THE HALOGENS 487 



On comparing chlorine as an element not only with nitrogen and 

 carbon but with all the other non- metallic elements (chlorine has so little 

 analogy with the metals that a comparison with them would be super- 

 fluous), we find in it the following fundamental properties of the halogens 

 or salt-producers. With metals chlorine gives salts (such as sodium 

 chloride, &c.) ; with hydrogen a very energetic and monobasic acid HC1, 

 and the same quantity of chlorine is able by metalepsis to replace the 

 hydrogen ; with oxygen it forms unstable oxides of an acid character. 

 These properties of chlorine are possessed by three other elements,. 

 bromine, iodine, and fluorine. They are members of one natural family. 

 Each representative has its peculiarities, its individual properties and 

 points of distinction, in combination and in the free state otherwise they 

 would not be independent elements ; but the repetition in all of them 

 of the same chief characteristics of the family enables one more quickly 

 to grasp all their various properties and to classify the elements them- 

 selves. 



In order to have a guiding thread in forming comparisons between 

 the elements, attention must however be turned not only to their points 

 of resemblance but also to those of their properties and characters in 

 which they differ most from each other. And the atomic weights of 

 the elements must be considered as their most elementary property, since 

 this is a quantity which is most firmly established, and must be taken 

 account of in all the reactions of the element. The halogens have the 

 following atomic weights 



F = 19, Cl = 35-5, Br = 80, . J = 127. 



All the properties, physical and chemical, of the elements and their 

 corresponding compounds must evidently be in a certain dependence 



chlorine, the chlorine being displaced by the oxygen disengaged. Spring and Procfc 

 (1689) represent the evolution of oxygen from KC1O,-, as due to the salt first splitting up 

 into base and anhydride, thus (1) 2MC1O 5 = M a O + C1 2 O 5 ; (2) C1 2 O 5 =C1 2 + O 5 ; and (8) 



I may further remark that the decomposition of potassium chlorate as a reaction 

 evolving heat easily lends itself for this very reason to the contact action of manganese 

 peroxide and other similar admixtures ;- for such very feeble influences as those of 

 contact may become evident either in those cases (for instance, detonating gas, 

 hydrogen peroxide, &c.), when the reaction is accompanied by the evolution of heat, or 

 when (for instance, H 2 + I 2 , &c.) little heat is absorbed or evolved. In these cases it is 

 evident that the existing equilibrium is not very stable, and that a small alteration in the 

 conditions at the surfaces of contact may suffice to upset it. In. order to conceive the 

 modus operandi of contact phenomena, it is enough to imagine, for instance, that a thd 

 surface of contact the movement of the atoms in the molecules changes from a circular 

 to an elliptical path. Momentary and transitory compounds maybe formed, but their 

 formation cannot affect the explanation of the phenomena. 



