_Articles necessary._--One quart Wedgewood mortar and pestle, or evaporating dish; one glass rod; one pane of glass, large enough to cover the mortar or dish; one ordinary-sized pail two-thirds full of pure rain or distilled water, and at least ten times that quantity of water at hand; twelve ounces (by weight, avoirdupois) of nitrate of potash (Dupont's refined, pulverized); twelve ounces (by measure) of commercial sulphuric acid; and three hundred and forty grains of clean, pure cotton wool. _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.=