ATMOSPHERE. 



ATMOSPHERIC INFLUENCE. 



631 



atmosphere. In fact, if there be no chemical union, the above law of 

 the mixture of gases requires us to allow that each is an atmosphere 

 independent of the other, and that the two are most probably of 

 unequal height*. From some considerations, into which we cannot 

 here enter, Mr. Dalton thinks that the actual pressures exerted by the 

 oxygen and nitrogen are in the proportions of the volumes occupied by 

 them [AIR], that is as 1 to 4 ; and concludes that the oxygen, atmosphere 

 extends to 38 miles iu height, that of nitrogen to 54 miles, that of 

 carbonic acid to 10 miles, and that of aqueous vapour to 50 miles. It 

 must however be observed, that the state of the carbonic acid of the 

 atmosphere is very variable ; that there is not the same quantity by 

 night as by day, in moist weather as in dry ; and that the higher strata 

 of the atmosphere contain more of it than the lower, which may arise 

 from rapid absorption by the earth. This was observed by M. Saussure 

 in his ascent of Mont Blanc, who mentions the large quantity of car- 

 bonic acid at that height, which, with the rarity of the air, produced 

 in him haemoptysis from the lungs. 



Against the hypothesis just described, it might be asserted that the 

 air which Gay-Lussac brought down from a height of more than four 

 miles was not found to differ from that of the earth's surface in the 

 proportion of its oxygen to its nitrogen, which would be the case if the 

 oxygen atmosphere diminished in density more than in proportion to 

 the diminution of that of the nitrogen, or rice rend. We do not know 

 whether the experiment of M. Gay-Lussac was made, or even intended 

 to be made, with that degree of accuracy which would justify its being 

 'red a test of Mr. Dalton's theory; but in any case it is an 

 experiment which it was very desirable to repeat. Accordingly, in the 

 balloon ascents undertaken by Mr. Welsh and Mr. Green, under the 

 direction of the Kew Committee of the British Association, specimens 

 of air were collected at different elevations. A portion of air collected 

 in August, 1852, at an elevation of 18,000 feet, was analysed by 

 Professor Miller of King's College, and found to contain 20'88 per cent, 

 of oxygen by volume, while air collected at the same time at the 

 surface of the earth contained 20'92, a difference too slight to be of 

 any importance. Indeed, the results obtained by Regnault, Brunner, 

 Verver, and others, on air collected at different points of the earth's 

 surface, and subjected to different methods of analysis, do not vary 

 than s^th from the quantity of oxygen given by Dumas and 

 Boussingault as the result of numerous careful analyses, namely, 20'81 

 per cent, of oxygen per measure, and 79'19 of nitrogen, or by weight, 

 23'01 of oxygen and 76'99 of nitrogen. 



The weight of the whole atmosphere is calculated thus : The 

 weight of a column of it having a base = l sq. inch, is 14'7304 Ibs. ; 

 hence, taking the diameter of the earth = 7926 miles, we easily find 

 the weight of the atmosphere=ll'67085 trillions of pounds, which ia 

 al>out naAwxith Part f tne TO*B of the earth itself. 



The specific gravity of dry air is to that of mercury as 1 : 10513J, 

 and a cubic foot of it weighs 1'29 ounces ; hence, were the air of 

 uniform density throughout its mass, in order to exert the same 

 prMBure, it must be 26214 feet, or about 5 miles in height. This is 

 wliat is called the hd'ilit <>f a homoyeneoui atmosphere. 



We mentioned, under article AIR, that the average pressure on the 

 human body is nearly 15 tons. Now, since at one time, the barometer 

 in this country stands at about 31 inches, at another time at only 28 

 inches, it will be seen on calculation, that this will cause a difference 

 in pressure on the body of as much as 14 tons at different times. The 

 reason why we do not feel this change of pressure is, that when, in 

 very fine weather, the barometer stands at 31, the tone of the system 

 in strengthened, and we are more lively and active ; while in damp 

 muggy weather, on the other hand, we become sensible of listlessness 

 and inactivity. All the while too, the external and internal pressure 

 become very soon equalised, and the animal juices which in the one 

 case are secreted with increased energy, in the other arc but moderately 

 produced. 



For the colour of the atmosphere, see AIR and SKY. For the quan- 

 tity of moisture contained in it, see HYGBOMETRY. 



For the history of atmospherical researches, see the following names 

 in Bioo. Dry., HEBO, CTESIBIUS, GALILEO, TOBRICELLI, PASCAL, BOYLE, 

 MARIOTTE, PRIESTLEY, SCHEELE, BLACK, LAVOISIER, CAVENDISH, &c. 



The actual constitution of the atmosphere, whether composed of 

 molecules exerting a repulsive force upon each other or not, must 

 reiM.-uu unsettled until some mathematical hypothesis can be found 

 which shall satisfy all observed phenomena. That probabilities are at 

 present all on the side of the molecular or atomic hypothesis, is pretty 

 generally admitted ; and the repulsion of the several parts of air is a 

 fact of every-day experience. Newton entered upon this question, 

 .iii'l showed (' Principia,' book ii. prop. 23) that if the constitution of 

 the atmosphere be atomic, and if the force exerted by each particle 

 extend only to those nearest to it, and be either nothing or inconsider- 

 able as to all others, that then the observed proportionality of the 

 elastic force to the density is consistent with no hypothesis except that 

 of a repulsive force inversely proportional to the distances of the 

 {articles from each other; that is, which becomes double when the 

 distance is halved, and BO on. But in the scholium to the same pro- 

 pnHition, he takes notice of the imperfection of the hypothesis, and 

 describes his theory as a mathematical ' handle ' to induce philosophers 

 j ider the subject further. The molecular theory, on the suppo- 

 sition that every particle repels all the rest, or, which is as likely to be 



the case, has alternate spheres of attraction and repulsion, is beyond 

 the reach of the present state of mathematical analysis. 



