78 METHODS OF CULTIVATION OF BACTERIA. 



as there may be minute perforations in the candles sufficiently 

 large to allow bacteria to pass through. Filtered fluids keep 

 for a long time if the openings of the glass vessels in which 

 they are placed are kept thoroughly closed, and if these vessels 

 be kept in a cool place in the dark. "A layer of sterile toluol 

 about half an inch thick ought to be run on to the top of 

 the filtered fluid to protect the latter from the atmospheric 

 oxygen. 



Instead of being filtered off, the bacteria may be killed by 

 various antiseptics, chiefly volatile oils, such as oil of mustard 

 (Roux). These oils are stated to have no injurious effect on the 

 chemical substances in the fluid, and they may be subsequently 

 removed by evaporation. It is not practicable to kill the 

 bacteria by heat when their soluble products are to be studied, 

 as many of the latter are destroyed by a lower temperature than 

 is required to kill the bacteria themselves. 



The Observation of Bacterial Fermentations in Sugars. — 

 The capacity of certain species of bacteria to originate fermenta- 

 tions in sugars constitutes an important biological factor. The 

 end-products of such fermentations may be various. They 

 differ according to the sugar employed and according to the 

 species under observation, and frequently a species which will 

 ferment one sugar has no effect on another. The substances 

 finally produced, speaking roughly, may be alcohols, acids, or 

 gaseous bodies (chiefly carbon dioxide, hydrogen, and methane). 

 For the estimation of the two former groups complicated chem- 

 ical procedure may be necessary. The formation of gases is, 

 however, usually taken as the criterion of the possession of 

 fermentative properties. Generally speaking, it is reliable, and 

 the methods to be pursued are simple. It must not be forgotten, 

 however, that some organisnjs give rise to sulphuretted hydro- 

 gen by breaking up the proteid present. The formation of this 

 gas can be detected by the blackening of lead acetate when it is 

 added to the gas-containing medium. The following are the 

 chief methods for detecting the formation of gas : — 



(i) Gelatin Shake Cultures (Fig. 44, a). — The gelatin in the 

 tube is melted as for making plates ; while liquid it is inoculated 

 with the growth to be observed, and shaken to distribute the 

 organisms throughout the jelly. It is then allowed to solidify, 

 and is set aside at a suitable temperature. If the bacterium 



