Occurrence.—Argon makes up 1.3 per cent by weight of the atmosphere or 0.933 per cent by volume. It has been found in mineral waters, in the volcanic gases of Mount Pelee and also in fire damp and in coal as well as in gases given out on numerous minerals.
Preparation.— The various methods for the preparation of argon use air as the raw ma terial: (1) Sparking air with oxygen, the method originally used Cavendish in 1785. Know-• ing that air contained a considerable quantity of nitrogen, Cavendish raised the question whether all of the apparently nitrogenous part of the air °could be reduced to nitrous acid, or whether there was not a part of a different nature from the rest which would refuse to undergo that change." To decide this point' he added an excess of oxygen to air and passed electric sparks through the mixture (precisely as Rayleigh and Ramsay did) until no further diminution of volume occurred. He then re moved the excess of oxygen, together with the oxide of nitrogen that had been formed, and found that only a small bubble remained un absorbed, which, he says, was not more than one-120th part of the bulk of the original nitro gen. The bubble that he thus obtained and the nature of which he did not further question must have been nearly pure argon.
(2) Separation from °atmospheric" nitro gen, the nitrogen being absorbed by a metal. Red-hot magnesium shavings were used by Rayleigh and Ramsay, but this material is no longer used, as a mixture of five parts pure lime and three parts of magnesium dust has been found preferable. 13y passim- "atmos pheric" nitrogen over this red-hot mixture and the residual gas over metallic calcium at a dull red heat, Maquenne readily effected complete absorption of the nitrogen ; metallic lithium has also been utilized for this secondary absorption.
(3) The most convenient laboratory method for preparing argon is to absorb oxygen and nitrogen from the air by passing it slowly over a mixture of nine parts calcium carbide and one part calcium chloride, heated to 800° C., the remaining gases being led over hot copper oxide to remove the hydrogen, hydro carbons and carbon monoxide present and finally removing the water vapor and carbon dioxide. It is stated that by using seven kilo grams of carbide, 11 litres of argon may be ob tained from air in two days.
(4) Argon is more soluble in water than nitrogen; this property is the basis of some re cent patents for the separation o: argon and nitrogen, but there has been no production by this method to date.
As obtained from the atmosphere argon always contains traces of neon, krypton, xenon and helium. The total quantity of these gases present amounts to only about 025 per cent of the argon, 85 per cent of this impurity being neon.
Properties.— Argon is 1,379 times as heavy as air. The critical temperature of argon is —117.4° C. and the critical pressure 52.9 at mospheres. At atmospheric pressure the boil ing point of liquid argon is —186.1° C., the
freezing point —187.9° C. and the melting point of solid argon —188° C. The density of liquid argon at —186.1° C. is 1.4046.
Since its discovery argon has been treated with the most violent reagents known to chem ists, but it refuses to react with any of them. Rayleigh and Ramsay stated that °argon does not combine with oxygen in the presence of alkali under the influence of the electric dis charge, nor with hydrogen in the presence of acid or alkali, nor when sparked, nor with phosphorus at a bright red heat nor with sulphur. Tellurium may be distilled in it and also sodium and potassium. Red-hot sodium peroxide has no effect. Persulphides of sodium and calcium have no effect at a red heat. Platinum sponge does not absorb it. Aqua regia, bromine water, bromine and alkali and potassium permanganate are all without in fluence. Mixtures of metallic sodium and silica or of sodium and boric acid are likewise without influence and hence also nascent silicon and boron.)) Moissan found that even fluorine does not act upon argon at any temperature. Villard claims to have formed a crystalline hydrate with water, but this result requires confirmation.
The chemical inertness of argon has made it impossible to study its atomic weight and molecular weight by methods other than the determination of its density which is 19.14 =I= (0=-16). On this basis the molecular weight is 39.88. The atomic weight is the same, as has been deduced from a comparison of the specific heats of the gas under constant pressure and at constant volume.
The spectrum of argon is very characteris tic. The most permanent lines are two in the red, a yellow line, two green lines and a violet line. It varies slightly depending on the method of preparation, but this is doubtless due to the presence of traces of impurities introduced by the reagents used in removing the large quan tities of nitrogen, etc., from the argon.
Manufacture and Uses.— Commercial pro duction of argon in the United States began in the latter part of the year 1914, when it was being manufactured for use as an inert gas in incandescent electric light bulbs. The fact was soon developed that it possessed distinct advan tages in connection with incandescent electric lighting, and the demand for this purpose grew rapidly. At the present time commercial pro duction in the United States amounts to thou sands of cubic feet of pure argon per month.
The commercial product sold as argon con tains 25 to 30 per cent pure argon. This is purified by the consumers to a product contain ing approximately 80 per cent argon and 20 per cent nitrogen, in which form it is intro duced into lamps.
All of the argon used commercially at the present time is made by air liquefaction proc esses. Both of the commercial air liquefaction processes in use to-day, the Linde and the Claude, are readily adaptable to tots production of argon on a commercial scale.