ATMOSPHERIC AIR, a distinction which has been preserved after 

 the necessity for it has ceased. In the time of Priestley all gases were 

 called airs, and common air was called atmospheric to distinguish it 

 from other gases. It is sufficient simply to refer to the word AIR. 



ATMOSPHERIC INFLUENCE. In the following remarks it is 

 proposed to confine attention exclusively to the action of the atmos- 

 phere upon the materials usually employed in the arts of construction. 

 All organic bodies are no doubt affected, in a greater or less degree, 

 by the same influences which affect building materials ; but the myste- 

 rious power of life so distinctly modifies the action of the laws of mere 

 inorganic matter, that a sufficiently broad line of demarcation may be 

 observed between the chemistry of organic and inorganic bodies, to 

 justify the exclusive investigation of the action of the atmosphere on 

 inorganic matter in this place. Moreover, the striking circumstances 

 under which many of our recently erected public buildings have 

 decayed, and many valuable works of art have been lost, has of late 

 years attracted so much attention as to require more than a passing 

 notice. 



The atmosphere affects the materials used in the arts chemically 

 and mechanically ; and at the same time it acts as tho medium 

 through which moisture, heat, electricity, and other natural powers are 

 able to attack the elements of which those materials are composed. 



The chemical decompositions determined by the influence of atmos- 

 pheric agents, depend of course, upon the composition of the atmos- 

 phere itself ; and this is known to vary in an extraordinary manner in 

 different localities. The normal composition of air is considered to be 

 of 208 parts of oxygen to 792 of nitrogen, but there are also numerous 

 other gases present in it, such as carbonic acid, ammoniacal, hydro- 

 chloric, nitric, sulphuric, and sulphuretted hydrogen, the proportions 

 of all which are affected by local causes in every imaginable manner. 

 Thus the quantity of carbonic acid gas has been ascertained to vary 

 within the range of from 3 to 6 parts (in volume) in 10,000 ; the amount 

 of ammonia taken up by rain water in falling through the atmosphere, 

 is said to vary notably with the position in which the observations are 

 made, within a range of from 1 to 5'45, even according to M. Bous- 

 singault. The atmosphere of the large towns of England especially, 

 contains large quantities of sulphuric acid arising from the combustion 

 of coal, and according to Dr. Angus Smith, its amount may occa- 

 sionally attain to as much as '0042 per cent, in weight of the atmos- 

 phere. In Paris, the acetate and bydrosulphate of ammonia replace 

 the sulphuric acid of Engh'sh cities ; and on the sea shores hydrochloric 

 acid may be very distinctly traced, in consequence of the evaporation 

 from the sea water. 



The meteorological conditions of the atmosphere also are subject to 

 periodical variations ; but these are, generally speaking, of great regu- 

 larity. Thus the electrical state of the atmosphere attains, in clear 

 weather, two maxima and minima, in the course of the day ; the first 

 maximum occurring between 7 and 9 A.M. ; and the second, between 

 7 and 9 P.M. ; but of course, the hygrometric state of the air must 

 interfere with this phenomenon. The intensity of the sun's light, 

 and, as a necessary consequence, its actinic power, attains its maximum 

 rather before midday; and its minima, a few minutes before, and 

 after the termination or commencement of twilight. Upon the 

 average of the year, the maximum of the daily temperature occurs 

 about 2 P.M.; whilst the minimum appears to exist when the sun 

 occupies a position of about 14 47' below the horizon in the morning. 

 The atmosphere is driest about midday, and it contains the greatest 

 amount of moisture by night : but the deposition of dew takes place 

 with the greatest copiousness between midnight and sunrise, on 

 account of the cold then prevailing. In our latitudes the horary 

 differences in the barometric pressure are hardly perceptible ; but it 

 would appear that there is a normal tendency to produce a rise of the 

 barometer in the morning, and that the mercuiy falls about midday, 

 to rise again about sunset, and to fall again at midnight. These facts 

 lead to the inference that the meteorological fluctuations of every day 

 are affected by the relative positions of the sun and earth ; and that 

 they correspond, more or less closely, with the cardinal positions of the 

 former at its rising and setting, at midday and midnight. 



The rate of evaporation, and the amount of humidity in the atmos- 

 phere are amongst the most important conditions of its action ; and 

 it is to be observed, that the intensity of their respective actions is the 

 greatest at directly opposite seasons of tho year ; for evaporation takes 

 place to the greatest extent during the summer months, and is the 

 least ivpparent in winter, whilst the amount of humidity in the atmos- 

 phere is greatest in winter and least in summer. It thus happens that 

 the greatest amount of moisture is in suspension in the atmosphere, in 

 a state to be absorbed by porous materials, precisely at the season of 

 diminished temperature and exposure to the attacks of frost; and 

 again, evaporation takes place at the period when the conditions 

 of temperature are such as are most favourable to the production 

 and development of the salts generated by the previously absorbed 

 moisture acting upon the earthy bases. 



The various external agents above noticed produce their destructive 

 effects upon materials used in the arts, either by the new combinations 

 they superinduce between the earthy bases, the metals, ;ind the 

 metalloids ; or by the solution and removal of the combinations 



