460 PRINCIPLES OF CHEMISTRY 



Thus u 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 O + C1 2 =2HC1 + O belongs to 

 the number of reversible reactions, and hydrogen will distribute itself 

 between oxygen and chlorine if it comes into contact with both these 

 elements. These relations determine many of the properties of 

 chlorine that is, its relation 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. With 

 a large surface of metal, and if it be slightly heated, 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 lft burns 

 in chlorine, thus 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 con- 

 verted into a powder. 16 Even such metals as gold and platinum, 17 



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

 Thomson's, and, for Na. 2 O, Beketoff 's results) : 



f 2XaCl, 195 ; CaCL, 170 ; HgCl,, 03 ; 2AgCl, r,i). 



1 NaoO, 100 ; CaO, 131 ; HgO, 42 ; Ag._,O, 6. 



f 2AsCl 3 , 143; 2PC1 5 , 210 ; CC1 4 , 21 ; 2HC1, 44 (gas). 



\ AsoO 3 , 155 ; PoOs, 370 ; CO.,, 97 ; H 2 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 oxygen 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. If 

 affinity be measured by heat, then in the former the affinity for chlorine is greater than 

 that for oxygen, whilst in the latter it is the reverse. But as the physical states of the 

 substances are different, and as chlorine displaces oxygen as well as oxygen displaces 

 chlorine, the affinity cannot be determined from thermo-chemical data without a number 

 of corrections which are still subject to doubt. 



14 This has been already pointed out in Chap. III. Note 5. 



15 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 may be conducted as follows : 

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

 gas-conducting tube furnished with a glass cock is placed in the cork closing it, and the 

 air is pumped out of the globe. The gas-conducting tube is then connected with a vessel 

 containing chlorine, and the cock opened; the chlorine rushes in, and the metallic leaves 

 are consumed. 



17 The behaviour of platinum to chlorine at a high temperature (1400) is very 



