CULTURE METHODS. 285 



the second degree II. The plates are placed upon suitable shelves, 

 under bell- jars, with observance of the needful precautions. In 

 very accurate work, to prevent as far as possible accidental inocu- 

 lations, the processes should be carried on in a quiet room, and as 

 rapidly as possible. The gelatine upon the original undiluted 

 plate generally becomes turbid even after twenty-four hours at a 

 room temperature. In the next few days the separate bacterial 

 colonies become visible to the naked eye, not only upon the 

 original plate, but usually also upon those which had received 

 diluted material. Differences in the behaviour of the individual 

 colonies soon arise, in their form, their colour, and the liquefaction 

 of the gelatine, \\hich either may not ensue, or may ensue in 

 greater or less degree. In order to be able to examine the colonies 

 upon such a plate with a high power, a cover-glass must be laid 

 upon the part to be examined. If the cover-glass be subsequently 

 raised, a " cast" of the surface colonies often clings to it, and such 

 a " cast" can now be treated as a cover-glass preparation. Such 

 preparations naturally cannot be expected till the gelatine ha$ 

 been liquefied by the bacteria. 



Tube Cultures. Plate cultures form the starting-point for 

 tube cultures. From such a plate a portion of a definite colony 

 can be removed with a platinum needle. The gelatine tube which 

 is to be inoculated is held upside down, the wadding plug carefully 

 withdrawn, and the platinum needle inserted upwards into the 

 gelatine according to the purpose in view, either with a " prick " 

 or a " scratch ". In the prick inoculation it is stuck right into 

 the nutrient gelatine almost to the bottom, in the scratch inocula- 

 tion we make a shallow scratch on the surface of the gelatine. 



It has been long determined that direct sunlight exerts an 

 injurious influence upon bacteria, and often kills even their spores 

 in a very short time. Hence bacteria cultures must under all 

 circumstances be protected from this injurious influence. 1 



The macroscopic aspect of the cultures gives, as we have more 

 than once said, many starting-points for distinguishing bacteria. 

 The nutrient solution either remains clear, or it becomes turbid 

 and forms clouds or precipitates of various characteristic ap- 

 pearances. The solution becomes diffluent, or becomes slimy, 

 viscous, often changes colour and chemical reaction. This occurs 

 still more notably in cultures upon transparent substrata. We 



1 See especially the researches of H. Marshall Ward in Phil. Trans., vol. 

 clxxxv., p. 961, 1895. 



