One remarkable peculiarity of some of these epidemics is, that natives of the region in which they prevail are for the most part unsusceptible to them. This is true however only of particular forms of pestilence. Some of them acknowledge no acclimation. Cholera, for example, attacks equally natives and new comers. On the other hand, yellow fever rarely attacks the natives who reside permanently within its zone. Its chief victims are strangers who have recently arrived within its sphere, particularly the inhabitants of northern climates. The susceptibility to its influence appears to be strictly proportionate to the degree of northern latitude from which the stranger has arrived, and the shortness of the interval that has passed since he left the European for the Equatorial regions. We see something of the same kind in the wide-spread epidemics of our own country. During the prevalence of Cholera it was observed over and over again, that persons coming directly from the pure air of the country into the infected part of a town, were seized with the disease. The explanation is not obvious. It would seem, however, to be connected with the suddenness of the shock on the system. Priestley found, that after shutting up a mouse in a given quantity of air a considerable time, it seemed to be weak, and to be slowly dying. If at this period he put a fresh mouse into the same air, it instantly died. It seems as if the system can bear a pestiferous atmosphere better when gradually than when suddenly exposed to it. I do not know that I can give a more vivid picture of a tropical epidemic than that which is afforded by the outbreak of Cholera in the 86th regiment at Kurrachee in June, 1846. On this occasion the atmosphere was very peculiar,—damp, hot, stagnant, and oppressive. Not a breath of air was stirring. A few isolated cases of cholera had occurred for some days. The utmost alarm was excited in the minds of experienced persons, who felt certain that an epidemic was at hand. Their fears were too fully realized. On the night of the 15th, upwards of 40 men were seized with cholera in its severest form; in two days more 256 were attacked, of whom 131 were already dead. “The floors of the hospital,” says Dr Thom, the surgeon of the regiment, “were literally strewed with the livid bodies of men labouring under the pangs of premature dissolution. Many were brought in with the cold and clammy damp of death; as if sudden obstruction of every vital function had taken place, and the fountains of life had been arrested by an invisible but instantaneous shock. It was indeed a sight never to be forgotten, to behold the powerful frames of the finest men of a fine corps, who had that morning been in apparent good health, and most of them on the evening parade, as if at once stricken down, and striving, with the last efforts of gigantic strength, to resist a death-call that would not be refused.”