124 PRINCIPLES OF CHEMISTRY 



for this purpose ; from it the hydrogen is displaced by many metals with 

 incomparably greater facility than directly from water, and such a 

 displacement is accompanied by the evolution of a large amount of 

 heat. 15 By the action of zinc or iron on sulphuric acid, hydrogen is 

 evolved, because the metal replaces it. When the hydrogen in sulphuric 

 acid is replaced by a metal, a substance is obtained which is called a 

 salt of sulphuric acid or a sulphate. Thus, by the action of zinc on 

 sulphuric acid, hydrogen and zinc sulphate, ZiiSO^, are obtained. 

 The latter is a solid substance, soluble in water. In order that the 

 action of the metal on the acid should go on regularly, and to the end, 

 it is necessary that the acid should be diluted with water, which dis- 

 solves the salt as it is formed ; otherwise the salt covers the metal, 

 and hinders the acid from attacking it. Usually the acid is diluted 

 with from three to five times its volume of water, and the metal is 

 covered with this solution. In order that the metal should act 

 rapidly on the acid, it should present a large surface, so that a maxi- 

 mum amount of the reacting substances may come into contact in a 

 given time. For this purpose the zinc is used as strips of sheet zinc, 

 or in the granulated form (that is, zinc which has been poured from a 

 certain height, in a molten state, into water). The iron should be in 

 the form of wire, nails, filings, or cuttings. 



The usual method of obtaining hydrogen is as follows : A certain 

 quantity of granulated zinc is put into a double- necked, or Woulfe's, 

 bottle. Into one neck a funnel is placed, reaching to the bottom of 

 the bottle, so that the liquid poured in may prevent the hydrogen from 



itself, even at the ordinary temperature, but that the action is limited by small 

 masses and only proceeds at the surface. In reality, zinc, in the form of a very 

 fine powder, or so called ' zinc dust/ is capable of decomposing water with the 

 formation of oxide (hydrated) and hydrogen. The oxide formed acts 011 sulphuric acid, 

 water then dissolves the salt produced, and the action continues because one of the 

 products of the action of water on zinc, zinc oxide, is removed from the surface. One 

 might naturally imagine that the reaction does not proceed directly between the metal 

 and water, but between the metal and the acid, but such a simple representation, which 

 we shall cite afterwards, hides the mechanism of the reaction, and does not permit of its 

 actual complexity being seen. 



15 According to Thomsen the reaction between zinc and a very weak solution of 

 sulphuric acid evolves about 38,000 calories (zinc sulphate beinjj; formed) per (55 parts 

 by weight of zinc ; and 56 parts by weight of iron which combine, like (55 parts by 

 weight of zinc, with 16 parts by weight of oxygen evolve about 25,000 calories (forming 

 ferrous sulphate, FeSO 4 ). Paracelsus observed the action of metals on acids in the 

 seventeenth century; but it was not until the eighteenth century that Lemery 

 determined that the gas which is evolved in this action is a particular one which differs 

 from air and is capable of burning. Even Boyle confused it with air. Cavendish 

 determined the chief properties of the gas discovered by Paracelsus. At first it was 

 called 'inflammable air'; later, when it was recognised that in burning it gives water, 

 it was called hydrogen, from the Greek words for water and generator. 



