Ointments are fatty compounds, of the consistence of butter, for external application. It is supposed that they should soften or liquefy when applied to the skin, so as to permit the medicament contained in them to exert its influence upon the cuticle or even to be absorbed, but a class of ointments, known as cerates, maintain their firm consistence when so applied, these being intended more as protectives than as medicinal agents. An ointment of the former class, such as zinc ointment, when applied to the skin upon lint, melts completely and soaks into the lint; while one of the second class, such as boric ointment, remains soft and like a plaster even when removed after a day. These considerations scarcely come within the compounder's province, except in so far as he should see that all ointments which he dispenses are capable of being spread easily upon lint. This is partly a matter of melting-point, but not entirely so, seeing that cocoa-butter, a substance as hard as wax, has a lower melting-point than lard; but it is generally recognised that ointments should melt at about 100° F., and cerates at about 120° F.

The most commonly employed bases or vehicles for ointments are (1) lard, (2) mixtures of oil and wax, (3) soft paraffin or vaseline, and (4) wool-fat or lanoline. Of these the least changeable through atmospheric influence or the action of chemicals is soft paraffin, next to it comes wool-fat, then lard and oil-and-wax mixtures, which are extremely liable to become rancid. Bases also differ in properties ; thus, in respect to absorptive power they are notably distinct. Physiological experiments by several observers place the bases in the following order, as regards rapidity of absorption of chemicals from them: first, wool-fat; second, lard and combinations like simple ointment; and a bad third, vaseline. It has been found that chemicals compounded with lanoline applied to the skin show themselves in the urine before corresponding compounds with other bases. Again, a strychnine ointment made with lard (1 in 4) applied to the shaved scalp of a 12-lb. dog killed it in twenty minutes, but a similar ointment made with vaseline produced no effect. On the other hand, Dr. A. P. Luff experimented with ointments of potassium iodide, by an exosmosis process- the ointment was put in a bladder, the bladder in water, and the test-reagent added to the water.

It was found that the vaseline ointment gave the iodide reaction in one hour, the lard one in nine hours, and the lanoline none at all at the end of twenty-four hours. The experience of others is entirely opposed to Dr. Luffs conclusion that paraffin ointments are most absorptive.

Another characteristic which distinguishes the ointment-bases is their relative miscibility with water. A good deal of work has been done on this subject, but the following results by Scoville and Loftus are conclusive. They represent parts by weight of the respective fluids absorbed by 100 parts of the bases after diligent mixing:

In comparing these figures the fact should not be overlooked that the volumes do not differ so greatly, owing to the greater densities of the salt solutions compared with water.