which they

Under the

which the yellow fever is characterized. Thus I have shewn by a long series of experiments*, in which the galvanic excitation, serves to measure the state of irritability of the organs, that chemical agents excite the nerves not only from the qualities which are peculiar to them, but also from the order in are applied after one another. torrid zone, where the barometrical pressure and temperature of the air, are nearly the the same throughout the whole year, and where the clectrical tides, the direction of the wind, and all the other meteorological variations succeed one another with an immutable uniformity, the organs of the man, habituated from his birth in his native climate, to the same impressions, become sensible to the smallest changes of the surrounding atmosphere. From this extreme sensibility, the inhabitant of the Havanah, transported to Vera Cruz while the vomito is committing the most cruel rarisk vages there, runs sometimes the same as persons not seasoned to the climatet. I

* Experiments on the irritation of the muscular and nervous fibre (in German) v. ii. p. 147. The second volume of this work which appeared after my departure from Europe, has never been translated into French.

+ M. Pugnet (sur les fievres de mauvaise charactere, p. 346) made the same observations with respect to the natives of Sainte-Lucie, who visited the neighbouring islands.

say sometimes, for in general the examples are as rare, of persons born in the West Indies being attacked with the yellow fever. at Vera Cruz, the United States, or Cadiz, as of negroes falling victims to this disease*.

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It is also a very remarkable phenomenon, that in the equinoctial regions, at Vera Cruz, the Havanah and Portocabello, the natives have nothing to fear from the yellow fever, while in the temperate zone, in the United States, and in Spain, the natives are as much exposed to it as strangers. Are we not to seek for the cause of this difference, in the uniformity of the impressions received by the organs of the inhabitant of the tropics, surrounded by an atmosphere which varies very little in its temperature and electrical tension? Perhaps also the mixture of putrid emanations is always the same, on a soil constantly heated by the rays of the sun, and covered with organic wrecks. The inhabitant of Philadelphia sees a winter like that of Prussia, succeeded by a summer equal in heat to that of Naples; and notwithstanding the extreme flexibility which we observe in the organization of the people of the north, he can never as it were

* Luzuriaga, T. i. p. 133. M. M. Blane and Carey state that fifteen negroes, male and female, died of the yellow fever in the island of Barbadoes, and at Philadelphia.

season himself to the climate of his native country.

The whites and the mestizoes who inhabit the interior table land of Mexico, of which the mean temperature is 16° or 17°*, and where the thermometer sometimes falls below the freezing point, are more liable to contract the vomito, when they descend from l'Encero to the Plan del Rio, and from thence to la Antigua and the port of Vera Cruz, than the Europeans or inhabitants of the United States, who come by sea. The latter passing by degrees into the southern latitudes, are gradually prepared for the great heats which they experience on landing; but the Spanish Mexicans on the other hand, change suddenly their climate, when in the space of a few hours, they are transported from the temperate region, to the torrid zone. The mortality is very great, especially among two classes of men very different in their habits and modes of living; the muleteers (arrieros) who are exposed to extraordinary fatigues in descending with their beasts of burden, by tortuous roads like those of Saint Gothard, and the recruits destined to complete the garrison of Vera Cruz.

In late times every imaginable care has been bestowed on these unfortunate young men, born

* 60°. 8 and 62°. 6 of Fahr.

on the Mexican table land at Guanaxuato, Toluca, or Puebla, for the purpose of preserving them from the influence of the deleterious miasmata of the coast, without success; they have been left for several weeks at Xalapa, to season them gradually to a higher temperature; they have descended on horseback, and by night to Vera Cruz, that they might not be exposed to the sun in crossing the arid plains of la Antigua; they have been lodged at Vera Cruz in well aired apartments; but it has never yet been observed, that they were attacked with the yellow fever with less rapidity and violence, than the soldiers for whom these precautions had not been taken. A few years ago, from a combination of extraordinary circumstances, of three hundred Mexican soldiers all between the age of eighteen and twenty-five, two hundred and seventy-two perished in the course of three months; and at my departure from Mexico, the government began at last to think of entrusting the defence of the town and castle of San Juan d'Ulua, to companies of negroes and men of colour seasoned to the climate.

In the season when the vomito rages with great violence, the shortest stay at Vera Cruz, or in the atmosphere which surrounds the city, is sufficient to communicate the disease to persons not seasoned to the climate. The

inhabitants of the city of Mexico when they propose to sail for Europe, dreading the insalubrity of the coast, generally remain at Xalapa, till the moment of the departure of their vessel. They set out on their journey during the cool of the night, and cross Vera Cruz in a litter, to embark in the boat which awaits them at the mole; and yet these precautions are sometimes useless, and it happens that these very persons are the only passengers who sink under the vomito, during the first days of the passage. We might admit that in this case, the disease has been contracted on board the vessel, which remained in the port of Vera Cruz, and which contained deleterious miasmata; but the celerity of the infection is more incontestibly proved, by the frequent examples of the better sort of Europeans, dead of the vomito, though in arriving at the mole, they may have found litters ready to commence immediately the journey to Perote. These facts appear on a first view in favour of the system, according to which the yellow fever is considered as contagious under all the zones. But how are we to conceive that a malady is communicated at great distances*, while at Vera Cruz it is decidedly not contagious by immediate contact+? Is it not easier * Contagium in distans.

+ Contagium per intimum contactum.