Sulphur dioxide and sulphuretted hydrogen are present in volcanic exhalations and in many mineral waters.
The dioxide has been known since the earliest times and is found as a naturally occurring product in the gaseous exhalations of volcanoes and in solution in some volcanic springs.
The miasmatic exhalations caused by the sun playing on stagnant waters after the floods give rise to the "Sennar fever," which drives even the natives from the plains to the southern uplands.
Owing to the noxious exhalations of the surrounding forests the town is so extremely unhealthy during the hot weather as to have acquired the title of the "Abode of the Plague."
There are neither sulphuric exhalations nor escapes of steam at present, and it would seem that this great volcano is permanently extinct.
It is, however, a curious question how, considering the increase of carbonic acid by the decomposition of organic bodies and possible submarine exhalations of volcanic origin, the water has not in some places become saturated and a precipitate of amorphous calcium carbonate formed in the deepest water.
The amount of carbonic acid in solution may also be increased by submarine exhalations in regions of volcanic disturbance, but it must be remembered that the critical pressure for this gas is 73 atmospheres, which is reached at a depth of 400 fathoms, so that carbonic acid produced at the bottom of the ocean must be in liquid form.
Though the Boeotian climate suffered from the exhalations of Copais, which produced a heavy atmosphere with foggy winters and sultry summers, its rich soil was suited alike for crops, plantations and pasture; the CopaIs plain, though able to turn into marsh when the choking of the katavothra caused the lake to encroach, being among the most fertile in Greece.
Only of certain districts, however, can it be said that they are positively unhealthy; to this category belong some parts of the Holland provinces, Zeeland, and Friesland, where the inhabitants are exposed to the exhalations from the marshy ground, and the atmosphere is often burdened with sea-fogs.
Though rain seldom falls, exhalations from the river, especially when the flood has begun to subside, render the districts near the Nile damp during September, October and November, and in winter early morning fogs are not uncommon.
At high-tide, accordingly, the town presents a very attractive appearance, but at low-tide, when the mud banks are exposed, it seems dirty and repulsive, and the noxious exhalations are extremely trying.
They are not exhalations like our daily colloquies and vaporous breath.