788 



METHODS OF INVESTIGATING BACTERIA. 



treated, not with acetic acid or alcohol, hut with a weak 

 solution of carbonate of potash, and under these circum- 

 stances the nuclei and plasma cells, and in fact all 

 animal tissues, lose the stain, and the bacteria alone 

 retain it (Koch), 



Methods of 

 cultivation. 



Vessels. 



II. The Artificial Cultivation of the Loiuer Organisms. 



Artificial cultivation is absolutely essential for the 

 minute study of the properties of the various micro- 

 organisms. 



The vessels most commonly employed for this purpose 

 are thick- walled test tubes ; or vessels 5 cm. in diameter 

 with narrow necks ; or flasks of various sizes, the best 

 of which are the so-called Erlenmeyer's flasks with flat 

 bottoms. In order to exclude the 

 organisms plugs of cotton wool are 

 employed, which extend for about 

 3 cm. into the neck of the vessels, 

 and project about 1 cm. above it; this 

 must not press too closely on the 

 walls lest channels be left on the sur- 

 face of the compact mass. Pasteur 

 employs small flasks (matras), on the 

 neck of which a small helmet fits (J5), 

 which contains the plug of wool (a). 

 These vessels are move especially 

 suitable for culture fluids where one wishes to re-inocu- 

 late frequently ; in this case it is not necessary to take 

 out the cotton wool plug with the particles of dust ad- 

 hering to it the helmet itself is lifted. Fol* employs 

 a complicated plan consisting of an external cotton wool 

 plug which remains untouched, in the interior of which 

 is a glass tube which becomes narrower at the lower part. 

 This tube contains at the lower part an asbestos, and at 

 the upper part a cotton wool plug, and when a specimen 

 is required only the latter is removed, while the asbestos 

 plug is perforated by a heated canula. As a rule, how- 



* Arch, des sc. pTiys. et natnr., Geneve, 1884, t. 11. 



Fig. 140. Pasteur's 

 culture vessels. 



