CULTURE METHODS AND APPARATUS 89 



potatoes to cultivate the color-producing^bacteria. Koch 

 later used this medium for isolating bacteria, and it was this 

 work that suggested to him his gelatin plate method which 

 made him, as well as the science which he founded, famous. 

 He tried nine different substances before he hit upon the use 

 of gelatin. Gelatin is used simply to give solidity, and not 

 for any nutritive value. Klebs and Breffeld had previously 

 used it in object glass preparations to prevent evaporation 

 while studying the fungi, but they did not use it as an isolat- 

 ing medium. The method is as follows. The sterilized gela- 

 tin is melted, and while it is still warm and in a fluid state it 

 is inoculated with the mixture of bacteria, properly diluted. 

 The medium is then poured on to a glass plate, previously 

 sterilized, and placed in a dish or under a bell jar to protect it 

 from contamination. These glass plates were frequently 

 placed one above the other on glass benches and stored in a 

 moist chamber. For a good many years the original glass 

 plate of Koch has not been used, but in its place the Petri 

 dish, which consists of a flat-bottomed glass dish with a similar 

 glass cover (Fig. 36). Esmarch suggested the use of what is 

 known as Esmarch roll. In this case the gelatin instead of 

 being poured out of the tube is rolled in a thin layer on the 

 interior of the tube. For detailed descriptions of these pro- 

 cesses the reader is referred to laboratory guides. 



The gelatin plate has a limited use on account of the fact 

 that many of the disease-producing bacteria grow readily 

 only at temperatures above that at which gelatin will remain 



