TEXT-BOOK OF BACTERIOLOGY. 99 



is, we may say, nothing else than a plate bent into a cylindrical 

 form. When a test-tube has been inoculated, we first endeavor, by 

 slanting it gently up and down, to produce a thorough distribution 

 of the germs; then an India-rubber cap is drawn over the cotton 

 plug, and the test-tube is laid horizontally in a vessel full of iced 

 water. While holding the neck of the test-tube with the left hand, 

 *we endeavor to turn it quickly round on its own axis with the right 

 hand, not allowing one end to dip down lower than the other, as 

 the liquid in that case flows toward the lower end and causes 

 irregularities. In a few moments the gelatin is hard, the tube is 

 removed, the India-rubber cap taken off', and if the operation has 

 succeeded, the thin transparent covering of the walls can scarcely 

 be seen. The wider the test-tube and the greater the surface over 

 which the gelatin can spread the better, as a rule, is the result. 



This process has its great advantages. One is the great saving 

 of space, the possibility of working with precision on journeys, in 

 a campaign, and wherever ease of transportation is a desideratum. 

 Another is the fact that all the germs which were introduced by 

 inoculation must necessarily develop, while if the contents of the 

 tube be poured out upon a plate or dish, some parts of the gelatin 

 always remain sticking to the walls of the tube, and consequently 

 some of the germs escape observation and examination. 



On the other hand, this process has also its disadvantages, 

 some of which, however, may be easily avoided. 



It sometimes happens that in one such tube no growth takes 

 place, while it develops luxuriantly in the others. On examination 

 it is found that the gelatin covered the lower end of the cotton 

 plug with a thick layer, and prevented the access of air to the in- 

 terior. Aerobic bacteria, therefore, cannot thrive in it, but if the 

 plug be taken out and the crust pierced with a sterilized platinum 

 needle, the growth of colonies will soon take place. 



Again, sometimes such numbers of air-bubbles pass from the 

 cotton-wool into the interior of the tube during the consolidation 

 of the gelatin that the latter is completely filled with them. To 

 avoid this, allow the gelatin to cool almost to a semi-solid state 

 before putting it into the iced water. The tubes cannot be prevented 

 from soon becoming useless when they contain a large number of 

 liquefying bacteria. The dissolved gelatin runs down the sides of 

 the glass and presently forms an opaque mass, with which nothing 

 can be effected, while on a flat plate the horizontal position insures 

 a much longer period of usefulness. 



The employment of agar-agar in the glass-plate process is at. 

 tended with much greater difficulties than in the case of gelatin 



