CHOLERA. 173 



increase in size sufficiently to allow of their being distin- 

 guished without the aid of a lens ; the power of growth to 

 the size of a colony visible to the naked eye being attributed 

 by Koch to the presence of the small quantity of available 

 oxygen that remained in the nutrient gelatine, growth 

 ceasing as soon as this oxygen is exhausted, the colony 

 remains of small size. 



A second experiment was made by inoculating nutrient 

 jelly contained in a small glass with comma bacilli ; this " was 

 placed under the receiver of an air pump, another glass pre- 

 pared in a similar way being placed outside the air pump as 

 a control experiment. It then appeared that those bacilli 

 under the air pump did not grow, while those outside the 

 receiver did well. But if those which had been under the 

 receiver were afterwards exposed to the action of the air, they 

 then began to grow. They therefore had not been destroyed ; 

 they only wanted the necessary oxygen to be able to continue 

 their growth. A similar result is obtained when cultivations 

 are placed in an atmosphere of carbonic acid ; whilst those 

 cultivations placed for control purposes outside the carbonic 

 atmosphere grow in the usual manner, those subjected to the 

 action of a stream of carbonic acid remain inactive and undergo 

 no development. Here also, however, they do not die ; for 

 after they have been exposed to the carbonic acid for a con- 

 siderable time, they again begin to grow as soon as they are 

 taken out of it." In place of carbonic acid, hydrogen may 

 be used, and the result is much the same. 



It cannot be assumed from these experiments that the organisms require 

 to obtain free oxygen from the air in order that they may continue their 

 growth,- for, as has been pointed out by Hueppe and Wood, if they are 

 grown on a suitable medium, such as unchanged normal albumen, they are 

 able not only to exist, but to produce unusually large quantities of their 

 specific poison, as they are able by the dissociation of such a medium to 

 obtain all that they require for their growth and development. 



All that can be argued from Koch's experiments is that the bacillus has 

 a saprophytic stage (although this was apparently not at first recognized by 

 Koch), during which it grows and multiplies, but only under ordinary con- 

 ditions where free oxygen can be obtained from the atmosphere. During 

 its parasitic existence it becomes so far modified, that, still growing with 

 great vigour, it produces its toxine more easily or in greater quantities in 

 the absence of oxygen, other conditions being favourable. During this 

 stage, however, it is much more readily affected injuriously by acids and 

 other germicidal reagents, and is therefore much more easily destroyed 

 than when it has had time to acquire its full saprophytic faculties. 



