Of this I have already treated in reference to its origin, properties, effects when internally administered, and those of its vapour when inhaled. (See pages 630 and 694.) Applied to the skin, it is slightly irritant, and produces a mild rubefacient effect; but it is not used in its undiluted state. in the form of ointment, either through its stimulant property, or by an alterative influence, it has a most favourable effect in obstinate chronic diseases of the skin, in which it is a very useful remedy. I have found it peculiarly efficacious in the scaly affections, in the advanced stages of eczema and impetigo, and in porrigo, favus, or scald-head. I know no local remedy superior to it in these affections, if any equal. it should not be employed in the early stages, nor during the existence of any general excitement; and, for fear of retrocession, it is best to apply it on different parts of the surface successively, when the disease is extensive. it is also sometimes useful as a stimulant to chronic and indolent ulcers. Tar-water is occasionally employed for the same purposes, but is less effectual.

Tar Ointment (Unguentum Picis Liquidise, U.S.) is made by melting together equal parts of tar and suet. it is sometimes proper to dilute it with a little lard, upon its first application in skin diseases, especially when there is any doubt of their sufficiently chronic character.