VARIABILITY OF GELATIN. 3 1 



said, is added to certain table gelatins to increase their body. Gelatin also contains 

 a variety of decomposition products due 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 varial)le sulxstance. 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 dr}-plates, who use it in large quantities. It comes in 

 two grades, a hard and a soft, and costs about #1.25 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 nuich cell detritus, etc., and filters 

 with difficulty. Other expensive gelatins, said to be of quite uniibrm quality, are 



Fig. 19* 



Lichtdruck gelatin, made b}- 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 (Cockayne;. 



Nutrient Agar. 



Ao'ar, 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, 

 )'ellowish-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. 2,^)- 

 It is reputed to be the product of species of Gelidium (figs. 31 and 32). 



*Fic. 29. — The kind of agar-agar usually employed in bacteriological work. This is a manu- 

 factured product known to the Japanese as slender " Kanten." The figure represents first quality 

 " Kanten," in unbroken package. (Courtesy of Dr. Hugh M. Smith, Deputy Commissioner of the 

 United States Bureau of Fisheries, who brought the package with him from Japan.) 



