Rubefacients are characterized by producing inflammation of the skin, without vesication. Though most of the more active substances belonging to the class will blister, if allowed to remain long in contact with the surface, they are less disposed to produce this effect, with the same amount of inflammation, than the epispastics, and are not used for the purpose. As before stated, there is a set of external irritants which, though they do not vesicate in the strict sense of the term, give rise to a pustular eruption. These, for the sake of convenience, I include as a subsection under the present head, as they fulfil similar indications.

The principles upon which the rubefacients operate therapeutically are the same, in general, as those of the epispastics, though with some differences which will be pointed out. But first it is necessary to understand that there are grades of power in the rubefacients, from the feeblest up to the most violent; so that we can obtain from them a speedy and powerful, or a slight and prolonged effect, as we may desire.

In relation, then, to their principles of operation, they agree with the epispastics in stimulating generally as well as locally, in producing the effects of a shock on the system, in their revulsive influence, their property of substitution, and their capability of causing pain. But they do not deplete materially, and cannot, of course, be used for obtaining a denuded surface.

In relation to their effects on the system at large, when a permanent and energetic effect is wanted, blisters are generally preferable to rubefacients; but for a very prompt and very powerful impression, we must have recourse to the most active of the latter class. Hence, these are preferably used in cases of sudden and extreme but transitory prostration; as in the cold stage of malignant and pernicious fevers, cholera, protracted syncope, and the sudden collapse from violent injuries; while blisters are more effectual, by their sustained effect in supporting the system, when support is necessary, after reaction; and are more efficient in breaking up morbid associations, as in paroxysmal diseases.

The breadth of surface over which the operation of the rubefacients may be extended, their more superficial action, and their less liability to be followed by sloughing, adapt them better to the treatment of that condition of debility, in the advanced stage of low fevers, in which the skin is cold, and the capillary vessels generally inert. They are, under these circumstances, greatly preferable to blisters.

In regard to the local effects of the two classes, when operating on the principle of revulsion, blisters are much more efficacious in inflammation, rubefacients in the more violent forms of nervous or functional disease. Thus, in a case of severe spasm of the stomach, or excessive nausea and vomiting, it is better to have recourse to the speedy and powerful operation of a mustard cataplasm; in gastritis, to the longer continued, though less energetic action of a blister.

It often happens that one of the active rubefacients may be used for its prompt effect, while blisters may afterwards come in to maintain the revulsive impression. Thus, in cases of sudden cerebral congestion, we apply a pair of sinapisms to the extremities, but afterwards, should the affection continue, put a blister on the back of the neck and between the shoulders.

At the very commencement of inflammation, when there may be some hope of breaking up the disease by a prompt and violent revulsive impression, the active rubefacients are sometimes preferable to blisters, not only from their greater quickness and energy, but also from the brief duration of their action, which is not carried onward with the disease, like that of blisters, should the effort to arrest it fail.

In chronic cases of slight but persistent inflammation, and in prolonged internal nervous disorder of a mild character, in which it is desirable to maintain a long-continued and steady, but feeble counter-irritation, the gentler rubefacients are better calculated than blisters to meet the indication, as less uncomfortable to the patient, and less disturbing to the system, while perhaps equally effective in eradicating the complaint.

The depletion produced by blisters, which is often considerable and effective, adapts them much better than rubefacients to subcutaneous inflammations, whether in the areolar and adipose tissue, or in the blood-vessels, nerves, or absorbents.

For obtaining the effects of pain on the system, the more powerful rubefacients are not less appropriate than blisters. The extreme pain of a sinapism is more likely to divert the mind from an imaginary trouble even than that of a blister, though the impression may not be so permanent.

The following are rubefacient remedies, which, in accordance with the plan of the work, though not properly medicines, require a brief notice in this place.