POUR

POUR, &c. For the cause of the elasticity of the atmosphere, see ELASTICITY; and for an explanation of its various operations, see METEOROLOGY. See also the articles ATMOMETER, CLIMATE, DEW, and METEOROLOGY, in the SUPPLEMENT.

The uses of the atmosphere are so various, that it is impossible to enumerate them. One of the most essential is its power of giving life to vegetables, and supporting that of all animated beings. For the latter purpose, however, it is not in all places equally proper: we shall therefore conclude this article with some remarks on

The Salubrity of the Atmosphere.—The air on the tops of mountains is generally more salubrious than that in pits. Dense air indeed is always more proper for respiration than such as is more rare; yet the air on mountains, though much more rare, is more free from phlogistic vapours than that of pits. Hence it has been found, that people can live very well on the tops of mountains where the barometer sinks to 15 or 16 inches. M. de Saussure, in his journey upon the Alps, having observed the air at the foot, on the middle, and on the summits of various mountains, observes, that the air of the very low plains seems to be the least salubrious; that the air of very high mountains is neither very pure, nor, upon the whole, seems so fit for the lives of men, as that of a certain height above the level of the sea, which he estimates to be about 200 or 300 toises, that is, about 430 or 650 yards.

Dr White, in the 68th volume of the Phil. Trans. giving an account of his experiments on air made at York, says, that the atmospherical air was in a very bad state, and indeed in the worst he had ever observed it, the 13th of September 1777; when the barometer stood at 30.30, the thermometer at 69°; the weather being calm, clear, and the air dry and sultry, no rain having fallen for above a fortnight. A slight shock of an earthquake was perceived that day.

The air of a bed-room at various times, viz. at night, and in the morning after sleeping in it, has been examined by various persons; and it has been generally found, that after sleeping in it the air is less pure than at any other time. The air of privies, even in calm weather, has not been found to be so much phlogisticated as might have been expected, notwithstanding its disagreeable smell.

From this and other observations, it is thought that the exhalations of human excrements are very little if at all injurious, except when they become putrid, or proceed from a diseased body; in which case they infect the air very quickly.

Dr Ingenhousz, soon after he left London, sent an account of his experiments made in the year 1779 upon the purity of the air at sea and other parts; which account was read at the Royal Society the 24th of April 1780, and inserted in the 70th vol. of the Phil. Trans. His first observations were made on board a vessel in the mouth of the Thames, between Sheerness and Margate, where he found that the air was purer than any other sort of common air he had met with before. He found that the sea-air taken farther from the land, viz. between the English coast and Ostend, was not so pure as that tried before; yet this inferior purity seems not to take place always. The Doctor's general observations, deduced from his numerous expe-

riments, are, "That the air at sea and close to it, is in general purer, and fitter for animal life, than the air on the land, though it seems to be subject to some inconsistency in its degree of purity with that of the land: 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."

The Doctor in the same paper transcribes a journal of experiments, showing the degree of purity of the atmosphere in various places, and under different circumstances; which we shall insert here in an abridged manner.

The method used in those experiments was to introduce one measure of common air into the eudiometer tube, and then one measure of nitrous air. The moment that these two sorts of elastic fluids came into contact, he agitated the tube in the water-trough, and then measured the diminution, expressing it by hundredths parts of a measure; thus, when he says, that such air was found to be 130, it signifies, that after mixing one measure of it with one of nitrous air, the whole mixed and diminished quantity was 130 hundredths of a measure, viz. one measure and 30 hundredths of a measure more.

"The different degrees of salubrity of the atmosphere, as I found it in general in my country house at Southal-Green, ten miles from London, from June to September, lay between 103 and 109. I was surprised when, upon my return to town to my former lodgings in Pall Mall Court, I found the common air purer in general in October than I used to find it in the middle of summer in the country; for on the 22d of October, at nine o'clock in the morning, the weather being fair and frosty, I found that one measure of common air, and one of nitrous air, occupied 100 subdivisions in the glass-tube, or exactly one measure. That very day, at two o'clock in the afternoon (it being then rainy weather), the air was somewhat altered for the worse. It gave 102. October the 23d, it being rainy weather, the air gave 102. October the 24th, the weather being serene, the air at nine o'clock in the morning gave 100. October the 25th, the sky being cloudy at 11 o'clock in the morning, the air gave 102. At 11 o'clock at night, from five different trials, it gave 105. October the 26th, the weather being very dark and rainy, the air gave 105, as before."

