62 



Messrs. Barnard and Hewlett. Method of [Mar. 28, 



Cooling may conveniently be effected by means of liquid carbonic acid. 

 This may be obtained commercially, ready compressed in a steel cylinder, 

 from which the gas may be allowed to escape and impinge on to the side of 

 the metal pot. By varying the rate of escape of the gas the temperature 

 may be controlled at will. An alternative method is to have another 

 containing cylinder 1ST outside the vessel in which the balls rotate, and to 

 pack the space between this metal cylinder and the metal pot with a 

 mixture of ice and salt or any other convenient freezing mixture. 



If pathogenic organisms are being dealt with the whole machine may be 

 covered with a glass bell-jar. Around the base-plate a groove is cut, and 

 in this any fluid bactericidal agent may be put, the glass jar being then 

 placed over the whole, thus effectually preventing the possibility of the escape 

 of any part of the contents of the pot." 



The introduction of the material to be ground into the containing vessel is 

 effected by unscrewing the top cylinder F and taking out the small plug P at 

 the top of the steel cone. The spindle of the central steel cone is hollow, so 

 that the emulsified bacteria may be introduced into the vessel by means of 

 a pipette through this opening without disturbing the balls or any other 

 part of the apparatus. On completion of the grinding action, owing to the 

 cell contents of the bacteria having been expressed, the material is much 

 more fluid than when introduced ; and, therefore when the machine stops, it 

 all sinks to the depressed centre of - the containing vessel, and may be 

 pipetted off again without further trouble. The introduction into, and 

 removal of the material from, the metal pot without disturbance of any of 

 the parts is a matter of considerable moment when dealing with pathogenic 

 organisms. This arrangement also enables the ground material to be 

 removed from the containing vessel immediately the grinding action ceases. 

 As a further precaution the lid of the pot E is recessed towards its centre as 

 shown : it is therefore possible to fill the hollow top of the lid with a 

 bactericidal agent ; immediately the machine is started the speed of rotation 

 ensures that this fluid is thrown outwards by centrifugal action, and com- 

 pletely seals the two screw-joints where the lid of the pot and the outer 

 cylindrical cover meet. As a point in the construction it is necessary that 

 the side wall of the top cylindrical cover should be made of vulcanite or 

 some diamagnetic material, as otherwise the necessary magnetic pull on the 

 armature inside would not be effected. 



A simplified form of the appliance is one in which the whole apparatus is 

 mounted horizontally, and the pull is put on the central steel cone not 

 through the agency of an electromagnet, but by means of gravity (fig. 2). 

 The whole arrangement is exactly similar to that previously described, 



