Tetraboron trioxide, B₄O₃

When boron dioxide is prepared, it is found that about five per cent, of the product consists of a substance insoluble in hydrochloric acid or sodium hydroxide but readily soluble in nitric acid with conversion into boric acid. This insoluble product, when dried at 100°, appears to consist of a hydrated oxide of the composition B₄O₃.2H₂O