Temp. Time. Weight absorbed. H_{2}SO_{4} 4° C. (39° F.) 3 min 0·00495 HCl „ „ 0·00733 NaOH „ „ 0·02020 The molecular ratio of the absorption, in the two latter, is 3 HCl : 10 NaOH, and it is noteworthy that the same ratio was observed for silk. Cellulose removes barium hydrate from its solution in wafer to form with it an insoluble compound. On adding lead acetate to the solution of cellulose in the ammonio-copper reagent, so prepared as to contain no carbonate, a {8} precipitate is obtained consisting of a compound of cellulose with lead oxide, but in variable proportions. The compound C_{6}H_{10}O_{5}PbO is formed by the action of finely-divided lead oxide upon the above solution. Quite recently it has been shown (O’Shea, _Chem. News_, May 28th, 1886) that when dilute solutions of lead are passed through ordinary filter paper, a certain amount is retained which cannot be removed by washing. Cellulose does not combine with metallic salts, a fact which has been established incidentally to researches upon the mode of action of mordants. The combinations of cellulose with acid radicles (ethereal salts) are both definite and stable. _Triacetyl Cellulose_ [C_{6}H_{7} (C_{2}H_{3}O)_{3} O_{5}] is formed by treating cellulose with six times its weight of acetic anhydride at 180° C. (356° F.). The product of the reaction is a syrupy solution from which the compound in question separates on dilution with water as a white flocculent precipitate. Triacetyl cellulose is insoluble in alcohol and in ether, but soluble in glacial acetic acid. It is easily saponified by boiling with a solution of potassium hydrate, the cellulose being regenerated. No derivative containing more than three acetyl groups has been obtained; but a mixture of the mono-and di-acetyl cellulose is formed in treating cellulose with only twice its weight of acetic anhydride, the formation of these bodies being unattended by their solution. Whenever cellulose, in any form, is brought into contact with strong nitric acid at a low temperature, a nitro product, or a nitrate, is formed. The extent of the nitration depends upon the concentration of the acid, on the time of contact of the cellulose with it, and on the state of the physical division of the cellulose itself. Knop, and also Kamarsch, and Heeren, found that a mixture of sulphuric acid and nitric acid also formed nitrates of cellulose; and still later (1847), Millon and Gaudin employed a mixture of sulphuric acid and nitrates of soda or potash, which they found to have the same effect. {9} Several well characterised nitrates have been formed, but it is a very difficult matter to prepare any one in a state of purity, and without admixture of a higher or lower nitrated body. The following are known:—