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The principle by which potatoes, paper, and many other substances can be hardened like parchment or horn is curious. Potatoes consist of about seventy per cent. water and twenty-five per cent. of starch, the remainder being salts and cellulose, which forms cells surrounded by the grains of starch. “When such a substance is for some time brought into contact with diluted sulphuric acid, that which results is simply a contraction of the cells” (i.e., a hardening), “or a kind of parchmenting.” Thus soft paper is converted into parchment.

It is evident that chemistry is as yet in its infancy as regards the conversion of cellulose by acid into hard substances. Since cotton, paper, and potatoes all produce by this process different substances, it is probable that hundreds of organic, or at least vegetable, substances will all yield new forms.

There is a marked difference between paste made of starch or flour, each having its peculiar merits. The former is principally prepared from potatoes. To prepare the cement we mix it with a very little water, stirring it very thoroughly till it assumes a bluish appearance. A little more hot water is then added, and the mass left till an opal-like tinge indicates that it has formed. To this then add hot water ad libitum. As it is almost colourless in very thin coats, it is largely used to glaze and give body or weight to, and often to simply falsify, woven fabrics, which by its aid seem heavier. To increase this weight white lead and other substances are used.