DESCRIPTION OF APPARATUS AND METHODS. 



25 



In fig. 5 is shown the manner of mounting the cell-holder, C, in ver- 

 tical guide F, upon the movable spectrometer arm, A. (See also fig. i.) 

 A double-walled sheet-iron and asbestus shutter moves in the vertical 

 guide, W. The heater of a Nernst lamp is at H, while the spectrometer 

 slit is at F. 



The cell for the gases was of glass tubing, with rock-salt plates for 

 windows. It is shown in fig. 10, while fig. 1 1 shows the manner in which 

 it is mounted in its holder by means of the key, k. This holder could 

 then be placed in the vertical guides shown in fig. 5. 



The shutter and the cell holder were operated from the observing 

 telescope by means of cords and pulleys, so that it was necessary to 

 enter the room only in making the spectrometer settings. 



SOURCE OF RADIATION. 



The question of the source of radiation, with several curves showing 

 the distribution of the energy in several kinds of electrically heated 



Fig. 5. 



clay radiators is given in Appendix I, in which is shown the great adapt- 

 ability of the " heater " of a Nernst lamp for this work, on account of 

 its strong radiation beyond 4 ix, and also on account of the ease with 

 which the constancy of the radiation can be maintained. Although two 

 I lo-volt heaters were provided, only one was generally used. Current 

 was obtained from a storage battery of 90 cells, which brought the 

 " heaters " to a rich cherry red. The " heater " thus used would last 

 for several months. But when used on a battery of 120 cells it lasted 

 but a few weeks, when it was found that the platinum conducting wire, 

 under the clay surface, had vaporized, and the surface of the heater 

 was covered with beautiful microscopic crystals of platinum. 



