| Colloidal solutions of boron may be prepared from boron obtained by the action of magnesium, sodium, or potassium on boron sesqui-oxide, by extracting the products of the reaction with hydrochloric acid and washing the residue repeatedly with water until the water acquires a decided colour. Two boron hydrosols have been obtained by the magnesium method: (i.) a relatively unstable sol, and (ii.) a comparatively stable sol. The first is dark brown in colour, and may be kept in a closed vessel for not more than three or four weeks. It is immediately coagulated by electrolytes, and rapidly oxidised by dilute nitric acid or hydrogen peroxide. It cannot be purified by dialysis. The second resembles the first in appearance, but may be kept for nine weeks and purified by dialysis. Two sols have been obtained by the potassium method; they resemble those just described. Three sols have been obtained by the sodium method, and are deep blue in colour. Two are unstable, but the third is stable and may be purified by dialysis. Boron hydrosols have a high electrical resistance; the boron particles are negatively charged. |