THE PREPARATION OE CULTURE MEDIA 7 



growth of some are utterly unfitted for the cultivation 

 of others. 



Thus, whilst some organisms are unable to thrive 

 and multiply excepting when surrounded with the most 

 nutritious and subtle foodstuffs, others absolutely refuse 

 to grow unless bathed in a liquid from which such 

 organic materials have been most carefully banished. 

 The ingenuity of the bacteriologist is, in fact, severely 

 tried in endeavouring to cater for the organisms which 

 he has under his charge, and every year, or even 

 month, sees many additions to the menu from which 

 he has to select, and which already includes such a 

 medley as living animals, blood serum, bouillon, beef- 

 jelly, agar-agar, potatoes, numerous purely mineral so- 

 lutions, &c. In some cases, moreover, it is necessary 

 that these food materials should be varied from time 

 to time, or degeneration of the vitality of the micro- 

 organisms which are cultivated on them often takes 

 place. 



We shall now describe the preparation of some of 

 the more important culture media in detail. 



Solid culture media. — To Eobert Koch belongs the 

 honour of having adapted solid media to the cultiva- 

 tion of micro-organisms ; for although such media had 

 been used previously, it is the particular methods of 

 employing them devised by Koch which have secured 

 for these solid culture materials the extended applica- 

 tion and universal reputation which they now enjoy ; 

 and it is by these methods that such brilliant results 

 have been achieved in so short a period of time. 



Already in the year 1881 Koch observed that if a 

 slice of cooked potato was exposed to the air, and after- 

 wards preserved at a suitable temperature in a damp 

 chamber, small isolated dots began in the course of a 

 few days to make their appearance. Of these little 



