[263]. The same reasoning will explain why English hops, that contain more Gallic Acid and Tannin than those imported from the Continent, are found to be superior as preservatives of beer.
[264]. Therapeutics, vol. 2. p. 470.
[265]. This subject has been ably illustrated by Mr. R. Phillips, in his translation of the London Pharmacopœia, by a series of vary striking and instructive diagrams.
[266]. See my work on Medical Chemistry: Sect. Precipitation.
[267]. An ingenious application of this law has been made for the purpose of purifying Epsom Salts. See Magnesiæ Sulphas; and also my work on Medical Chemistry, Art: Solution.
[268]. Amœnitates Academ; T. 7. p. 307.
[269]. See also a paper in the Medical Transactions, vol. 2. entitled, “Several extraordinary instances of the cure of Dropsy, by George Baker, M. D. Read September 9, 1771.”
[270]. The most subtle of all poisons,—the matter of febrile contagion,—is certainly modified in activity by the degree of moisture in the atmosphere influencing its solubility; the Plague is said to be most common in Egypt after the inundation of the Nile, a period at which the atmosphere is necessarily saturated with water; according to the account of Sir Robert Wilson, the English and Turkish armies that marched to Cairo escaped contagion, while the troops that remained stationary on the moist shore of Aboukir, were very severely visited. On the other hand, the Harmattan, a wind experienced on the western coast of Africa, between the Equator and fifteen degrees North Latitude, blowing from north-east towards the Atlantic, and which, in consequence of its passage over a very extensive space of arid land, is necessarily characterized by excessive dryness, puts an end to all Epidemics, as the Small Pox; and infection at such a time does not appear to be easily communicable even by art. Philosophical Transactions, vol. 21.
The difficulty of communicating infection to animals during a dry state of the air, as remarked on the Western Coasts of Africa, during the blowing of the Harmattan, agrees with some observations on Plague by the French physicians, as this complaint first made its appearance in the French army during a moist state of the air in Syria, when it lay under the walls of Jaffa in February, 1800.
It is a well known fact that volatile bodies are sooner converted into a gaseous state by the presence of water in the atmosphere; this is strikingly exemplified by the greater rapidity with which Limestone is burnt and reduced to quick-lime in moist weather, and by the assistance which is rendered in a dry season, by placing a pan of water in the ash-pit; so again the perfume of flowers is most sensible when the air is humid, as during the fall of the evening dew, or in the morning when the dew evaporates, and is dissipated by the rays of the rising sun; for the same reason the stench of putrid ditches and common sewers, is conveyed to the organs of smell much more speedily in summer previous to rain, when the air is charged with moisture.