'Willesden Canvas' appears to resist the attack of moisture and of animal organisms: the copper-compounds which it contains do not interfere with its use as a painting ground, but the absence of soluble salts (chiefly sulphates) from it should be ascertained. This test is easily made by soaking a piece of the canvas weighing 50 grains in distilled water overnight, pouring off the clear liquid next morning, heating it to boiling, and added a few drops of barium chloride solution*. If a distinct precipitate be formed the canvas cannot be employed safely, but a mere cloudiness may be disregarded. It is just as well to test the watery extract for free acid by means of litmus paper, because an attempt has been made to remove the green colour of the Willesden canvas by a bath of dilute sulphuric acid, the presence of which is, on all accounts, to be avoided. 'Willesden paper' possesses properties similar to those of Willesden canvas, and may sometimes be found serviceable for work in oil-colours.,