HANDLING SMALL AMOUNTS OF MATERIAL: FILTRATION 287 



is stopped and before there is time to prevent it by closing the 

 upper end of the tube. These difficulties may be overcome by a 

 modification of the simple filtering tube, 1 consisting of the intro- 

 duction of an inner tube or trap. A glass tube about 3^ milli- 

 meters internal diameter has fused into its vertical axis a tiny 

 tube about i millimeter in diameter and 7 to 8 millimeters long. 

 The lower end of the main tube is caused to flow together until 

 the central opening is about 2 'millimeters in diameter, and it is 

 then ground so as to give a perfectly flat surface. The apparatus, 

 which is 30 millimeters long, is attached to a rubber tube and is 

 employed in the same manner as the previously described filter- 

 tube. It is obvious that as the filtrate rises in the tube it over- 

 flows into the small trap and is held in the space between the 

 walls of the outer and inner tubes. The tube through which 

 the liquid rises is therefore free, and any air bubbles entering 

 cannot cause a loss of the filtrate, nor can the liquid flow back 

 if suction is stopped. The filtrate can be removed either by 

 means of a drawn-out pipette or by inverting the tube and in- 

 ducing the liquid to flow by means of a platinum wire. 



Savage 2 introduces the filter paper within the tube, making 

 the manipulation somewhat simpler, the filtering of liquids from 

 very fine precipitates somewhat easier and permits of han- 

 dling larger volumes of liquid. But this method fails to handle 

 as tiny quantities of liquid as that of Behrens and the residue 

 is not so readily separated from the filter. Savage describes 

 his method as follows: 



"A glass tube of about 4 millimeters inside diameter is drawn 

 out as abrupt as possible, and the narrow portion of the tube 

 should extend from 15 to 30 millimeters from this point, with 

 parallel sides and an inside diameter of about eight-tenths of a 

 millimeter. The entire tube is 8 or 9 centimeters long, and both 

 ends are rounded in the lamp flame. From a piece of soft filter 

 paper of smooth surface and long fiber a triangular piece is torn 

 (not cut), 2 to 2\ centimeters long and i centimeter wide at 

 the base. This is rolled between the fingers into a slightly taper- 



1 Chamot, Jour. Appl. Micros., 3, 854. 



2 Savage, Jour. App. Micros., 3 (1900), 678. 



