BAXTER AND JESSE. — ATOMIC WEIGHT OF CHROMIUM. 427 



tion of the nitric acid. This was effected by passing the current of air 

 containing the moisture and nitrogen compounds through two weighed 

 U-tubes, one containing a concentrated solution of potassium hydrox- 

 ide and solid potassium hydroxide and the other resublimed phospho- 

 rus pentoxide. The air current passed first through the potassium 

 hydroxide tube in order that moisture vaporized from the hydroxide 

 might be retained by the pentoxide tube. That the absorption of ox- 

 ides of nitrogen was complete was shown by the fact that no test for 

 nitric acid could be obtained beyond the phosphorus pentoxide tube 

 either with moist litmus paper or with diphenylamine. 



Since the three samples of silver dichromate were crystallized from 

 nitric acid of different concentrations, it was necessary to make separate 

 determinations of the moisture and nitric acid content with each sample. 

 Extreme purity of material was unnecessary, and, as rather large quan- 

 tities of salt were desired, three samples were prepared from ordinary 

 silver nitrate and potassium dichromate and then were crystallized from 

 nitric acid of the concentrations 3 normal, 0.8 normal, and 0.16 normal, 

 respectively, glass vessels being employed throughout. 



Weighed portions of the silver dichromate were heated for four hours 

 at 200° in a current of pure dry air exactly as in preparing the salt for 

 the silver analyses. Then the weighed potassium hydroxide and phos- 

 phorus pentoxide tubes were attached to the hard glass tube, with a 

 protection tube containing phosphorus pentoxide at the end. The 

 silver dichromate was gradually heated to complete fusion, and the air 

 current was allowed to pass through the system for one half hour in 

 order to make certain that all the vapors expelled from the dichromate 

 were carried into the absorbing tubes. The absorption tubes were then 

 reweighed. 



Before the tubes were weighed, they were carefully wiped with a 

 clean damp cloth and were allowed to stand near the balance case for 

 one hour. The tubes were provided with ground glass stopcocks lubri- 

 cated with Ramsay desiccator grease. During the weighing one stop- 

 cock in each tube was open to equalize the air pressure within and 

 without the tubes. In order to lessen the error in weighing, as well as 

 to save time and labor, the tubes were not weighed separately, but to- 

 gether as one system. Counterpoise tubes of the same shape and size 

 were always employed. Blank determinations showed that the air 

 current and manipulation of the tubes caused an increase in weight of 

 0.00010 gram in one half hour. This quantity is applied as a correc- 

 tion in every case. 



In place of a platinum boat a superficially oxidized copper boat was 

 used in these experiments. At the low temperature of fusion of silver 



