178 



CHEMICAL TECHNIQUES 



parts rosin melted together) is used to bind the glass to the metal. 

 A brass collar, C, engages B and is threaded to A. For visibility, the 

 center part of the collar is cut away on two sides. The ends of the 

 capillary tubes are flared and ground fiat, and a piece of collodion 

 membrane is held tightly between the ground surfaces at D. The 

 ultrafilter is used with positive pressure. 



D. STIRRING DEVICES 



Many investigators have used a stream of carbon dioxide-free 

 air bubbles ejected from a fine glass tip to obtain stirring in manipu- 

 lations of a submacro order. However, when very small volumes of 

 liquid are to be stirred, recourse must be had to other means. When 

 dealing with a drop of liquid in a capillary tube, a practical method 

 of agitation appears to be the simple expedient of moving the drop 

 back and forth in the tube by means of controlled air pressure. 

 Schmidt-Nielsen (1942) devised a centrifuge for sealed capillary 

 tubes which rotates them so that liquid contained within will be 

 thrown from one end to the other to effect mixing. The apparatus, 



, rt R ^ 



■ Ampullae 



63. Centrifuge apparatus for mixing and extracting small amounts 

 of liquid in ampules. Length of the apparatus about 25 cm. 

 From Schmidt-Nielsen (1942) 



shown diagrammatically in Figure 63, is attached to the motor 

 shaft and revolves with it. During the revolutions the plate with 

 the ampules is slowly turned, being connected by means of a 

 rubber band to a small rubber wheel which in turn is being driven 

 by its frictional contact with the motor housing. The rubber wheel 



