The investigation of gases anda their properties were late in the study of chemistry. Firstly, the term "gas" wa introduced by Van Helmont and Boyle was probably the first to collect a gas. in the middle of 17" century, some of chemists, such as Hales, Black, Cavendish, etc; described several experiments on gases. 
  1. van Helmont's theory on gases
  2. Black's experiments
  3. Cavendish's experiments
  4. Scheele's experiments
  5. Priestley's experiments
Van Helmont
Johann Babtista van Helmont was born in Brussels in 1579 and died on December 30, 1644. Van Helmont’s works in chemistry were published in 1648 as Ortus Medicinae (translated by John Chandler (as Oriatrike or Physick Refined, appear in 1662) and reprinted in Henry M. Leicester and Herbert S. Klickstein, A Source Book in Chemistry).

Figure. Johann Babtista van Helmont
Van Helmont’s chemical work is quantitative works. He made extensive use of balance, expressed clearly the law of indestructibility of matter and emphasized that metals when dissolved in acids are not destroyed but can be recovered again by suitable means.
Van Helmont was the first who used the term “gas”. The name gas is almost certainly derived by van Helmont from the Greek word “chaos”. He also the first clearly to realize the production of gas in various chemical processes. He says that gas is composed of invisible atoms which can come together by intense cold and condense to minute liquid drops. He recognized that gas may be contained in bodies in a fixed form, set free again by heat, fermentation or chemical reaction.
The kinds of gas mentioned by van Helmont are :
  1. The poisonous gas, extinguishing a candle flame, which collects in mines and in the Grotto del Cane, i.e. carbon dioxide.
  2. The gas carbonum, formed by burning charcoal and other combustibles, which is usually carbon dioxide but sometimes carbon monoxide.
  3. The gas forming in cellars, especially from fermenting wine (carbon dioxide).
  4. Gas formed by effervescence of sulfuric acid and salt of tartar or distilled vinegar and calcium carbonate (carbon dioxide).
  5. A poisonous red gas formed when aqua fortis acts on metals such as silver. This was nitric oxide.
  6. The gas evolved from aqua fortis and sal ammoniac in the cold (chlorine and nitrosyl chloride).
  7. The gas evolved in bubbles from Spa water, which then deposits an ochry sediment (carbon dioxide).
  8. The gas ventosum, which evolved in eructations, i.e. carbon dioxide.
  9. Gas pingue, which is inflammable, is evolved in putrefaction.
  10. A combustible gas, formed on dry distillation of organic matter (a mixture of hydrogen, methane and carbon monoxide)
  11. A sulphurous or acid gas (i.e. sulphur dioxide) which flies off from burning sulphur.
  12. Gas sylvestre from fused saltpetre and charcoal (carbon dioxide)
  13. An ethereal or vital gas, a kind of vital spirit of a gaseous nature, which is the reason why other gases act so swiftly and powerfully on the body.
Boyle Law
Robert Boyle was probably the first chemist to collect a gas. The most important thing that we can concluded from Boyle’e experiments is his law of the effect of pressure on air.
His interest in this phenomenon led him in 1662 to discover the law named after him, that the volume of a gas in inversely proportional to the pressure. He proved this experimentally both for pressures greater than atmospheric and pressures less than atmospheric, in the first case using the well known U-tube arrangement with mercury, and in the second case a straight glass tube containing some air confined over mercury, which could be raised in a vessel of mercury.

This is sometimes called Mariotte’s law, but Mariotte does not state it until 1679 and does not claim it as new. To explain the compressibility of air, Boyle compared its particles with small coiled springs, fleeces of wool, or little sponges, the lower “springs” in a column of air being compresses by the weight of those above them. (Mariotte uses the same analogies). He also remarks that the pressure could be explained as due to the motion of the particles, as assumed by Descartes: “The restless agitation of that celestial matter [the ether] wherein the particles swim, so whirls them round each other that each corpuscle endeavors to beat off all others from coming within the little sphere requisite to its motion about its own centre” – a good statement of the famous Theory of Vortices of Descartes.
Boyle concluded (General History of the Air, 1692) that air was composed of at least three parts:

(1) Vapours from water and living animals
(2) A very subtle emanation from the earth’s magnetism, producing the sensation of light
(3) A fluid, compressible and dilatable, having weight and able to refract light

Stephen Hales (1677 – 1761) studied at Corpus Christi College, Cambridge. His principal work, actually, is botanical. In his two books, Vegetable Staticks (1727) and Haemastaticks (1733), however, he has some interesting chemical observations. His chemical works is entirely qualitative.
Hales attempted to determine the amount of “air” which could be extracted from various substances by heating in a gun-barrel and collecting over water. The material he used were coal, red lead and saltpetre, iron filings and dilute sulphuric acid, and iron filings and dilute nitric acid. The various gases, all of gases which produced, were “air” to Hales. They were measured and then thrown away.

Figure 15. Stephen Hales (1677 – 1761)
In his book, Vegetable Staticks, Hales came to the vague conclusion that air abounds in animal, vegetable, and mineral substances and that its presence leavens them. He regarded air as an element.

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