466 PRINCIPLES OF CHEMISTRY 



reaction of chlorine on hydrogen occupies (at the original temperature 

 and pressure) a volume equal to the sum of the original volumes , that 

 is, a reaction of substitution here takes place H 2 + C1 2 = HC1 4- HC1 

 In this reaction twenty-two thousand heat units are evolved for one 

 part by weight [1 gram] of hydrogen. 13 



These relations show that, the affinity of chlorine for hydrogen is 

 very great and analogous to the affinity between hydrogen and oxygen. 

 Thus l4 on the one hand by passing a mixture of steam and chlorine 

 through a red-hot tube, or by exposing water and chlorine to the sun- 

 light, oxygen is disengaged, whilst on the other hand, as we saw above, 

 oxygen in many cases displaces chlorine from its compound with 

 hydrogen, and therefore the reaction H 2 + C1 2 = 2HC1 + belongs 

 to the number of reversible reactions, and hydrogen will distribute 

 itself between oxygen and chlorine. This determines the relation 

 of Cl to substances containing hydrogen and its reactions in the 

 presence of water, to which we shall turn our attention after 

 having pointed out the relation of chlorine to other elements. 



Many metals when brought into contact with chlorine immediately 

 combine with it, and form those metallic chlorides which correspond 

 with hydrogen chloride and with the oxide of the metal taken. This 

 combination may proceed rapidly with the evolution of heat and 

 light ; that is, metals are able to burn in chlorine. Thus, for example, 

 sodium l6 burns in chlorine, synthesising common salt. Metals in the 

 form of powders burn without the aid of heat, and become highly 

 incandescent in the process ; for instance, antimony, which is a metal 

 easily converted into a powder. 16 Even such metals as gold and 



15 In the formation of steam (from one part by weight [1 gram] of hydrogen) 29,000 heat 

 units are evolved. The following are the quantities of heat (thousands of units) evolved in 

 the formation of various other corresponding compounds of oxygen and of chlorine (from 

 Thomson's, and, for Na^O, BeketofFs results) 



C2NaCl, 195; CaCL, 170; HgCl 2 , 68- 2AgCl, 59. 



t Na. 2 0, 100; CaO, 181; HgO, 42; Ag,O, 6. 



<2AsCl 3 ,148; 2PC1 5 ,210; CC1 4 , 21 ; 2HC1, 44 (gas). 



\ As.jO 3 , 155 ; P 3 5 , 370 ; CO* 97 ; H 3 O, 58 (gas). 



With the first four elements the formation of the chlorine compound gives the most 

 heat, and with the four following the formation of the oxygon compound evolves the 

 greater amount of heat. The first four chlorides are true salts formed from HC1 and the 

 oxide, whilst the remainder have other properties, as is seen from the fact that they are 

 not formed from hydrochloric acid and the oxide, but give hydrochloric acid with water. 



w This has been already pointed out in Chapter III., Note 5. 



13 Sodium remains unaltered in perfectly dry chlorine at the ordinary temperature, 

 and even when slightly warmed ; but the combination is exceedingly violent at a red heat* 



16 An instructive experiment on combustion in chlorine maybe conducted as follows: 

 leaves of Dutch metal (used instead of gold for gilding) are placed in a glass globe, and a 



