6 



THE ATOMIC WEIGHTS OF 



filtered. ' ' Quantitative " paper and a platinum funnel were used. 

 The paper had been specially washed with hydrochloric and 

 hydrofluoric acids, followed by large quantities of water; finally 

 it was treated with a hot solution of sodium carbonate before the 

 main solution was filtered. 



Traces of calcium carbonate which may contaminate the 

 filtrate were effectively removed by a subsequent operation, con- 

 sisting in the exposure of the solution of the sodium carbonate 

 (contained in a large platinum-lined dish) to an atmosphere of 



FIG. 3. 



carbon dioxide for many days. The resulting sodium bicarbonate 

 was then drained and washed centrifugally and the precipitation 

 as bicarbonate repeated three times. Needless to say, platinum 

 utensils were used throughout. Finally the salt was gently ignited 

 to the normal carbonate. Spectroscopic tests showed the prepa- 

 ration to be free from calcium. 



Figure 3, in vertical projection, shows the apparatus used in 

 precipitating the bicarbonate. The arrangement was found quite 

 convenient for conducting the precipitation under pressure and 

 practically without interruption. The air in the upper reservoir 



