488 PRINCIPLES OF CHEMISTRY 



on this fundamental point, if the grouping in one family be natural. 47 b!s 

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

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

 chlorine, 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 

 or nitrogen in this respect differs widely from the halogens. Although 

 the form C10 2 corresponds with N0 2 and C0 2 , yet the last is the 

 highest oxide of carbon, whilst that of nitrogen is N 2 5 , and for chlorine, 

 if there were an anhydride of perchloric acid, its composition would 

 be C1 2 7 , which is quite different from that of carbon. In respect to 

 the forms of their compounds the halogens, like all elements of one 

 family or group, are perfectly analogous to each other, as is seen from 

 their hydrogen compounds : 



HF, HC1, HBr, HI. 



Their oxygen compounds exhibit a similar analogy. Only fluorine 

 does not give any oxygen compounds. The iodine and bromine com- 

 pounds corresponding with HC10 3 and HC1O 4 are HBr0 3 and HBrO 4f 

 HI0 3 and HIO 4 . On comparing the properties of these acids we 

 can even predict that fluorine will not form any oxygen compound. 

 For iodine is easily oxidised for instance, by nitric acid whilst 

 chlorine is not directly oxidised. The oxygen acids of iodine are com- 

 paratively more stable than those of chlorine ; and, generally speaking, 

 the affinity of iodine for oxygen is much greater than that of chlorine. 

 Here also bromine occupies an intermediate position. In fluorine 

 we may therefore expect a still smaller affinity for oxygen than in 

 chlorine and up to now it has not been combined with oxygen. If any 

 oxygen compounds of fluorine should be obtained, they will naturally be 

 exceedingly unstable. The relation of these elements to hydrogen is the 

 reverse of the above. Fluorine has so great an affinity for hydrogen 

 that it decomposes water at the ordinary temperature ; whilst iodine 



47 bif gee, for example the melting point of NaCl, NaBr, Nal in Chapter II. Note 27. 



According to P. Freyer and V. Meyer (1892), the following are the boiling points of some 



of the corresponding compounds of chlorine and bromine : 



BC1 S 17 BBr 3 90 



.SiCl 5 59 SiBr 4 153 



PC1 3 76 PBr 3 175 



SbCl 3 228 SbBrs 275 



BiCl 3 447 BiBr r , 458 



SnCl 4 606 SnBr 4 619 



x ZnCl 2 780 ZnBr 2 650 



Thus for all the more volatile compounds the replacement of chlorine by bromine 

 raises the boiling point, but in the case of ZnX.> it lowers it (Chapter XV. Note 19). 



