§ v.] Origin and distribution. 41 



impossible to be quite certain whether we are dealing with dead 

 or with living objects. Yet it is a matter of the first importance 

 to determine whether germs capable of development are present 

 or not, and this would require further and very complex modes 

 of procedure. 



Hence various other methods have been tried with the object 

 of making the investigation easier and more trustworthy in both 

 directions. It was Koch who at length cracked the e.^g, like 

 Columbus. Starting from the empirical fact that gelatine, com- 

 bined with other nutrient substances easily prepared and in a 

 state of solution, is a very favourable substratum for the develop- 

 ment of most Fungi that are not strictly parasitic, and also 

 of Bacteria, he distributes quantities of the substances intended 

 for examination, earth, fluids, &c., in properly prepared gefetine, 

 liquefying at a temperature of about 30° C, and then makes the 

 gelatine stiffen by lowering the temperature. The quantities 

 may be exactly determined. Each germ is fixed in the 

 stiffened mass and so developes, and the products of the 

 development are at least at first also fixed and not liable to. 

 displacement in the medium. If the transparent gelatine is 

 spread in a thin layer on glass slides at the commencement of 

 the investigation, the germs and the products of their develop- 

 ment can be found with certainty with the microscope, and if 

 necessary be counted. If the object is to examine the air, the 

 best plan is to draw it in slowly by means of an aspirator 

 through glass tubes, coated inside with a layer of gelatine. If 

 the stream is properly regulated, the greater part at least of the 

 germs which are mixed with the air sink downwards and are 

 caught in the gelatine, where they may then undergo further 

 development. If experiments of this kind are properly con- 

 ducted and disturbing impurities excluded, distinct groups of 

 Bacteria, Fungi, &c., will be found after a few days in the 

 gelatine. Each group originates in a germ, or in some cases in 

 an assemblage of germs, which made its way to the particular 

 spot at the commencement of the experiment, as may often 



