METHODS OF INVESTIGATING BACTERIA. 789 



ever, these precautions are quite unnecessary ; when the 

 vessels are opened it is sufficient to burn the outer part 

 of the cotton wool, which contains the dust, in the flame 

 of the Bunsen burner in order to remove almost entirely 

 the danger of the introduction of germs. 



In many cases where we wish to have a larger surface 

 of nutrient material we may employ, besides Erlenmeyer's 

 flasks, small flat glass capsules with vertical walls 1 to 

 2 cm. in height, and with a diameter of 4 to 6 cm. the 

 so-called crystallisation capsules. These are preserved 

 in a wider cylindrical vessel, 15 to 20 cm. in height, in 

 which the capsule can be readily raised and lowered by 

 means of a bent strip of metal ; the cylinder is plugged 

 with cotton wool in the usual manner. If microscopic 

 slides or glass plates are employed for cultivation it is 

 sufficient to preserve them in glass dishes which are well 

 closed, and it is best for the cover to overlap the lower 

 part* of the dish. 



The nutrient substrata. These must, in correspondence The nutrient 

 with what has been said above with regard to the con- Sl 

 ditions of life of the lower fungi and of the necessary 

 nutrient materials, consist of carbonaceous, nitrogenous, 

 and mineral substances ; the excellence of the nutrient 

 solution depends on the nutritive value of the materials 

 employed, and also on whether the quantities present 

 approach as nearly as possible the optimum of con- 

 centration, on whether the reaction is that which is best 

 for the organism in question, on whether, and in what 

 quantity, oxygen is present, &c. 



If we wish to cultivate mould fungi, and to avoid as far as Formouldand 

 we can the entrance of bacteria, we must employ a substratum yeas 

 which is solid, which contains a small amount of water, and 

 of which the reaction is markedly acid ; in the case of the 

 yeast fungi the best conditions are obtained by the employ- 

 ment of fluids which are not so markedly acid, but are still 

 distinctly so, and which contain a considerable amount of 



* A more precise description of the apparatus and the utensils 

 necessary for the cultivation of bacteria will be found in the special 

 catalogues of the firms from .which all these articles can be obtained. 

 The following may be recommended : E. Muencke, Berlin, N.W. ; H. 

 Eohrbeck, Berlin, N.W. 



