AS AFFECTED BY TEMPERATURE. 7 



THE MAKING OF A SPECTROGRAM. 



The apparatus used throughout the entire study of the effect of high 

 temperature has already been discussed. Two cells were used, one 10 cm. 

 and the other 1 cm. in length, both having the same general design and 

 differing only in length. 



The cell, placed in a bath suitable for keeping the temperature constant, 

 was arranged as indicated in the diagram, and the source of light so located 

 that the rays, reflected from a concave mirror, passed longitudinally through 

 the cell and formed an image of the Nernst glower on the slit of the camera. 

 The position of the prism is so adjusted as to fill the grating uniformly with 

 light. Holding the eye directly above the grating, in a position later to be 

 occupied by the photographic plate, we could easily tell when the light was 

 falling properly upon the grating. When the cell was correctly adjusted 

 the lights were extinguished and the photographic plate inserted. With 

 the plate in position, the light was turned on and an exposure made at room 

 temperature. The position of the plate was then moved a given distance, 

 and the temperature of the cell raised very slowly, this process being repeated 

 at intervals of about 20 or 25. 



It is clear that, with such pressures as are developed by heating water to 

 200, it is very difficult to obtain a tight joint between glass and metal. 

 This difficulty, however, has been partly overcome by the special form of 

 apparatus designed by Dr. Strong and described on page 6 of this mono- 

 graph. We were not able to secure a closing that would hold above 200, 

 but once a good closing was secured it was not necessary to remove the ends 

 for several operations. 



Great care had to be taken in heating the cell, on account of the difference 

 in expansion of the glass ends, and the metal in contact with them. When 

 the temperature was raised more than 40 an hour the glass ends usually 

 broke. At such high temperatures as we were employing the glass was 

 rapidly attacked by the water; later, when we were using the clear uviole 

 glass, a single heating rendered the glass ends almost opaque, especially if 

 they were allowed to stand for any length of time. 



It was found that in many cases precipitates would appear in the cell 

 at a temperature slightly above 100. This precipitate, however, formed 

 rapidly, once it began, and almost as quickly disappeared. By properly 

 regulating the intervals at which exposures were made, the effect of the 

 precipitate could be avoided; hence this effect does not appear on any of 

 the strips photographed. 



It is probable that slight hydrolysis took place at first, as 



2NdCl 3 + 3H 2 = 2Nd(OH) 3 + 3HC1 



The presence, then, of a slight excess of hydrochloric acid would hinder the 

 reaction in the direction indicated above by the arrow. Since most hydrox- 

 ides lose water at temperatures above 100, it is possible that the following 

 reaction would take place: 



2Nd(OH) 8 = Nd 8 O a + 3H 8 



