Air as medium is surprisingly prevalent in both modern and contemporary art, fascinating artists by its semi-presence.
Invisible to the eye, it yet possesses properties which verge on the tangible: it can be heated or cooled; gusted into galleries; scented, compressed or even cursed.
Marcel Duchamp was probably one of the first to (literally) capture such paradoxes with his 1919 'Air de Paris', a labelled ampoule of Parisian ether (left).
Unlike most air works produced by subsequent artists, however, Duchamp's bottled essence remains permanently trapped; unwitnessed in any physical sense, yet nonetheless 'felt' due to the nostalgic, emotionally-driven associations with place engendered by its presence.
Yves Klein, whose work is in various ways fundamental to the history of the immaterial art object, was also fascinated by the properties of air, seriously envisaging it as a potential building block within a utopic architecture of elemental forces.
Hundreds of sketches and plans, as well as many essays, articulate Klein's earnest investigations into the use of air and other natural forces as a revolutionary new form of construction.
Film footage exists of the artist using compressed air to deflect a flow of water - an experiment mounted to demonstrate the viability of a 'toit d'air', or air roof (above and below) - and for several years Klein liaised with leading architects and designers in a quest to investigate the possibilities of elemental architecture. 
Yet none of these unlikely grand designs ever came to fruition before Klein's untimely death, leaving other artists to turn his architecture of air into something of a reality.
1. Particularly fascinating - and more or less feasible - plans drawn up with fellow artist Jean Tinguely included unrealised schemes to employ aeromagnetics or air/hydrogen mixes to create levitating sculpture that would do away with the need for the classical plinth.
Dutch artist Marinus Boezem (born 1934) was arguably the first to work with air in something of the sense that Klein had anticipated.
His contribution to the 1965 Amsterdam exhibition 'Show V' - one of the first to introduce conceptual art to the Netherlands - consisted of three 'Air Doors' defined by currents of warm and cold air directed into the gallery space.
Although a modest intervention compared to Klein's more ambitious plans, the work successfully demarcated space using wholly immaterial means.
Boezem's interest in air as a sculptural medium continued throughout the 1960s and into the '70s, although frequently combined with other materials, such as cloth made to undulate by natural or artificially produced wind currents (left).
The 1969 piece Signing the sky above the port of Amsterdam with an aeroplane can, however, be seen as another work entirely created in air and characterised by (eventual) absence. Exactly as stated in its title, an aircraft's condensation trails were used to spell out Boezem's surname in the cloudy sky above Amsterdam's harbour, the ephemeral wording disappearing almost as soon as it was created .
In 1966, U.S. artist Michael Asher used self-contained air blower units to create his Vertical Columns of Accelerated Air in the Newport Harbor Art Museum, California and New York's Whitney Museum of American Art.
These invisible yet palpable works were installed with close regard to each venue's existing architecture, the intention being to subtly redirect visitors' routes through each building while simultaneously modifying their experience of its presence.
See also: the empty gallery
The Air Conditioning Show first took place in 1972 at The Visual Arts Gallery, New York, six years after its initial proposal by British artists Terry Atkinson and Michael Baldwin, founding members of the Art & Language collective.
Underpinned by theoretical and investigative critique, the installation was intended to be as imperceptible as possible, the air conditioning units simply serving to maintain a steady temperature in the empty gallery.
The point of the work was to focus attention on the multitude of 'hidden' economic, social and cultural contexts surrounding the commercial gallery as a site of art display; a thesis expounded at length in the weighty texts which accompany the piece (left).
Interestingly, the wordplay evoked for today's audiences by the term 'conditioning' (although probably fortuitous) provides a particularly neat entry point into the work's insistence that all art consumption is essentially mediated - and is certainly more readily assimilable than the Art & Language texts themselves.
In contrast to the theoretically weighty artworks outlined above, Jeppe Hein's Smoking Bench (above) might seem something of a one-liner.
It's certainly true that the Danish artist's insistence on playful interactivity is intentionally conceived as a refreshing antithesis to the high-minded, often somewhat alienating earnestness of much conceptual art. But beneath the crowd-pleasing immediacy lie stealthy critiques of institutionalised space and its insistence on a particular kind of rationality and control; a subtext that aligns Hein's practice far more closely to that of artists such as Michael Asher and Art & Language (above) than its vastly differing execution might suggest.
Smoking Bench grants participants momentary invisibility by swathing them in a fine mist as they seat themselves, a process that can be viewed in a large mirror hung opposite.
A work which - with its neutrally toned, unadorned seating plinth and metallic expanse of mirror - appears to adopt a minimalist vocabulary, also turns out to 'minimalise' participants during their brief experience as vanishing art objects, while equally turning the movement's rigourous formalism abruptly on its head.
Invisible Labyrinth (above) is equally an exercise in absence/presence, but departs from (literal) smoke-and-mirrors effects by using a grid of infrared signals to demarcate various maze-like layouts.
Visitors to the labyrinth are equipped with digital headphones that vibrate whenever they collide with one of the invisible 'walls', thus guiding their path through apparently empty space.
Aire/air (2003) by Mexican artist Margolles fills space with air that is both physically and emotionally charged: water that has been used to wash the bodies of murder victims in Mexico City's morgues is either dispensed as a fine mist via humidifiers, or (in a variant presumably intended to counteract the unease of viewers confronted with a too-stickily-tangible manifestation of violent death), used to cool the air blown out of air-conditioning units.
Margolles, who studied forensic science and once worked in the morgues themselves, turned to art as a means of confronting the evidence of brutality and misery her former occupation involved: "I didn't know how to express myself in relation to human death".
Aire serves as an unseen yet tangible testament to lives themselves rendered invisible by violent crime.
Although French artist Loris Gréaud is perhaps most closely associated with extravagant gesamtkunstwerke such as his acclaimed 'Cellar Door' project, the notion of absence somewhat paradoxically underpins many of his most interesting works to date.
'Les Résidents', a piece featured in Gréaud's first important solo show (2005), consisted of a two-part, two-site installation.
Having first circulated a rumour via local radio that one of Paris' oldest mansion blocks was haunted, an apartment in the building was reconfigured by a team of architects in order to induce extreme unease in guests volunteering to spend a night on the premises.
The resultant floor plan was simultaneously replicated in a gallery space using blasts of cold air. A nod towards Klein's 'architecture of air', the permeable layout likewise transformed visitors into 'ghosts' capable of walking through walls, elliptically fulfilling the claims for the 'haunted' house on which it was based.