NUTRIENT MEDIA 319 



Steaming for i hour at two or three successive intervals of twenty- 

 four hours. 



Cooked potatoes afford a solid nutrient medium which is 

 quickly prepared and which is particularly adapted for the 

 culture of chromogenic bacteria. Potatoes free from wounds 

 are selected and scrubbed in water until they are perfectly 

 clean, and the eyes and any unsound spots, if these could not 

 be avoided, are cut out with a knife. Then the potatoes are 

 placed for an hour in a solution of i part of mercuric chloride 

 in 500 parts of water to disinfect the surface. They are next 

 steamed for about an hour in a steam sterilizer, and after twenty- 

 four hours the steaming is repeated for about half an hour. 

 The sterilized potatoes are then placed in glass Petri dishes, 

 are cut in halves with a sterilized table-knife, and the cut sur- 

 faces are inoculated. If the source of the inoculation is not a 

 pure culture, an isolation of forms may be approximated by 

 making long scratches over the surface of the potato with a 

 sterilized platinum needle which has been in contact with the 

 source of the inoculation. It will add to the security of the 

 process of sterilization if each potato, before being placed in 

 the bath of mercuric chloride, is wrapped in a piece of tissue 

 paper, and so protected until it is cut open for inoculation. 



Another method of preparing potatoes which is, on the whole, 

 more convenient and certain, is to cut out long cylindrical 

 plugs from sound potatoes by means of a cork-borer or any 

 metal tube of the proper size, and then to cut the potato cylin- 

 ders very obliquely in two pieces, each of which is then to be 

 placed in the bottom of a test-tube so that the oblique surface 

 stands uppermost. After plugging the tubes with baked cotton, 

 the potato cylinders are subjected to a temperature of ioo° C. 

 in the steam sterilizer for one hour at three successive intervals 

 of twenty-four hours. A sterilized paste 'made from potatoes 

 or bread serves well for the culture of molds as well as of bacteria. 



A decoction of horse-dung furnishes a good medium for the 

 culture of Mucor and various other molds. The decoction is 

 prepared by boiling the dung in water, then filtering and sterilizing 



