5i8 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



formed, Deville was able to recombine the gases completely. 

 Deville wrote : 



" If ammonia gas is sparked during several hours, until 

 its volume appears to be exactly doubled, no sensible 

 absorption is observed on introducing drops of water 1 

 into the eudiometer : the decomposition appears to be 

 complete. But if, instead, one passes in some bubbles 

 of gaseous hydrochloric acid, a slight cloud obviously 

 obscures the mixture of nitrogen and hydrogen into which 

 the ammonia has been transformed. This transformation 

 is then not absolute. This affords an easy explanation of 

 the following experiment. After having decomposed one 

 volume of ammonia as perfectly as possible by sparking, 

 giving two volumes of a mixture of nitrogen and hydrogen, 

 one volume of gaseous hydrochloric acid is introduced into 

 the eudiometer and the sparking is renewed during 8 to 10 

 hours. At the end of this time the upper part of the 

 apparatus is covered with a layer of sal-ammoniac and the 

 mercury has risen to the platinum wires" (Comptes rendus, 

 1865, 60, 324). 



The decomposition and recomposition of ammonia may 

 be represented by the equations : 



2NH 3 N 2 + 3H 2 . N 2 + sH 2 + 2 HC1 -> 2NH 4 C1. 



The recombination is now usually demonstrated by 

 sparking over sulphuric acid, a method already used by 

 Deville in combining sulphur dioxide with oxygen. 



Bineau (1838) investigates the vapour-densities of ammo- 

 nium and phosphonium salts. During the half-century which 

 followed the enunciation of Avogadro's hypothesis, great 

 difficulty was experienced in accepting his view that equal 

 numbers of molecules, under similar physical conditions, always 

 occupied equal volumes. One difficulty arose from the fact 

 that one " equivalent "occupied a different volume in different 

 gases. Thus if i volume of oxygen was required to convert 

 a given weight of potassium into its oxide, the quantities of 



1 In Lessons on Dissociation, " concentrated sulphuric acid " is used. 



