128 Mixter — Heat of Combination of Acidic Oxides. 



may be briefly stated. When a mixture of impure boron and 

 carbon was burned in oxygen under pressure the boric oxide 

 produced volatilized and condensed as a fine white powder, and 

 considerable boron carbide was formed. Impure amorphous 

 boron and also crystalline boron containing aluminum reacted 

 with explosive violence with sodium peroxide. The heat of 

 oxidation of boron may, therefore, be easily found by the per- 

 oxide method when pure boron is available. 



Aluminium Oxide. 



The amorphous oxide nsed was prepared by igniting a pow- 

 dery form of hydroxide. For crystalline oxide, crystals of 

 corundum were taken. These were pulverized in a steel mor- 

 tar, the powder digested with hydrofluoric acid, then sulphuric 

 acid was added and the mixture heated until fumes of the latter 

 acid escaped. Then the oxide was washed. It was white and 

 was found to be free from lime, iron and silica, and to contain 

 a trace of magnesia. Both preparations were floated in water 

 and only the more finely divided portions retained. In order 

 to determine the alumina remaining after a combustion the 

 silver vessel containing the solid product was placed in half a 

 liter or liter of cold water. The fusion dissolved rapidly owing 

 to the presence of sodium peroxide. After solution the silver 

 piece was removed and an excess of nitric acid added and the 

 alumina filtered off. It was washed first with water and then 

 with ammonia to remove any silver chloride present. This 

 residue of alumina was deducted from that taken for an experi- 

 ment. There is nothing in the literature regarding the solu- 

 bility of ignited alumina in alkaline solutions other than the 

 statement that the more intensely the oxide is heated the slower 

 it is taken up by alkalies. In order to learn if the residue of 

 alumina mentioned in the experiment is likely to dissolve so 

 as to cause an error, the following tests were made with finely 

 divided alumina which had been heated in a platinum crucible 

 over a large blast lamp. In one test 4 grams of alumina and 

 20 grams of sodium peroxide were mixed and about 400 cc of 

 hot water were slowly poured upon the mixture. The violent 

 reaction between the peroxide and water gave at once a boil- 

 ing concentrated solution of sodium hydroxide. After a few 

 minutes an excess of nitric acid was added and the solution 

 filtered. The alumina found in the filtrate was 2*5 per cent 

 of the quantity taken. In another test about the same quan- 

 tities of the mixed oxides were added to the surface of warm 

 water. In this case no alumina went into solution. Since the 

 solid residues from the combustions were chiefly sodium car- 

 bonate and aluminate, and contained much less sodium peroxide 



