154 PRINCIPLES OF CHEMISTRY 



sorbing and envolving oxygen, by which a definite quantity of free 

 oxygen is preserved in the entire mass of the atmosphere. 4 



Free oxygen may be obtained by one or another method from all 

 the substances in which it occurs. Thus, for instance, the oxygen of 

 many substances may be transferred into water, from which, as we 

 have already seen, oxygen may be obtained. 5 We will first consider 

 the methods of extracting oxygen from air as being a substance every- 

 where distributed. The separation of oxygen from it is, however, 

 hampered by many difficulties. 



From air, which contains a mixture of oxygen and nitrogen, the 

 nitrogen alone cannot be removed, because it has 110 inclination to 

 combine directly or readily with any substance ; and although it does 

 combine with certain substances (boron, titanium), these substances com- 

 bine simultaneously with the oxygen of the atmosphere. 6 However, 



4 The earth's surface is equal to about 510 million square kilometres, and the mass of 

 the air (at a pressure of 760 mm.) on each kilometre of surface is about 10 J thousand millions 

 of kilograms, or about 10^ million tons ; therefore the whole weight of the atmosphere 

 is about 5100 million million ( = 51xl0 14 ) tons. Consequently there are about 2 x 10 15 

 tons of free oxygen in the earth's atmosphere. The innumerable series of processes 

 which absorb a portion of this oxygen are compensated for by the plant processes. Count- 

 ing that 100 million tons of vegetable matter, containing 40 p.c. of carbon, formed from 

 carbonic acid, are produced (and the same process proceeds in water) per year on the 100 

 million square kilometres of dry land (ten tons of roots, leaves, stems, &c. per hectare, or 

 YO of a square kilometre), we find that the plant life of the dry land gives about 100,000 

 tons of oxygen, which is an insignificant fraction of the entire mass of the oxygen of 

 the air. 



5 The extraction of oxygen from water may evidently be accomplished by two pro- 

 cesses : either by the decomposition of water into its constituent parts by the action of a 

 galvanic current (Chap. II.), or by means of the removal of the hydrogen from water. 

 But, as we have seen and already know, hydrogen enters into direct combination with very 

 few substances, and then only under special circumstances ; whilst oxygen, as we 

 shall soon learn, combines with nearly all substances. Only gaseous chlorine (and 

 especially, fluorine) is capable of decomposing water, taking up the hydrogen from it, 

 without combining with the oxygen. Chlorine is soluble in water, and if an aqueous 

 solution of chlorine, so-called chlorine water, be poured into a flask, and this flask be 

 inverted in a basin containing the same chlorine water, then we shall have an apparatus 

 by means of which oxygen may be extracted from water. At the ordinary temperature, 

 and in the dark, chlorine does not act on water, or only acts very feebly ; but under 

 the action of direct sunlight chlorine decomposes water, with the evolution of oxygen. 

 The chlorine then combines with the hydrogen, and gives hydrochloric acid, which dis- 

 solves in the water, and therefore free oxygen only will be separated from the liquid: 

 and it will only contain a small quantity of chlorine in admixture, which can be easily 

 removed by passing the gas through a solution of caustic potash, which retains the 

 chlorine. 



6 A difference in the physical properties of both gases cannot be here taken advantage 

 of, because they are very similar in this respect. Thus the density of oxygen is 1(5, and 

 of nitrogen 14 times greater than the density of hydrogen, and therefore porous vessels 

 cannot be here employed the difference between the times of their passage through a 

 porous surface would be too insignificant. 



Graham, however, succeeded in enriching air in oxygen by passing it through india- 



