170 CHEMICAL TECHNIQUES 



having a drawn-out piece of glass tubing to plug one end (Fig. 46) 

 so that the larger air space would prevent the displacement of the 

 liquid film, which was across the tube, when the stopper was fitted 

 on. To protect solutions from atmospheric carbon dioxide, Linder- 

 str0m-Lang, Weil, and Holter (1935) employed stoppers containing 

 soda lime tubes (Fig. 47). 



Tube Holders. Perhaps the simplest holder for a small reaction 

 tube is a short length of thick- walled rubber tubing into which the 

 bottom of the tube may be placed, as in Figure 50. It is more con- 

 venient to use a small wooden or metal block with three flexible 

 metal prongs to hold the tube. For titration, where the color of the 

 solution is to be matched with a color standard, a single block with 

 prongs to hold two tubes is used (see Fig. 64, page 180; A. H. Thomas 

 Co. and E. Petersen, Carlsberg Laboratory). 



Rediiclor. Kirk and Bentley (1936) devised a small glass volu- 

 metric flask of either 0.1 or 0.2 ml. capacity for use as a reductor 

 in their method for the estimation of iron (Fig. 48). The reductor 

 is made from heavy-walled 2 mm. bore capillary tubing. In the iron 

 method (page 277) cadmium amalgam is employed to reduce the 

 iron. 



Tube and Pestle. For the grinding of bits of tissue, Glick ( 1937) 

 used a small pestle with a 250 /xl. tube having the inner bottom sur- 

 face ground as shown in Figure 49. 



B. MICROLITER PIPETTES 



Pipettes of various designs have been employed for measuring 

 microliter volumes. The chief among these will be described. 



Fixed Pipettes. One of the pipettes developed by Linderstr0m- 

 Lang and Holter (1931) is shown in Figure 50 (A. H. Thomas Co. 

 and E. Petersen, Carlsberg Laboratory). It consists of a capillary 

 tube drawn out to a tip which is slightly bent so that contact can 

 be made with the wall of a vessel. The pipette is calibrated by first 

 weighing it, and then filling it with water to a little more than the 

 i-equired volume. The i)ipette is again placed on the pan of the 

 balance and water is removed by touching a piece of filter paper to 

 the tip. When the desired weight of water remains in the pipette, it 

 is removed from the balance and a mark is placed at the meniscus, 

 either by etching with hydrofluoric acid or using a piece of gummed 



