This grinding process was at first adopted by Harden and Young in their experiments but was afterwards abandoned in favour of Buchner's hand-grinding process, as it was found liable to yield juices of low fermenting power, probably on account of inefficient cooling during the grinding process. A slight modification of Buchner's process has, however, been introduced, the hand-ground mass being mixed with a further quantity of kieselguhr until a nearly dry powder is formed, and the mass packed between two layers of chain cloth in steel filter plates and pressed out in a hydraulic press at about 2 tons to the square inch (300 kilos. per sq. cm.). The press and plates are shown in section in Fig. 2. It has also been found convenient to remove yeast cells and kieselguhr from the freshly pressed juice by centrifugalisation instead of by filtration through paper, and to wash the yeast before grinding by means of a filter-press. Working with English top yeasts Harden and Young have found the yield of juice extremely variable, the general rule being that the amount of juice obtainable from freshly skimmed yeast is smaller than that yielded by the same yeast after standing for a day or two after being skimmed. The yield for 1000 grams of pressed brewer's yeast varies from 150 to 375 c.c., and is on the average about 250 c.c. Very fresh yeast occasionally presents the peculiar phenomenon that scarcely any juice can be expressed from the ground mass, although the latter does not differ in appearance or consistency from a mass which gives a good yield. EXTRACTION OF ZYMASE FROM UNGROUND YEAST. /1. Maceration of Dried Yeast./ A valuable addition to the methods of obtaining an active solution of zymase was made in 1911 by Lebedeff [1911, 2; 1912, 2; see also 1911, 3, 7, and 1912, 1]. This investigator had been in the habit of grinding dried yeast with water for preparing samples of yeast-juice of uniform character and observed that when the dried yeast was digested with sugar solution and the mixture heated, coagulation [p025] took place throughout the whole liquid, the proteins of the yeast having passed out of the cells. Further examination revealed the interesting fact that dried yeast readily yielded an active extract when macerated in water for some time. The quality of the resulting "maceration extract" depends on a considerable number of factors, the chief of which are: (1) the temperature of drying of the yeast; (2) the temperature of maceration; (3) the duration of maceration; and (4) the nature of the yeast, as well as, of course, the amount of water added in maceration. In general the yeast should be dried at 25°-30° and then macerated with 3 parts of water for 2 hours at 35°.