THE HALOGENS 481 



On comparing chlorine as an element not only with nitrogen and 

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

 little analogy with the metals that a comparison would be superfluous), 

 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 (con- 

 taining one H in its molecule) acid HC1, and the same chlorine is able 

 by metalepsis to replace the hydrogen ; with oxygen it forms oxides of 

 in acid character. These properties of chlorine are possessed 1 by three 

 other elements, bromine, iodine, and fluorine. They are members of 

 one natural family. Each representative has its peculiarities, its indi- 

 vidual 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 signs of the family 

 -enables one to foretell from one element the properties of another, 

 and thus abbrev^Bk; an acquaintance with all the differences of their 

 elementary propeMfc and a system isation of the elements themselves. 



In order to have a guiding thread in forming comparisons between 

 the elements, attention must be turned to those of their properties and 

 signs in which they differ most from each other, because it is only 

 under this condition that the comparison ceases to be artificial. And 

 the atomic weights of the elements must be counted as their most 

 elementary property ; it being a quantity which is most undoubtedly 

 established, and which acts in all the manifestations of the element. 

 The halogens are endowed with atomic weights 



F = 19, 01 = 35-5, Br=80, 1 = 127. 



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

 corresponding compounds must evidently be in a certain dependence 

 on this fundamental point, if the grouping in one family be natural. 

 And we find in reality that, for instance, the properties of bromine, 

 whose atomic weight is almost the mean between those of iodine and- 

 ohlorine, occupy a mean position between those of these two elements. 

 The second measurable property of the elements is their equivalence or 

 their capacity for forming compounds of definite forms. Thus carbon 



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 feeble alteration in it 

 proceeding at the surfaces of contact may suffice to cause its destruction. In order to 

 conceive the modus operandi of contact phenomena, it is enough to imagine, for instance, 

 that at the borders of contact the movement of the atoms in the molecules changes from 

 a circular to an elliptical path. Momentary and transitory compounds may be formed, 

 but their formation cannot alter the explanation of the phenomena. 



VOL. I. I I 



