394r I'\}ote and Langley — Indirect Method for 



The problem consisted in preparing various known mixtures 

 of the two oxides under definite conditions and determining 

 the densities of the mixtures. 



Jlethods and Errors. 



We first investigated the method to be used to determine 

 the density. The mixed oxides as they are obtained in anal- 

 ysis are very fine powders. After some preliniinary work the 

 following method for determining density was adopted. The 

 ordinary form of bottle pycnometer of 10'^'' capacity was nsed. 

 The stopper was carefully ground to fit, using fine carborundum 

 powder. The capacity was redetermined from time to time, 

 but it lemained nearly constant. The unweiglied material was 

 finely powdered and placed in a small beaker half full lof 

 water. The water was boiled hard for a half hour by passing 

 an electric current through a fine spiral of platinum wire sus- 

 pended in the water, the powder being stirred up a number of 

 times during the operation. The water could not be boiled in 

 a beaker over a flame on account of severe bumping, as the 

 powder settles rapidly. The boiling can be accomplished over 

 a flame, however, if a rougli platinum dish is substituted for 

 the beaker and the liquid stirred continually. The contents 

 of the beaker were cooled, most of the water poured off, and the 

 residue washed through a funnel into the pycnometer with 

 boiled water. Any powder on the neck of the pycnometer 

 was washed down and the pycnometer filled to overflowing. 

 Any powder or air bubbles floating on the top were swept off 

 with a glass rod. Air bubbles did not usually adhere to the 

 sides, but were always looked for and removed with a wire if 

 present. The pycnometer was placed in a tank of water and 

 kept at 20° C. for twenty minutes. The stopper was then 

 inserted, the top quickly wiped off, and the whole dried and 

 weighed at once. The contents Avere transferred to a platinum 

 dish, evaporated to dryness, and ignited finally over a blast for 

 five minutes. In transferring the contents of the pycnometer 

 to the dish it was found that all loss could be avoided if the 

 pycnometer were inverted over the dish and shaken slightly till 

 the powder left the bottom. By then bringing the mouth 

 against the side of the dish the mixture will run out quietly. 

 Two or three rinsings with water are necessary. The pycnom- 

 eter usually contained a trace of oxide sticking to the sides, 

 which might weigh as much as two milligrams. It was there- 

 fore dried at 120° C. and weighed to determine the small 

 amount of oxide. This method gave results which were en- 

 tirely' satisfactory for our purpose. The average difference 

 between duplicates in forty determinations of density amounted 



