6 MICROSCOPIC TECHNIQUES 



per is then fitted over the whole and, after testing the unit, elec- 

 trical connections to the outside are made through tungsten glass 

 seals. Small glass projections on the outside of the drying tube 

 near the bottom serve to support the heating unit in its proper 

 position. 



Two glass boats {H) contain the phosphorus pentoxide used as 

 the drying agent. They are placed in tube G, which is 4 in. in di- 

 ameter, through the opening at J. The closures at A and J are 

 grease joints fitted with springs in the usual manner. All connect- 

 ing tubes on the low pressure side of the apparatus have a diameter 

 of 1.5 in. The vapor trap {K) has a 1.5 in. inner tube and a 0.5 

 in. annular space, and the inner tube projects about 5 in. below the 

 level of the solid carbon dioxide in butyl alcohol used as a re- 

 frigerant to surround the trap. 



The two-stage diffusion pump (L) and the single-stage booster 

 pump {N) employ Octoil-S {Distillation Products Co.) instead of 

 mercury since the former has a very low vapor pressure (claimed 

 to be < 10"*^ mm. of mercury at 15°), effects a very high pumping 

 speed, and is much cheaper than mercury. The pump (L) has an 

 intake speed of 10 liters per second ; it was designed by Professor A. 

 L. Hughes of the Washington University Department of Physics. 

 The boilers of the vapor pumps were protected from drafts, etc. by 

 a covering of several layers of wet asbestos paper and over these 

 a thin aluminum cone (M) . Turned-under tabs of the cone support 

 a flat circular 300 watt heater ( Chromolox) . A small electric blower 

 is used to cool the upper chamber of the boiler of pump L for 

 maximum efficiency. A single-stage Welch mechanical pump is con- 

 nected by rubber pressure tubing at U to the booster pump {N). 

 The phosphorus pentoxide trap (0) prevents contamination of the 

 oil when the vacuum is broken by stopcock P. The stopcock Q 

 has a 1 cm. bore and is used to isolate the high-vacuum section 

 from the mechanical pump while the space E is being exhausted. 

 The two-way stopcock R connects to the air at T and to E through 

 S. 



The type of support employed for the glass apparatus is shown in 

 Figure 2. The main standard is a rectangular aluminum box 4 ft. 

 high bolted to a % in. iron base plate 2 ft. square. The aluminum 

 box is made by welding together at W two heavy 8 in. aluminum 

 channels (V). The glass is supported by copper rings silver- 



