TIN-: H.\LM<;F,.\S 483 



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 (which 

 forms ozone if the temperature be not high), F., + H 2 O = 2HF + O, 

 therefore 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 fruit- 

 less. 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 was set free by 

 the action of oxygen or an electric current on incandescent and fused 

 calcium fluoride, but it then, at the high temperature, acted even on 

 platinum, and was therefore absorbed, leaving only oxygen. When 

 chlorine acted on silver fluoride, AgF, in a vessel of natural fluor spar, 

 CaF.,, fluorine was also liberated ; but it was mixed with chlorine, and 

 it was impossible to study the properties of the resultant gas. Brauner 

 also obtained fluorine by igniting cerium fluoride, 2CeF 4 = 2CeF 3 + F. 2 ; 

 but this, like all preceding efforts, only showed fluorine to be a gas 

 which decomposes water, and is capable of acting in a number of 

 instances like chlorine, but gave no possibility of testing its properties. 

 It was evident that it was necessary to avoid as far as possible the 

 presence of water and a rise of temperature ; this Moissan succeeded 

 in doing in 1886. He decomposed anhydrous hydrofluoric acid, liquefied 

 at a temperature of 23 and contained in a U-shaped tube (to which 

 a small quantity of potassium fluoride had been added to make it a 

 better conductor), by the action of a powerful electric current (twenty 

 Bunsen's elements in series). Hydrogen was then evolved at the 

 negative pole, and fluorine appeared at the positive pole (of iridium 

 platinum) as a colourless gas which decomposed water with the forma- 

 tion of ozone and hydrofluoric acid, and combined directly with silicon 

 (forming silicon fluoride, SiF 4 ), boron (forming BF 3 ), sulphur, <kc. But 



4S 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 

 a 1 *o proceeded, CaO + F 2 = CaF., + O that is, the calcium distributed itself between the 

 i and ihiorine. MnK,. which is capable of splitting up into MnF., and Fo, is without 

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

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

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

 efforts (Davy, Knox, Louget, Fremy, Gore, and others) made 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 there were pure results. Probably Fremy obtained 

 fluorine, but it was not pure. 



ii 2 



