APEiL 28, 1911] 



SGIENCI] 



667 



All this, however, does in no way interfere 

 with our actual practise of grading. Our 

 division lines between the different grades 

 may be drawn somewhat arbitrarily, the rela- 

 tive credit given for the different grades may 

 be somewhat arbitrary, the injustice done the 

 students under a system which enforces uni- 

 formity of grading is nevertheless small com- 

 pared with the injustice when each instructor 

 is left to grade according to his own fancy. 



Max Meyer 



Unr^rsity op Missouri 



AN EFFICIENT AND RAPID MERCURY STILL 



It is with some hesitancy that the ■writer 

 attempts a description of so simple and com- 

 monplace an apparatus as a mercury still. 

 Nearly every laboratory has its own method 

 of purifying mercury. However, of the vari- 

 ous " dry " methods that are usually employed 

 two stand out prominently — Weinhold's 

 method and Hulett's method.^ The principle 

 of the first is to distil under reduced pressure 

 or partial vacuum; while the second in addi- 

 tion to this makes use of a small jet of air 

 bubbling up through the mass of mercury. 

 The jet of air plays the role of oxidizing the 

 metallic impurities. Both methods are good. 

 The purifying effect of air bubbling through 

 mercury, even at room temperature, is now 

 well recognized. 



In 1905 the writer described a new form of 

 still,° in a brief paper read before the Amer- 

 ican Association for the Advancement of 

 Science, at its New Orleans meeting, in which 

 use was made of the mercury vapor lamp. 

 A description of the apparatus" as at present 

 modified and perfected by several years' use 

 in the laboratory is the object of this paper. 



It is common observation that mercury con- 

 denses on the walls at the cooler parts of the 

 lamp. Now by fusing to the mercury vapor 

 lamp a properly shaped condensing chamber, 

 mercury of a high degree of purity may be 



> Hulett, Phys. Rev., Vol. XXI., December, 1905. 

 = Science, Vol. XXIII., March 16, 1906. 



= Letters Patent, U. S. A., Nos. 891,264, 891,265; 

 Germany, No. 201,017. 



obtained. For the apparatus to be a practical 

 working still the lamp must have additional 

 modifications. Tig. 1 shows all of the essen- 

 tial parts. The mercury are is maintained 

 between the electrodes A and B. These elec- 

 trodes are of mercury and are in communica- 

 tion, through the narrow barometric legs BE^ 

 and AE,, with the vessels E^ and E, contain- 



AAA/WW 



ing the supply mercury. These vessels are 

 connected directly through an adjustable re- 

 sistance and an inductance to some convenient 

 source of direct current. It is well to include 



