UTaiic/ies/er Meviflirs, Vol. Iv. (191 1), A^^. 10. 7 



sented by metaboric acid results. If this heatins:^ has, 

 however, taken place in a vacuum, the water evolved 

 being absorbed by a drying reagent, the metaboric acid is 

 a dry amorphous powder. Further, when a mixture of 

 boric anhydride and orthoboric acid in equimolecular 

 proportions is fused under pressure, a crystalline mass 

 results. 



The changes in vapour pressure on heating orthoboric 

 acid were next examined. 



The first experiments were carried out at 70°C. The 

 vapour tension of the acid at this temperature remained 

 almost perfectly constant for many days, when it suddenly 

 dropped to about one-fifth its original value. When this 

 drop in pressure occurred, a portion of the material was 

 withdrawn and analysed, when it was found to contain 

 23'4% water. (Theorj^ for metaboric acid 20'5 %.) 



The rest of the material was then heated to i8o°C. 

 and the further changes in vapour pressure observed. It 

 was necessary to employ this high temperature as the 

 vapour pressure of the meta acid would otherwise have 

 been too small to observe. 



The observations at i8o°C. were inconclusive as 

 regards th formation of other compounds. The vapour 

 pressure steadily decreased with time, and, though this 

 decrease was not at a constant rate, no definite and 

 marked changes occurred. 



Some experiments were carried out by the freezing 

 point method on the molecular conditions of the boric 

 acids in aqueous solution. Three portions of the ortho 

 acid were heated till one had the composition of metaboric 

 acid, the second pyroboric acid and the third was com- 

 pletely dehydrated. Solutions of these portions, as well 

 as the ortho acid, were then prepared of such a strength 



