. Carnegie Institution of Washington publication. VARIABILITY OF GELATIN. 3! said, is added to certain table gelatins to increase their body. Gelatin also contains a variety of decomposition products diie to the growth in it of various fungi and bacteria while it is in the vats or in the drying-house. If there is any delay in the drying it is spotted all over with molds and bacteria. It also contains some wax or grease, used to anoint the surface on which it is spread to dry, and this wax or grease is probably also a variable substance. Gelatins also polarize, it is said, in many different way
. Carnegie Institution of Washington publication. VARIABILITY OF GELATIN. 3! said, is added to certain table gelatins to increase their body. Gelatin also contains a variety of decomposition products diie to the growth in it of various fungi and bacteria while it is in the vats or in the drying-house. If there is any delay in the drying it is spotted all over with molds and bacteria. It also contains some wax or grease, used to anoint the surface on which it is spread to dry, and this wax or grease is probably also a variable substance. Gelatins also polarize, it is said, in many different ways. An absolutely pure gelatin of uniform character for bacterio- logical purposes is not to be had. That which perhaps comes the nearest to it and which is here recommended is Nelson's gelatin, made in London and well known to the makers of photographic dry-plates, who use it in large quantities. It conies in two grades, a hard and a soft, and costs about $ per pound. No. i, that which I like best, comes in shreds resembling " excelsior " used for packing (fig. 28). No. 3, which comes in long, broad strips, contains much cell detritus, etc., and niters with difficulty. Other expensive gelatins, said to be of quite uniform quality, are. Fig. 29* L,ichtdruck gelatin, made by Carl Creutz, Michelstadt, in Hesse, and Geneva Red Cross gelatin made by Winterthur, in Switzerland, under direction of Dr. Eder, of the Imperial Institute of Vienna (Cockaynej. NUTRIENT AGAR. Agar, or agar-agar, as it is usually called, from a Malay word meaning "vege- table," is a manufactured product obtained from various sea-weeds growing in Chinese and Japanese waters. Various species are used as food and the trade is con- siderable. It usually comes into the hands of the bacteriologist as long, slender, yellowish-white strips (fig. 29) or as blocks (fig. 30), or more especially in recent years, in the form of a gray-white fine powder of European manufacture (fig. 33). It is reput
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