The alarming fatality consequent upon an epidemic reign of disease demands the closest scrutiny upon the part of communities, large or small, to guard against its approach or prevalence. Medical skill is unable to cope with the fearful onslaught of epidemics, and in many cases epidemic diseases are of so violent a character that the most vigorous constitutions succumb to the assault, and the profoundest medical skill and most rational medical treatment are unavailing. It is questionable if medical science will ever be able to materially decrease the rate of mortality that usually ensues upon epidemic reign; the subtle ethereal poison causing epidemics being of too violent a character to allow ascendency to be gained by material medicinal agents. Since it is questionable that mastery can be gained by medicinal or therapeutic agents, the proper remedy is suggested, not by investigating the best agents of cure, but in measures of prevention, as the author is quite certain that by proper knowledge and concerted action the spread of an epidemic can be limited, and its onset prevented. Epidemic diseases belong to the class which has been conveniently but inaccurately designated "zymotic."  They are generated, according to the most modern physiological doctrine, by a specific poison, introduced into the body from without, which is capable of causing morbid changes in the blood, and of destroying life. The poisons of varioius epidemic diseases are distinct inter se; the contagion of typhus, for instance, being altogether different from that of small-pox, and the contagion of cholera from that of diphtheria, and yet it is plain that they are all somehow related, and capable of gradual transmutation from one type into another. Some ancient types have died out -- the black-death, the sweating sickness, and the plague; but new types, undescribed by the old physicians, have arisen. We are able to note remarkable "waves of disease;" at one time the great mortality is from typhus, at another from small-pox, at another from scarlatina. In England they have recently had a succession of epidemic. The outbreak of cholera in 1866 was followed by typhoid fever, and as the latter began to abate in violence, scarlatina appeared in the most malignant form, and attacked the metropolis. This disease had begun a year and a half ago to decline in London, but at the same time it began to spread through other parts of the kingdom, where it has since raged distructively. A little later than the scarlatina, relapsing fever, which has been rare in these countries since 1849, broke out with great severity, also attacking London first, and, when it had spent its force there, extending itself into the provincial towns. Lastly, they have been visited with an epidemic of small-pox more severe than any outbreak of that disease which has been recorded in England during the present generation. And no sooner has the small-pox begun to abate its violence than they are threatened with a return of cholera. This periodicity of disease is yet to be explained; but it is established that, given the same conditions for the reception and propagation of contagion, about the same proportion of lives will be carried away, whether the prevailing epidemic be scarlet fever or typhus, or relapsing fever or small-pox.

The blood-poisoning of the zymotic diseases, which is thus various and changing in type, is traceable, however, to the same class of causes. In some epidemics the germs of contagion are far more volatile than in others, but, in all, we know by experience that, if we can isolate the patient and submit his immediate surroundings to disinfectant agents, we check the spread of the disorder.

Pure air and pure water are irreconcilably hostile to contagious disease. The first duty, therefore, of sanitary administration is the enforcement of effective ventilation, the supply of a fixed quantity of fresh air to every person in every house. This is an innovation which will of course be resisted both by ognorance and self-interest, but no infraction of real liberty will be committed in preventing ignorant or self-interested persons from doing mischief to the community by sowing the seeds of disease broadcast. The next step in the work of prevention is to insist upon a free and well-distributed system of sewers to carry away at once from every habitation the impurities which poison the air, and which, even when they do not directly progagate contagion, insidiously weaken the constitution of those subjected to their influence and prepare them for the reception of the germs of disease. Most country villages and many small towns are almost wholly destitute of systematic drainage, and cesspools, which are the commonest substitutions, are merely traps for infection. The enforcement of drainage and the abolition of cesspools are reforms which experience has shown will never be carried out by the local authorities, and is especially an improvement which ought to be and can be carried by pressure from a strong central executive office. A third precaution is systematic disinfection, not only of everything connected with and surrounding a person suffering from contagius disease, but of all places where dirt unavoidably accumulates, and whence at any time effluvium can be perceived to proceed.

These precautions, howver, though valuable tin themselves, and also as tending to effect the further object to which we are now able to refer, are quite unavailing unless supplemented by securities for a pure supply of water. Cholera, as we have seen, is held to be propagated almost exclusively through polluted water, and there is scarcely a form of epidemic that is not to some extent disseminated in the same way.

We have stated that it is doubtful if sanitary reform can ever be properly enforced by local authority, and hence, advocate that its requirement should be insisted upon by national statutes. The health of any country is as much a principle of political economy as its freedom and just as worthy, if not more so, of vigilance; and it is to be hoped that the day is not far distant when legislators in every land will see the absolute necessity to enact such laws rendering thorough disinfection and drainage obligatory upon all its citizens. Physicians have long advocated so desirable a reform, and neglected no opportunity to teach the people the virtue of and benefits to be derived from disinfection; but the absence of any epidemic gives a false sense of security, and the advices are unheeded until the deathly blast of the epidemic is upon them, when their folly is exposed and the wisdom of precaution established.

Of the disinfectants, the following are the best: -- Chloride of lime, Labarraque's solution, carbolic acid, and bromo-chloralum. Chlorine gas is probably the best, but not so practicable for universal use. Most of them are comparatively cheap, and no household should be without a sufficient quantity.

All cesspools, sinks, etc., should be thoroughly disinfected whenever they become offensive and exhale noxious vapors, and no pools of stagnant water or other filthy places should be permitted to remain undrained for any space of time. If such a desirable reform could become of universal operation, the reign of epidemics would be over and become things of the past.