trary direction to the lower air; and that all this may probably contribute more to keep such a place healthy, provided the passages are open enough to receive the benefit of the circulation, than all the noxious fumes that are raised there can do to infect the atmosphere, and render it unwholesome. I am aware, indeed, that the smoke of common fuel (not to say any thing of the breathing of animals and other phlogistic processes,) is, besides the water which rises first, an heterogeneous matter capable of ignition; in short, an inflammable air. I know, likewise, that coal yields near a third of its weight in air, and that near five hundred thousand tons of this air is calculated to be dispersed in the atmosphere over London, and that it is supposed to be a fourth part of the whole quantity of air which is respired by the inhabitants of London in the course of a year. The levity of the mephitic air, and the constant rarefaction of the atmosphere of great towns, however, will always be the certain means of an incessant renovation of good air from the country, to replace that which shall have been exhausted. At the same time it must be acknowledged, that the pure air in the atmosphere of the country, exceeds that in the atmosphere of a populous city. Berg Bishop Watson. man, man, from experiment, declares, that the air of the country is two-thirds of fixed and phlogisticated airs, and one third of dephlogisticated air; but that the air in or near cities is much more contaminated. But, the very accurate and ingenious observations of Doctor Ingenhouz shew, that it is not from the country, or even by residing in it, that the air most beneficial to human existence is to be found: for he declares, that the air at sea and close to it, is in general purer, and fitter for animal life, than the air on land, though it seems to be subject to greater diversity in its degree of purity than that of the land and that probably, the air will be found in general much purer, far from the land than near the shore, the former being never subject to be mixed with land air. But, yet I should doubt, whether local situation, particularly low ground, might not be adverse to this doctrine in a general sense. By the heat of the sun, an acid vapour, we know, is exhaled from the sea; for such is positively proved, from the different states of the atmosphere in hot and cold climates. For instance, in Hudson's Bay and Russia, metals hardly ever rust; whereas in warmer regions they * Priestley. they always do, as well as in places abounding with salt springs. Be this, however, as it may, it is the variation in the pressure of the atmosphere, which in common affects animals, especially those of the human species. Thus in serene, clear weather, when the atmosphere is more heavy, we are generally more chearful and vigorous; the greater weight of the atmosphere bracing our fibres much better: but in foul and rainy weather, when the barometer is low, we are generally languid and dull; the air enclosed within the body being counterbalanced by a less external weight, distends the body, and occasions flatulencies, pains, and uneasiness. Besides this, the symptoms observed in our bodies in bad weather, may be in a great measure supposed to be owing to the dampness of the air at the time.* In the winter, when the weather is clear and frosty, the air has been generally found to be in the purest state; probably, because the corruption of so many substances is at that time retarded, and in a great measure hindered by the cold. In autumn, on the contrary, when a vast variety of vegetable and animal substances are hastened in dissolution and putrefaction by heat and moisture, the air is less pure. * Cavallo. The The rarefaction of the air has at all times a surprizing effect upon the animal constitution. At the height of six or eight thousand feet above the level of the sea, this effect becomes striking. The muscular force becomes exhausted; an universal faintness and lassitude prevails; the limbs, without frequent repose, refuse to perform their functions; the heart beats with an oppressive rapidity. This is all occasioned by the diminution of the compressive force of the air on the surface of the frame, whence the vessels suffer an unnatural dilatation. For instance, we may be supposed, ordinarily to bear a weight upon our bodies of 22033 pounds of air; but at the height of 7500 feet, this weight is diminished 5508 pounds, seven ounces, or exactly one quarter. And hence the absolute necessity of having a fresh circulation of atmospheric air in apartments, which have had fires, lights, and company, each of which act powerfully in exhausting the heavy, and in augmenting the light air. Not that fires, in all instances, are to be considered as injurious, but the contrary; for although they may owe their energy to the good air which they destroy, they yet must be allowed to be agents well qualified for the purification of the atmosphere, for the acid they furnish combines with the putrid particles, and destroy's De Saussure. their their power. The Egyptians, it should seem, were very early acquainted with this efficacy of fires. The Greeks also were early enlightened on the subject. Acron, we are told, reaped the greatest honour, during the raging of a plague at Athens, by ordering fires to be kept up near each person that was affected. Hippocrates went further, and had fires lighted up in every street, and in every public place.* Bergman, in considering the situations most probably endowed with the purest atmospheres, says, "that from the gravity of fixed air, it follows, that the lower strata of the atmosphere must abound more with it than the higher; for the prodigious quantity which is generated by fermentation, putrefaction, effervescence, and cther natural operations every day going on, for the most part remains close to the surface of the earth, and this must be again fixed by the continual generation of new bodies: and hence," concludes he," the reason I imagine is plain, why higher situations are in general more healthful than lower ones." The long life of man, we know, greatly depends upon the goodness of the air which he daily respires. The best aliments are not able * Plutarch. to |