Remarks.—It is advisable that the mortar or dish be deep and narrow, as the mixture can be better formed in a vessel of this shape. If not convenient to procure a mortar, a common earthen bowl will answer; glass is objectionable, as the heat generated in the combination of the acid and nitre is liable to crack it. A new pail should not be used, especially if it is painted, as the acids attack the paint, and injure the cotton. I prefer one that has been used for some time, and has been frequently cleaned. A common earthen wash-bowl, or any large glass dish, will answer in place of the pail. Metal pails or vessels should not be used.
Nitrate of Potash (saltpetre) should be dry and finely-powdered. I use none other than Dupont's refined; this is very nearly, if not absolutely, chemically pure.
The commercial Sulphuric Acid (oil of vitriol) of America is of great uniformity of strength, as sold by druggists generally. I use a test-bulb graduated to the proper density, and have been very successful in my experiments.
In some twenty different samples of acid, used in different cities in the United States, I found only one that produced a poor cotton, and this might have been influenced by the moisture of the atmosphere, it being a very rainy day when I used it.
During my recent and somewhat extensive practice, I have thought that the fine long fibres of cotton wool do not make so desirable a soluble cotton as that which is heavy or common. Four or five very careful experiments upon this point, have had the effect to produce in me a strong belief that my ideas are entitled to some consideration. I should not select the finest cotton for making soluble cotton, but now invariably take that which is thick or coarse.
The result of my experience is (other things being equal), that cotton prepared in fine dry weather has a greater degree of solubility than when prepared in a moist atmosphere: hence I would recommend the experimenter to choose fine, clear weather for preparing it.
Manipulation.
Having at hand every article requisite, proceed as follows:—Put the nitrate of potash into the mortar or dish; be sure it is dry and well powdered, and then add the acid; stir them well with the pestle and glass rod, so that the lumps will be all (or nearly so) out, and a pasty solution formed. This operation should not occupy more than two minutes' time. Then put in the cotton, about one-quarter of the whole bulk at a time: it should be well picked apart, so that it may come immediately in contact with the acids, and should be kneaded, with the pestle and glass rod, into the solution, and as soon as wetted, another quarter should be added and wetted as soon as possible; so continue until all is in: then knead with the pestle and mortar for at least four minutes, or until every fibre of the cotton is saturated with the liquid; then the mortar should be covered over with the pane of glass, and allowed to stand for fifteen or twenty minutes; then the entire contents of the mortar should be thrown into the pail two-thirds full of water, and stirred with the glass rod as rapidly as possible: if this rapid stirring is omitted, the cotton will be injured by the action of the acids in combining with the water. The water should be poured off, and another change put into the pail.
After about three changes, the hands may be used in the farther washing. The hands should be perfectly clean, and free from all chemicals. The changes of water and washing should be continued until every trace of acid has disappeared, which can be seen by testing with blue litmus test-paper. After it is thought that the cotton has become free, the water may be squeezed out of a little lump about the size of a pea, and then placed between the fold of the test-paper, and if it reddens the paper, there is acid present, and the washing should be continued until there is no change in the paper. When this is done, the cotton can be put into the folds of a dry towel or cloth (which has been thoroughly rinsed, so that no soap be present), and wrung out as dry as possible, and then it may be picked apart and put aside, exposed to a moderate temperature (say 100° Fahr.) to dry, when it is ready for use.