14 MASS. EXPERIMENT STATION BULLETIN 341 



and solids and a partial vacuum is formed within the container. Thus, the 

 formation of a vacuum depends mainly on the venting of the jar. In addition 

 to inhibiting the growth of spoilage microorganisms, the vacuum serves to 

 maintain an effective seal. The function of the wire bail is mainly to direct 

 the glass cover down in close contact with the rubber ring to a correct setting 

 while^the vacuum is forming. 



Obviously, the normal method of determining vacuums by piercing the cover 

 with a pointed gauge, graduated for the purpose, cannot be carried out on glass- 

 covered jars. Consequently, two methods were used for determining vacuums 

 in glass-covered jars (7). 



Plate III. Apparatus for the Determination of Vacuums by the Vacuum Desiccator 



Method. 



Methods of Determining Vacuum 



Vacuum Desiccator Method. — In this method a desiccator is connected to a 

 water pump by means of vacuum tubing. (See Plate III.). The connections 

 are made tight so that a vacuum of 27 or 28 inches can be obtained. After 

 loosening the bail, the jar is immersed in a large glass jar of water, which is 

 then placed in the desiccator and the aspirator allowed to slowly exhaust the 

 air from the system. When the vacuum inside the desiccator becomes higher 

 than that in the jar of food, the glass cover will lift, breaking the vacuum and 

 allowing bubbles to escape from the jar. The vacuum gauge, which is set 

 between the faucet and the desiccator, is read at this time. The vacuum present 

 in the jar of food is slightly lower than the reading on the vacuum gauge. The 

 slight resistance of adhesion between rubber rings and jar is negligible in most 

 cases of freshly sealed jars. In this work most jars were examined within a 

 day after canning. 



