72 



AIR AIR-PIPES. 



the tendency of u volume of confined air to occupy a 

 greater space. Inconsequence, ii presses f(|iially in 

 ull directions on the sides of the vessel containing it, 

 mid tliis pressure increases or diminishes in propor- 

 tion as the enclosed air is condensed or rarefied, pro- 

 vuled the teiiipenitnre remains the siune. The 

 dilutability of air has, according to the preceding 

 experiment, no limits. A bladtier, almost empty, 

 vill become inflated if placed in an exhausted re- 

 ci i\er. Klasiicity being the property of a body to 

 resume its original form as soon as the force which 

 changes it ceases, it is evident, from what we have 

 said, tliut it is a property of air. The different ap- 

 plications made of air in the different branches of art 

 are so various and numerous, that we cannot possibly 

 enumerate them. Of the chemical properties of air, 

 it will be sufficient to mention the following : the 

 Micients believed it a simple body, one of the four 

 elements ; modem chemists have discovered that it 

 IN composed of two bodies, apparently elementary, 

 oxygen and azote. The most accurate experiments 

 have shown tliat this fluid, taken from different parts 

 of the globe, and even at a great height, is com- 

 posed ot 21 parts of oxygen, 78 of azote, one part 

 of carbonic acid, and some atoms of hydrogen. The 

 air refracts the rays of light, and its power of refrac- 

 tion is in the ratio of its density. (See Refraction.) 

 It is capable of acquiring electricity, and it refuses, 

 when very dry, a free passage to the electricity 

 which tends to escape from electrified bodies. (See 

 Electricity.) When subjected to great heat or cold, 

 it is dilated or condensed, but undergoes no change 

 of properties. If it is suddenly compressed, much 

 heat is disengaged, with a bright light. It enters 

 bodies through the most minute pores, and adheres 

 to them strongly ; coal, particularly, absorbs a great 

 quantity of air. (See Carbon.) Water and all 

 liquids always contain it, and it can only be expelled 

 by a strong heat. Almost all combustible bodies 

 decompose it at a high temperature, which varies 

 with the different substances. They absorb its oxy- 

 gen with the disengagement of more or less caloric 

 and light, and form acids or oxydes : phosphorus, 

 however, combines at a low temperature with the 

 oxygen and azote of the air, and produces, with the 

 former, phosphorous acid ; with the latter, phosphur- 

 eted nitrogen ; the moisture of the air and the 

 melting ot the phosphorus favour these combina- 

 tions. When the air is brought into contact with 

 animal and vegetable substances, it changes them 

 immediately, particularly if it is moist, and gives to 

 some of them acid properties ; it bleaches flax, hemp, 

 bilk, and increases the brilliancy of many colours. 

 1 1 is indispensable to the life of all organic beings ; 

 an imals respire it incessantly, and decompose it ; a 

 l\rt of its oxygen is transformed into carbonic acid, 

 und this combination produces caloric, which contri- 

 butes principally to the preservation of animal heat. 

 (See Respiration.) Vegetables imbibe the carbon, 

 which the carbonic acid, diffused through the air, 

 contains. The air is the agent of combustion ; the 

 particles of bodies combine with its oxygen, and 

 evolve heat and light. (See Combustion.) Finally, 

 the air is the principal medium of sound. (See 

 Acoustics.) For further information, see the articles 

 Atmosphere, Gas, and Contagion. 



AIR, in painting, deserves the most acoirate study 

 of the artist, particularly of the landscape painter, as 

 it is the medium through which all objects are seen, 

 and its density or transparency determines their 

 appearance, both in respect to size and colour. It 

 softens the local colours, and renders them more or 

 le-s decided or characterized, producing what is 

 technically called tone. The appearances produced 

 by the interposition of the air differ with the climate, 



the season, and the time of the day ; and landscape 

 painters, who, in other respects, are not masters, 

 ha\e yi\en the greatest charm to their pictures by a 

 happy imitation of these appearances even where 

 the objects painted ]H>ssess in themselves very little 

 attraction. Ilackert, a (ierman, who was a long 

 time painter to the late king of Naples, c\ceK, per- 

 haps, in this branch of art, all modern painters. His 

 views on this subject are given in his life by Goethe. 



AIR, fixed. See (,'nx. 



Am, in music (in Italian, aria), at the present day, 

 means a continuous melody, in which some lyric 

 subject or passion is expressed, li was originally 

 opposed to the irregular declamation of recitative, 

 or the more staid action of choral music. Sanmaise 

 regards the term as derived from the Latin ni-'m. 

 The air appertained, consequently, to measured 

 music, and, whether constituted of one or of more 

 voices, this measured style (if not choral) was deno- 

 minated air. But in modem times, by way of dis- 

 tinction, the lyric melody of a single voice, accom- 

 panied by instruments, is its proper form of composi- 

 tion. Thus we find it in the higher order of musical 

 works; as in cantatas, oratorios, operas, and also, 

 independently, in concertos. It should be consti- 

 tuted of euphonic simple lyric strains. An air former- 

 ly supposed as its ground-work a particular state of 

 feeling or emotion, of a certain duration, expression, 

 and interest, to which the recitative is generally 

 preparatory. Formerly, too, as essential to an air, 

 a symphony, expressing the burden of the stanzas or 

 couplets of the song (ritornello, or refrain), was in- 

 troduced as tributary to the leading melody, which 

 was followed by another and less elaborate part, 

 forming the antithesis, to which was subsequently 

 added a repetition of the first part. Since the days 

 of Gluck and Mozart, these have declined, and other 

 forms have been adopted, particularly by Mozart, 

 more conformable to poetry, and more expressive of 

 the sentiments and situation of the singer. Still 

 Mozart could not entirely withstand the prevailing 

 taste, with reference to which he produced numerous 

 bravura airs, not always in character, yet not want- 

 ing in expression and effect. Another form of airs 

 are the cavatinos (or single strains), lately introduced 

 by the Italians, and calculated to add grace and 

 embellishment to the song. At the present day, the 

 Germans either adopt this or make use of other forms, 

 as the subject may require. Arietta signifies a short, 

 less elaborate air, designed to express a more simple 

 and transient emotion. Arioso is also applied to 

 music resembling the aria, and is inserted in single 

 lyrical passages to vary the recitative. 



AIR-GUN; an instrument for the projection of 

 bullets by means of condensed air. The ancients 

 were acquainted with the principles of its construc- 

 tion, and an instrument of this description was in- 

 vented by Ctesibius of Alexandria, who flourished 

 about 120 B. C. The first modern account of an 

 air-gun, which we meet with, is in the Elements 

 a" Artillerie of David Rivaut, preceptor to Louis 

 XIII. of France. 



AIR-PIPES ; a recent invention for the ventilation 

 of ships by means of the rarefying power of heat. 

 Mr Sutton, a brewer of London, is the inventor. 

 If the usual aperture to any fire be closed up in front, 

 and another be introduced by the side of the fire- 

 place, it will attract the current of air into that 

 direction ; and the coppers, or boiling-places of ships, 

 are well known to be placed over two holes, sepa- 

 rated by a grate, the one for the fire, the other for 

 the ashes ; there is also a flue from the tops for the 

 discharge of smoke. Mr Button's pipes, now, are 

 introduced into the ash-place, and carried through 

 the hold to any part of the vessel. The two holes 



