We have had frequent occasion to remark, that all the metals possess a tonic power. We shall find that in this class of medicines there are many which seem to act chiefly by lessening irritability: there are some that produce tonic effects by their stimulus on the arterial system: others that more imperceptibly increase the general powers of the whole body. In the second class mercury must be arranged, and iron seems to unite the second and third: copper, arsenic, and probably silver, belong exclusively to the third.

While, then, mercury acts as a general stimulus, it seems to unite the tonic power of the other metals; and, from these considerations, all its powers may be explained. We mean not to prejudge the question of its anti-venereal influence: when we consider this subject we shall find some arguments in favour of its specific power. Yet we may now remark, that the effects just mentioned will in a great measure explain the changes it produces; and, though a specific power appears probable, it is scarcely necessary. We shall begin, however, with cutaneous diseases, as these are most nearly connected with the.principle we are endeavouring to lay down.

We took an early opportunity of observing, that we can seldom, in any instance, demonstrate any taint in the general mass; yet we added, that what appears in the secreted fluids must have been virtually, and perhaps formally, contained in it. The powers of nature separate these impure portions, and determine them to the most ready and extensive outlets, the urine and the skin; generally the latter. The constitutions most subject to chronic, cutaneous diseases, are distinguished by a general languor in the circulation: those subject to the acute kind, by an irregular determination to the surface. The former are our present subject; and we may add, that the obstructions appear on the skin, in the glandular system, or in the parts where, from the distance from the heart, its action has less power in consequence of this languor. If the circulation is properly supported, the matter is carried through the extreme vessels; if irregularly hurried, the depositions are more frequent and numerous. This position we shall have frequent opportunities of illustrating, parti-cularly in considering the eruptions of variola. By the steady, uniform, increased action, excited by mercurials,' the vessels obstructed in lepra and some similar diseases, regain their powers and throw off the accumulated masses: the same increased action prevents their recurring. Nor is this merely hypothetical; for one of the first effects of mercurials in these cases is to increase the eruption; in fact, to determine more copiously to the skin to throw off the offending matter. One difficulty indeed remains. If these humours possessed an assimilating power, the cause of their increase would continue, perhaps, in proportion to their evacuation. The general causes of cutaneous eruptions are not, however, of this kind; and the subject of syphilitic eruptions we reserve. When mercury has removed the more common species, a return must be guarded against. The matter seems to accumulate in spite of the powers of nature; and, as the continuance of mercurials would be inconvenient, less active medicines prevent a disease which they would not cure; and the saline or sulphureous mineral waters, sea water, or even laxative doses of salts, with a mild diet, will succeed. We need only add, that, though mercurials are alone sufficient, the addition of antimonials to assist their determination to the skin is eminently useful.

This enquiry, though it has detained us, will facilitate our future progress. Scrofula unites the cutaneous complaints with the common obstructions; and, in this case also, mercurials joined with, or followed, by neutral salts, arc particularly useful; and in our account of the effects of remedies, see Scrofula, we have hinted at the origin of the disease: a languor and want of irritability in the vessels. In gutta serena, indolent tumours of the viscera, particularly of the liver, in jaundice, constipation, and many obstructions, it is obvious that mercury must be useful on the same principle. In chronic inflammations of the liver, its action is particularly elucidated by a singular fact, the disease seldom yielding till the gums are affected by the mercurial; in other words, till the irritability of the vascular system is excited. Its use in old quar-tans seems owing to its influence on infarctions of the viscera; and in various dropsies independent of its evacuating powers, it is probably salutary by the same effects. In amenorrheca this action is peculiarly striking, since it is useful only in those languid inirri-table habits which we have been used to call phlegmatic and cachectic. In melancholy, the viscera arc commonly affected, and support a disease which other causes originally produced; and mercury is sometimes a very useful remedy. In chronic rheumatisms the inactive state of the vessels is sufficiently obvious, and mercurials are singularly useful.

One other class of diseases remains, viz. the spasmodic, as trismus, tetanus, and hydrophobia. On these subjects we can scarcely at present speak, but must refer to what we have said on the subject of 'irregular action,' as the effect of debility. The whole will be illustrated under the articles Convulsions and Fevers, q. v. But, if this idea be for a time admitted, the utility of mercury will be obvious, and experience has already established the fact. If useful in the croup, it is on the same principle; and the proof is the same as that adduced in speaking of its advantages in in-farcted liver: the complaint does not yield till we find proofs of the irritability of the vascular system being restored.

As an errhine and a sialogogue, (we now allude to the topical application of calomel,) it seems to act as a local stimulus only. As a sudorific and diuretic, it seldom acts without some assisting medicine more directly pointing to the different organs, and seems only to support the general action while the peculiar stimulus is supplied by the other ingredient of the formula. In general, its action in every disease is assisted by the medicines more peculiarly appropriated to it.