FREE BOOKS

Author's List




PREV.   NEXT  
|<   139   140   141   142   143   144   145   146   147   148   149   150   151   152   153   154   155   156   157   158   159   160   161   162   163  
164   165   166   167   168   169   170   171   172   173   174   175   176   177   178   179   180   181   182   183   184   185   186   187   188   >>   >|  
s flame below the incubator is never extinguished. The expansion of the metal walls of the capsule, which takes place upon the boiling of its contents, provides the motive force, transmitted through the rigid rod to raise the long arm of the lever B D, and as this expansion only takes place at a predetermined temperature, the lever will only be acted upon when the critical temperature is reached, no sensible effect being produced at even 1 deg. C. below that at which the capsule is destined to act. W is a weight sliding on the lever rod D; by increasing the distance between the weight and the fulcrum of the lower increased pressure is brought to bear upon the walls of the capsule with the result that the boiling-point of the liquid in the capsule is slightly raised, and a range of about two degrees can thus be obtained with any particular capsule. FOOTNOTES: [7] Made by the firm of Chas. Hearson & Co., 235 Regent St., London, W. XIV. METHODS OF CULTIVATION. Cultivations of micro-organisms are usually prepared in the laboratory in one of three ways: ~Tube cultures.~ ~Plate cultures.~ ~Hanging-drop cultures.~ These may be incubated either ~aerobically~ (i. e., in the presence of oxygen) or ~anaerobically~ (i. e., in the absence of oxygen, or in the presence of an indifferent gas, such as hydrogen, nitrogen, or carbon dioxide). With regard to the temperature at which the cultivations are grown, it may be stated as a general rule that all media rendered solid by the addition of gelatine are incubated at 20 deg. C., or at any rate at a temperature not exceeding 22 deg. C. (that is, in the "cold" incubator); whilst fluid media and all other solid media are incubated at 37 deg. C. (that is, in the "hot" incubator). Exceptions to this rule are numerous. For instance, in studying the growth of the psychrophylic bacteria, the yeasts and the moulds, the cold incubator is employed for all media. Tube cultivations are usually packed in the incubator in small tin cylinders, such as those in which American cigarettes are sold, or in square tin boxes. Beakers or tumblers may be used for the same purpose, but being fragile are not so convenient. Metal test-tube racks, long enough to just fit into the interior of the incubator and each accommodating two rows of tubes, are also exceedingly useful. ~AEROBIC.~ ~The Preparation of Tube Cultivations.~ The preparation of a tube cultivatio
PREV.   NEXT  
|<   139   140   141   142   143   144   145   146   147   148   149   150   151   152   153   154   155   156   157   158   159   160   161   162   163  
164   165   166   167   168   169   170   171   172   173   174   175   176   177   178   179   180   181   182   183   184   185   186   187   188   >>   >|  



Top keywords:
incubator
 

capsule

 

temperature

 

incubated

 

cultures

 
weight
 
expansion
 

presence

 
oxygen
 

cultivations


Cultivations

 

boiling

 
general
 

stated

 
accommodating
 

exceeding

 
addition
 
gelatine
 

rendered

 

hydrogen


preparation

 

interior

 

indifferent

 

cultivatio

 

nitrogen

 

regard

 

AEROBIC

 

Preparation

 

carbon

 

dioxide


cigarettes

 
American
 

cylinders

 

square

 

convenient

 
purpose
 

tumblers

 
fragile
 

Beakers

 
packed

Exceptions
 

numerous

 
exceedingly
 
instance
 

studying

 

yeasts

 
moulds
 

employed

 
bacteria
 

growth