moderately stable in acid solution, and its bleaching action is best in slightly alkaline solution. An acid sol bleaches too slowly, or not at all; an alkaline sol induces evolution of oxygen and consequent waste. The great disadvantage of peroxide of hydrogen is its great expense, which is enhanced by an increasing demand for it in other industries. A minor disadvantage is its instability, which leads to loss in transit and storage. It is sold usually in strengths indicated by the volume of oxygen obtained from unit volume of the solution, when treated with permanganate in a nitrometer (_e.g._ "15 vols. peroxide"). It is a fortunate feature of both the oxidizing and reducing agents usually employed in bleaching, that they have considerable antiseptic power. This assists materially in preserving the gelatine from putrefaction during the critical period between extraction and concentration. REFERENCES. "Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed., pp. 61-66, 78-82. "Gelatine, Glue, and Allied Products," T. Lambert, pp. 29, 30, 49, 51. "Chemical Engineering," _J.R. San. Inst._, No. 2, 1910. S. Rideal. On adsorption phenomena: 1. "Chemistry of Colloids," Dr. W. W. Taylor. 2. "Chemistry of Colloids," V. Pöschl. 3. "Chemistry of Colloids," Zsigmondy and Spear. 4. "Chemistry and Physics of Colloids," E. Halschek. 5. "Surface Tension and Surface Energy," Willows and Hatschek. SECTION VI.--EVAPORATION The evaporation of the weak gelatine sols (3-9 per cent.) obtained by the processes described in previous sections into sols of such concentration (20-55 per cent.) that they readily set to a stiff gel on cooling, is now an essential feature of gelatine manufacture, and is one of the most important processes. In the early days of this industry, manufacturers aimed at obtaining a concentrated sol, as this saved time in drying, and so reduced the possibilities of putrefaction. The advent of evaporation has reduced these possibilities to a minimum, and has also enormously reduced the space required and the capital outlay needed in the drying sheds. It has, in addition, given the practical advantages involved in dealing up to the last minute with a much less viscous liquor. As the liquors extracted are weaker, the extraction is more complete and the decolorization more easily effected. The earliest attempts at evaporation were not very successful, partly on account of the prolonged "stewing" which ruined the setting power, and partly because of the poor economy of heat. Thus in the open evaporators the sol was maintained at a high temperature for a long period, and this process only proved suitable for low-grade products.