is too low, being probably about 90°, instead of 130° to 140° (?). When the acid mixture has been brought to the proper strength by a few preliminary trials, proceed according to the directions given at the next page. PREPARATION OF NITRO-SULPHURIC ACID BY OIL OF VITRIOL AND NITRE.