88 THE GERM THEORY OF DISEASE. 



during all this process the plants are giving off waste products. 

 These waste products mingle with the remains of the potato, 

 giving to it its offensive qualities, and it is said to be rotting. 

 We learn from this study that it is not necessary that food 

 material be dissolved in liquid in order that it may be ap- 

 propriated by bacteria. They are able to furnish a solvent 

 which will bring it into a condition fitting it for osmosis. 

 Otherwise bacteria would be utterly incapable of attacking 

 solids or semi-solids. Yet the cultivation of the pathogenic 

 micro-organisms on solidified media has been rendered popu- 

 lar by Dr. Koch, of Berlin, and it is now fully established 

 that they grow well upon such food. This was described in 

 my last lecture, and is very similar to the growing of the 

 organisms on the potato just described, and illustrates the 

 same physiological laws. Indeed, it seems to have been the 

 observation of the growth on the potato that led Dr. Koch 

 to the dry slide method. By this method, solid food mate- 

 rials may be prepared that are suited to the wants of the par- 

 ticular organism ; and, so far as I am informed, any of the 

 micro-organisms may be successfully cultivated by this plan, 

 if sufficient skill is used in the selection and preparation of 

 the material. Under these conditions it could not, reasonably, 

 be expected that the food material could be imbibed by the 

 organisms unless they could furnish some solvent by which 

 it is brought into a condition fitting it for osmotic action. 



All these facts point unmistakably to the formation of 

 soluble ferments by the organisms, which serve the purpose, 

 not only of liquefying, but of changing the molecular form of 

 food material ; a process in which the analogy to digestion, or 

 conversion of food material into peptones in the stomachs of 

 the higher animals, is perfect. 



Many of the micro-organisms seem specially adapted to 

 certain foods, and their digestive solvents are such as to serve 