The air at Ostend was found by the Doctor to be generally very good, giving between 94 and 98. At Bruges, the air taken at seven o'clock at night gave 103. November the 8th, the air at Ghent at three in the afternoon gave 103.

November the 12th, the air of Brussels at seven o'clock P. M. gave 105½. The next day, the air of the lower part of the same city gave 106; that of the highest appeared to be purer, as it gave 104; which agrees with the common popular observation. November the 14th, both the air of the highest and that of the lowest part of the city appeared to be of the same goodness, giving 103. The weather was frosty.

November the 22d, the air of Antwerp in the evening gave 109½; the weather being rainy, damp, and cold. November the 23d, the air of Breda gave 106. The next day about 11 o'clock the air gave 102; the weather being fair, cold, and inclining to frost. At

seven o'clock it gave 103. Next day, being the 25th, the air gave 104; the weather being cold and rainy. The 26th it gave 103; the weather being very rainy, cold, and stormy. November the 27th, the air at the Moordyke close to the water gave 101$\backslash frac\{1\}\{2\}$; the weather being fair and cold, but not frosty. This spot is reckoned very healthy. November the 28th, the air of Rotterdam gave 103; the weather being rainy and cold. November the 29th, the air of Delft gave 103; the weather being stormy and rainy.

November the 30th, the air of the Hague gave 104; the weather being cold, and the wind northerly. The first of December the weather underwent a sudden change; the wind becoming southerly and stormy, and the atmosphere becoming very hot. The day after, Fahrenheit's thermometer stood at 54$^\{\backslash circ\}$; and the common air being repeatedly and accurately tried gave 116; and that preserved in a glass phial from the preceding day gave 117; and that gathered close to the sea gave 115.

December the 4th, the air of Amsterdam gave 103; the weather being rainy, windy, and cold. The day after, the weather continuing nearly the same, the air gave 102. December the 10th, the air of Rotterdam gave 101; the weather being rainy. December the 12th, being in the middle of the water between Dort and the Moordyke, the air gave 109; the weather being remarkably dark, rainy, and windy. December the 13th, the air of Breda in the morning gave 109; the weather continuing as the day before. And in the afternoon, the air gave 106$\backslash frac\{1\}\{2\}$; the weather having cleared up. December the 16th, the air of the lower part of the city of Antwerp gave 105, that of the higher part 104; the weather being rainy and temperate. December the 17th, the air of Antwerp gave 107; the weather continuing nearly as in the preceding day. December the 19th, the air of Brussels gave 109; the weather being rainy, windy, and rather warm. December the 21st, the air of Brussels gave 106; the weather being dry and cold. The next day the air and the weather continued the same. December the 23d, the air of Mons gave 104; the weather being rainy and cold. December the 24th, the air near Bouchain gave 104$\backslash frac\{1\}\{2\}$; the weather being cloudy and cold. December the 25th, the air of Peronne gave 102$\backslash frac\{1\}\{2\}$; the weather being frosty. December the 26th, the air of Cuvilli gave 103; the weather frosty. December the 27th, the air of Senlis gave 102$\backslash frac\{1\}\{2\}$; the weather frosty. December the 29th, the air of Paris gave 103; the weather frosty. 1780, January the 8th, the air of Paris gave 100; the weather frosty. January the 13th, the air of Paris gave 98; hard frost.

Thus far with Dr Ingenhousz's observations. His apparatus was a very portable one, made by Mr Martin, which in reality is the eudiometer-tube and measure as used by Mr Fontana before he made his last improvement. "The whole of this apparatus (says Dr Ingenhousz) was packed up in a box about ten inches long, five broad, and three and a half high. The glass-tube or great measure, which was 16 inches long, and

divided into two separate pieces, lay in a small compass, and could be put together by brass screws adapted to the divided extremities. Instead of a water trough, such as is used commonly, I made use of a small round wooden tub," &c.

