l6o RESEARCHES UPON ATOMIC WEIGHTS. 



weighed U -tubes, one containing a concentrated solution of potassium hy- 

 droxide and solid potassium hydroxide and the other resublimed phosphorus 

 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 oxides of nitrogen was complete was 

 shown by the fact that no test for nitric acid could be obtained beyond the phos- 

 phorus 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 quantities of salt were desired, 

 three samples were prepared from ordinary silver nitrate and potassium di- 

 chromate and then were crystallized from nitric acid of the concentrations 3 

 normal, 0.8 normal, and 0.16 normal, respectively, glass vessels being em- 

 ployed throughout. 



Weighed portions of the silver dichromate were heated for 4 hours at 200° 

 in a current of pure dry air exactly as in preparing the salt for the silver analy- 

 ses. Then the weighed potassium hydroxide and phosphorus pentoxide tubes 

 were attached to the hard-glass tube, with a protection tube containing phos- 

 phorus pentoxide at the end. The silver dichromate was gradually heated 

 to complete fusion, and the air current was allowed to pass through the sys- 

 tem for 30 minutes 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 lubricated with Ramsay 

 desiccator grease. During the weighing one stopcock 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 together 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 o.oooio 

 gm. in 30 minutes. This quantity is applied as a correction 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 dichromate there 

 is little danger of decomposition of nitric acid or oxides of nitrogen by the oxi- 

 dized copper. It is to be noted that if the nitric acid is decomposed during the 

 experiment according to the following equation: 



2HNO3 = H2O + 2NO2 + 0, 



and is absorbed by the potassium hydroxide as NO2, there is a slight loss of oxy- 

 gen. The proportion of nitric acid present being very small, however, this 

 error could have no appreciable effect on the results. 



