THE HALOGENS 489 



has so little affinity for hydrogen that hydriodic acid, HI, is formed 

 with difficulty, is easily decomposed, and acts as a reducing agent in 

 a number of cases. 



From the form of their compounds the halogens are univaknt 

 elements with respect to hydrogen and septivalent with respect to 

 oxygen, N being trivalent to hydrogen (it gives NH 3 ) and quinquivalent 

 to oxygen (it gives N 2 6 ), and C being quadrivalent to both H and 

 as it forms CH 4 and C0 2 . And as not only their oxygen compounds, 

 but also their hydrogen compounds, have acid properties, the halogens 

 are elements of an exclusively acid character. Such metals as sodium, 

 potassium, barium only give basic oxides. In the case of nitrogen, 

 although it forms acid oxides, still in ammonia we find that capacity to 

 give an alkali with hydrogen which indicates a less distinctly acid 

 character than in the halogens. In no other elements is the acid- 

 giving property so strongly developed as in the halogens. 



In describing certain peculiarities characterising the halogens, we 

 shall at every step encounter a confirmation of the above-mentioned 

 general relations. 



As fluorine decomposes water with the evolution of oxygen, 

 F 2 4> H 2 = 2HF + O, for a long time all efforts to obtain it in free 

 state by means of methods similar to those for the preparation of 

 chlorine proved fruitless. 48 Thus by the action of hydrofluoric acid 

 on manganese peroxide, or by decomposing a solution of hydrofluoric 

 acid by an electric current, either oxygen or a mixture of oxygen and 

 fluorine were obtained instead of fluorine. Probably a certain quantity 

 of fluorine 48 bis was set free by the action of oxygen or an electric 

 current on incandescent and fused calcium fluoride, but at a high 

 temperature fluorine acts even on platinum, and therefore it was not 

 obtained When chlorine acted on silver fluoride, AgF, in a vessel of 

 natural fluor spar, CaF 2 , fluorine was also liberated ; but it was mixed 



49 Even before free fluorine was obtained (1886) it was evident from experience gained 

 in the efforts made to obtain it, and from analogy, that it would decompose water (see 

 first Russian edition of the Principles of Chemistry). 



48 bu It is most likely that in this experiment of Fremy's, which corresponds with the 

 action of oxygen on calcium chloride, fluorine was set free, but that a converse reaction 

 also proceeded, CaO + F 2 = CaF 2 + O that 'is, the calcium distributed itself between the 

 oxygen and fluorine. MnF 4 , which is capable of splitting up into MnF 2 and F 2 , is without 

 doubt formed by the action of a strong solution of hydrofluoric acid on manganese per- 

 oxide, but under the action of crater the fluorine gives hydrofluoric acid, and probably 

 this is aided by the affinity of the manganese fluoride and hydrofluoric acid. In all the 

 attempts made (by Davy, Knox, Louget, Fremy, Gore, and others) to decompose fluorides 

 (those of lead, silver, calcium, and others) by chlorine, there were doubtless also cases 

 of distribution, a portion of the metal combined with chlorine and a portion of the fluorine 

 was evolved ; but it is improbable that any decisive results were obtained. Fremy probably 

 obtained fluorine, but not in a pure state. 



