THE COM POSITION OF WATER, HYDRO* i FA 



The properties <>f ]ii/<Ii-<>>/i'n. Hydrogen presents us with an example 

 of a gas which at first sight does not differ from air. It is not sur- 

 prising, therefore, that Paracelsus, having discovered that an aeriform 

 substance is obtained by the action of metals on sulphuric acid, did not 

 quite determine its difference from air. In fact, hydrogen, like air, is 

 colourless, and has no smell ; 21 but a more intimate acquaintance 

 with its properties proves it to be entirely different from air. The first 

 sign which distinguishes hydrogen from air is its combustibility. This 

 property is so easily observed that it is the one to which recourse is 

 usually had in order to recognise hydrogen, if it is evolved in a re- 

 action, although there are many other combustible gases. But before 

 speaking of the combustibility and other chemical properties of hydro- 

 gen, we will first describe the physical properties of this gas, as we did 

 in the case of water. It is easy to show that hydrogen is one of the 

 lightest gases. 22 If passed into the bottom of a flask full of air, 



carbonic anhydride C0. 2 (according to the equation 2H..O + C = 2H.> + CO. 2 ), and the result- 

 ing mixture is called water-gas ; we shall speak of it in describing the oxides of carbon. 



- 1 Hydrogen obtained by the action of zinc or iron on sulphuric acid generally smells 

 of hydrogen sulphide (like rotten eggs), which it contains in admixture. As a rule such 

 hydrogen is not so pure as that obtained by the action of an electric current or of sodium 

 on water. The impurity of the hydrogen depends on the impurities contained in the 

 zinc, or iron, and sulphuric acid, and on secondary reactions which take place simul- 

 taneously with the main reaction. Thus iron sulphide gives hydrogen sulphide 

 (FeS + HoSO4 = HoS + FeSO 4 ). However, the hydrogen obtained in this manner may be 

 easily freed from the impurities it contains : some of them namely those having acid 

 properties are absorbed by caustic soda, and therefore may be removed by passing the 

 hydrogen through a solution of this substance ; another series of impurities is absorbed 

 by a solution of mercuric chloride ; and, lastly, a third series is absorbed by a solution of 

 potassium permanganate. The hydrogen may be dried by passing it over sulphuric acid 

 or calcium chloride. The substances serving for purifying the hydrogen are either 

 placed in Woulfe's bottles, or in tubes containing pumice stone moistened with the 

 purifying agent. The surface of contact is then greater, and the purification proceeds 

 more rapidly. If it be desired to procure completely pure hydrogen, it is sometimes 

 obtained by the decomposition of water (previously boiled to expel all air, and mixed 

 with pure sulphuric acid), by the galvanic current. Only the gas evolved at the negative 

 electrode is collected. Or else, an apparatus like that which gives detonating gas is used, 

 only the positive electrode being immersed under mercury containing zinc in solution. 

 The oxygen which is evolved at this electrode then immediately, at the moment of its 

 evolution, combines with the zinc, and this compound dissolves in the sulphuric acid and 

 forms zinc sulphate, which remains in solution, and therefore the hydrogen generated 

 will be quite free from oxygen. 



'-'- An inverted beaker is attached to one arm of the beam of a rather sensitive 

 balance, and its weight counterpoised by weights in the pan attached to the other arm. 

 If the beaker be then filled with hydrogen it rises, owing to the air being replaced 

 by hydrogen. Thus, at the ordinary temperature of a room, a litre of air weighs 

 about 1'2 grams, and on replacing the air by hydrogen a decrease in weight of about 1 

 irrum per litre is obtained. Moist hydrogen is heavier than dry for aqueous vapour 

 is nine times heavier than hydrogen. In filling balloons it is usually calculated that (it 

 being impossible to have perfectly dry hydrogen or to obtain it quite free from air) 

 the lifting force is equal to 1 kilogram ( = 1,000 grams) per cubic metre ( = 1,000 litres). 

 VOL. I. K 



