What is the nature of the oxide of boron?
The most common oxide of boron is boron trioxide, $\ce{B2O3}$, which is the anhydride of boric acid:

$$\ce{H3BO3 -&gt; HBO2 + H2O}$$
$$\ce{2HBO2 -&gt; B2O3 + H2O}$$
$$\text{net} \ \ce{2H3BO3 -&gt; B2O3 + 3H2O}$$

As the anhydride of an acid, boron trioxide is acidic. It reacts with water to regenerate the acid:

$$\ce{B2O3 + 3H2O -&gt; 2H3BO3}$$

Boric acid's acidity does not derive from the dissociation of a proton (or abstraction of a proton by water), as does many acids, for example sulfuric acid:

$$\ce{H2SO4 &lt;=&gt; H+(aq) +HSO4- (aq)}$$
$$\ce{H2SO4 + H2P &lt;=&gt; H+(aq) + HSO4- (aq)}$$

Boric acid (written now as $\ce{B(OH)3}$, which is suggestive of its structure) instead reacts with water by forming a dative complex which is the active acid. A water molecule associates with the empty $p$ orbital on boron. This difference in behavior is enabled also by the electropositive character of boron. The acidic oxides of some metals behave similarly. 

$$\ce{H2O + B(OH)3 &lt;=&gt;H2OB(OH)3 &lt;=&gt;H+(aq) + B(OH)4^- (aq)}$$