The abbé Fontana, who has made a great number of Fontana's very accurate experiments upon this subject, gives his opinions in the following words: "I have not the least hesitation in asserting, that the experiments made to ascertain the salubrity of the atmospherical air in various places in different countries and situations, mentioned by several authors, are not to be depended upon; because the method they used was far from being exact (A), the elements or ingredients for the experiment were unknown and uncertain, and the results very different from one another.

"When all the errors are corrected, it will be found that the difference between the air of one country and that of another, at different times, is much less than what is commonly believed; and that the great differences found by various observers are owing to the fallacious effects of uncertain methods. This I advance from experience; for I was in the same error. I found very great differences between the results of the experiments of this nature which ought to have been similar; which diversities I attributed to myself, rather than to the method I then used. At Paris I examined the air of different places at the same time, and especially of those situations where it was most probable to meet with infected air, because those places abounded with putrid substances and impure exhalations; but the differences I observed were very small, and much less than what could have been suspected, for they hardly arrived at one-fiftieth of the air in the tube. Having taken the air of the hill called Mount Valerian, at the height of about 500 feet above the level of Paris, and compared it with the air of Paris taken at the same time, and treated alike, I found the former to be hardly one-thirtieth better than the latter.

"In London I have observed almost the same. The air of Islington and that of London suffered an equal diminution by the mixture of nitrous air; yet the air of Islington is esteemed to be much better. I have examined the air of London taken at different heights (for instance, in the street, at the second floor, and at the top of the adjoining houses), and have found it to be of the same quality. Having taken the air at the iron gallery of St Paul's cupola, at the height of 313 feet above the ground, and likewise the air of the stone gallery, which is 202 feet below the other; and having compared these two quantities of air with that of the street adjoining, I found that there was scarce any sensible difference between them, although taken at such different heights.

"In this experiment a circumstance is to be considered, which must have contributed to render the above-mentioned differences more sensible: that is, the agitation of the air of the cupola; for there was felt a pretty brisk wind upon it, which I observed to be stronger and stronger the higher I ascended; whereas in

in the street, and indeed in all the streets I passed through, there was no sensible wind to be felt. This experiment was made at four in the afternoon, the weather being clear. The quicksilver in the barometer at that time was 28.6 inches high, and Fahrenheit's thermometer stood at 54°."

A few lines after, Mr Fontana proceeds thus:—"From this we clearly see, how little the experiments hitherto published about the differences of common air are to be depended upon. In general, I find that the air changes from one time to another: so that the differences between them are far greater than those of the airs of different countries or different heights. For instance, I have found that the air of London in the months of September, October, and November, 1778, when treated with the nitrous air, gave II, I, 1.90, and II, II, 2.25, which is a mean result of many experiments which differed very little from each other. The 26th day of November last, I found the air, for the first time, much better; for it gave II, I, 1.80, and II, II, 2.20; but the 14th of February 1779, the air gave II, I, 1.69 and II, II, 2.21; from whence it appears, that the air of this 14th of February was better than it had been six months before. There can be no doubt of the accuracy of the experiments, because I compared the air taken at different times with that which I had first used in the month of September, and which I had preserved in dry glass bottles accurately stopped."

This difference in the purity of the air at different times, Mr Fontana farther remarks, is much greater than the difference between the air of the different places observed by him: notwithstanding this great change, as he observed, and as he was informed by various persons, no particular change of health in the generality of people, or facility of breathing, was perceived.

Mr Fontana lastly concludes with observing, that "Nature is not so partial as we commonly believe. She has not only given us an air almost equally good everywhere at every time, but has allowed us a certain latitude, or a power of living and being in health in qualities of air which differ to a certain degree. By this I do not mean to deny the existence of certain kinds of noxious air in some particular places; but only say, that in general the air is good everywhere, and that the small differences are not to be feared so much as some people would make us believe. Nor do I mean to speak here of some vapours and other bodies which are accidentally joined to the common air in particular places, but do not change its nature and intrinsic property. This state of the air cannot be known by the test of nitrous air; and those vapours are to be considered in the same manner as we should consider so many particles of arsenic swimming in the atmosphere. In this case it is the arsenic, and not the degenerated air, that would kill the animals who ventured to breathe it."