=268. Preparation of Diastase for Starch Solution.=—The methods of preparing malt extract for use in starch analysis have been described in paragraph =179=. If a purer form of diastase is desired it may be prepared by following the directions given by Long and Baker.[219] Digest 200 grams of ground malt for twenty-four hours with three parts of twenty per cent alcohol. Separate the extract by filtration and to the filtrate add about one and a half liters of ninety-three per cent alcohol and stir vigorously. After the precipitate has subsided the supernatant alcohol is removed by a syphon, the precipitate is brought onto a filter and washed with alcohol of a strength gradually increasing to anhydrous, and finally with anhydrous ether. The diastase is dried in a vacuum over sulfuric acid and finally reduced to a fine powder before using. Thus prepared, it varies in appearance from a white to a slightly brownish powder. Made at different times and from separate portions of malt, it may show great differences in hydrolytic power. =269. Estimation of Starch in Potatoes by Specific Gravity.=—A roughly approximate determination of the quantity of starch in potatoes can be made by determining their specific gravity. Since the specific gravity of pure starch is 1.65, it follows that the richer a potato is in starch the higher will be its specific gravity. The specific weight of substances like potatoes is conveniently determined by suspending them in water by a fine thread attached to the upper hook of a balance pan. There may be a variation of the percentage of other constituents in potatoes as well as of starch, and therefore the data obtained from the following table can only be correct on the assumption that the starch is the only variable. In practice, errors amounting to as much as two per cent may be easily made, and therefore the method is useful only for agronomic and commercial and not for scientific purposes. The method is especially useful in the selection of potatoes of high starch content for planting. The table is constructed on the weight in grams in pure water of 10000 grams of potatoes and the corresponding per cents of dry matter and starch are given. It is not always convenient to use exactly 10000 grams of potatoes for the determination, but the calculation for any given weight is easy.[220] _Example._—Let the weight of a potato in air be 159 grams, and its weight in water 14.8 grams. Then the weight of 10000 grams of potatoes of like nature in water would be found from the equation 159: 10000 = 14.8: _x_. Whence _x_ = 931 nearly. In the table the nearest figure to 931 is 930, corresponding to 24.6 per cent of dry matter and 18.8 per cent of starch. When the number found is half way between the numbers given in the table the mean of the data above and below can be taken. In other positions a proper interpolation can be made if desired but for practical purposes the data corresponding to the nearest number can be used.