GELATIN AND POTATO MEDIA. 171 



Make a mass culture (single potato) of each of the following: 

 Orange sarcine, Red bacillus of water, Violet bacillus of water. 



The inoculated potatoes are kept in the moist chamber, which 

 should be set aside where the sun will not strike. When moisture ac- 

 cumulates on the under side of the lid, it should be removed at once, 

 in the manner indicated. On the second day, the cultures are exam- 

 ined in hanging-drops, and on the day following they are used for 

 staining purposes. 



For modifications of the above method of making cultures on po- 

 tato see p. 182. 



Gelatin Plate Culture. 



It has been mentioned that cultivation on potatoes was 

 resorted to at a very early date. The far-reaching studies 

 of Pasteur led to the introduction of various liquid media, 

 such as beef, veal, or chicken broth. Nutrient solutions, 

 containing- various mineral salts, were likewise employed. 

 In order to isolate bacteria, which might be present in a 

 mixture, it was necessary to resort to excessive dilution, 

 and, even then, it was far from certain that a pure culture 

 was really obtained. One of the greatest aids to the ad- 

 vancement of bacteriology was supplied by Koch, when he 

 introduced the use of gelatin. By the addition of gelatin 

 to beef tea, a transparent medium was obtained which was 

 solid at ordinary temperature, and which could be. liquefied 

 without any difficulty. This transformation of a liquid me- 

 dium into a solid one rendered it possible to isolate bacteria 

 in a condition of absolute purity. The uncertainty regard- 

 ing the action of bacteria, whether they themselves pro- 

 duced certain changes, or whether it was due to something 

 else that accompanied them, was thus easily set aside. 



The object of the gelatin plate method, as with the 

 dilution potato culture already made, is to isolate the 

 several kinds of bacteria that may be present in a mixture. 

 The isolated organisms developing in a solid, transparent 

 medium, form colonies which are easily perceived, and 



