Used in the method above pointed out, sulphate of quinia is capable of interrupting almost any case of intermittent fever, from the mildest to the most violent. But different quantities are required in different varieties of the disease. The purely irritative intermittent, occurring independently of miasmatic influence, and sometimes ceasing spontaneously after a few paroxysms, may usually be checked by from six to twelve grains, given in each intermission. The ordinary miasmatic intermittent, though it will sometimes yield to the same quantity, is much more effectively treated with from twelve to twenty-four grains in the same time. The pernicious form of the same affection cannot be trusted to less than from twenty to sixty grains. The great rule, in the administration of the medicine, is to give enough to produce an obvious impression on the brain, and to maintain that impression until every vestige of a paroxysm shall have ceased. In many instances, the disease is at once arrested, and there is no return of the paroxysm, especially if the required impression has been produced some hours before the period for its recurrence. More frequently, perhaps, one additional paroxysm occurs, though with mitigated severity, and the succession is arrested at the second. Rarely does the disease pass on to a third. Sometimes, instead of having been quite set aside, the paroxysm returns in a very mild or partial form, without a distinct chill, and with very little fever, and altogether so slight that, in itself, it would scarcely attract attention. It is important, however, as indicating that the disease is not yet arrested, and that, unless the impression of the medicine is maintained, the paroxysms may again assume their original severity. It is not impossible that a quotidian, which has been once interrupted, may return on the following day as a tertian; and, in like manner, a broken tertian may assume the quartan type. These results may be guarded against by continuing the quinia beyond the period for the tertian or quartan paroxysm; though they are so rare that this caution may generally be dispensed with, and the plan only put in force in cases presenting this peculiarity. As a general rule, therefore, the medicine may be omitted immediately after the complete interruption of the paroxysms. It is of no advantage to continue with it regularly afterwards, with a view to prevent the septenary or bi-septenary recurrence of the paroxysms, to which the disease is liable; for a smaller daily amount than that originally necessary for arresting the disease, cannot be depended on for preventing these returns; and to persist in the use of so large a quantity may render the system at length insusceptible to its operation. The best plan for checking the weekly or bi-weekly returns, the latter of which are by far the most common, is, I think, to ascertain at what period, in any particular case, the paroxysm is disposed to recur, and to anticipate this by giving the patient, during the two days preceding that period, as much as was at first necessary to interrupt the disease; and to pursue this course every week afterwards for one or two months, or longer if necessary. The interrupted use of the medicine prevents the system from becoming accustomed to it; and, after a time, the disposition of the disease to recur ceases.

Writers speak of latent intermittent fever, in which the disease is masked under other forms. Thus, there may be a chill and perspiration, without fever, or a headache with loss of appetite, malaise, etc., recurring at the regular day and hour; or the effects of the cause may be shown in an attack of violent periodical neuralgia, or some special local affection, as diarrhoea or dysentery; and all such cases will yield to the antiperiodic treatment quite as readily as the regular disease. There seems, however, to be no necessity for considering these affections as masked intermittent fever. They are simply different morbid results of the same cause, and, like intermittent fever itself, are not exclusively of miasmatic origin, but may result also from other causes. But, whatever may be their source, they are equally amenable to the antiperiodic treatment.

Intermittent neuralgia is probably, next to intermittent fever, the most prevalent form of regular periodical disease. It may attack almost any part of the body, internal or external. As I have seen it. however, it is most common in or about the eye. Sometimes there is reason to believe that it is the effect of marsh miasm; but I have known it much more frequently quite independent of this cause. It is often probably of a rheumatic or gouty character, sometimes apparently the result of debility, occurring in the convalescence from other diseases; but quite as often, its source cannot be satisfactorily traced. I have never witnessed a case of this kind, if unconnected with organic disease, which has not yielded to sulphate of quinia. The ordinary doses of this medicine used in intermittent fever will often cure it; but they will often also fail. Double or even triple the quantity may be necessary to produce the desired effect.

Intermittent headache, especially hemicrania, may be considered as closely analogous with neuralgia, and is treated in the same way. But organic lesions of the brain are not unfrequently attended with severe pain in the head, assuming a somewhat regular periodical character, and liable, without caution, to be mistaken for pure functional neuralgia. In these cases, quinia may sometimes afford partial relief; but it is often quite powerless, at least in any quantity in which I have ventured to prescribe it; and, I believe, may do harm by over-exciting the already irritated nervous centres Rheumatism and gout are not unfrequently intermittent, in their nervous forms, whether as neuralgia, or painless disorder of function; and sometimes, also, in their inflammatory state. In either case, they yield in general readily to quinia.