260 FLUORINE. 



Let us treat common salt, the chloride of sodium, with sulphuric acid. 

 You see that it gives at once an abundant disengagement of gaseous 

 hydrochloric acid. 



We will do the same with sodium fluoride. Let us add, in a leaden 

 vessel, sulphuric acid to the alkaline fluoride. We shall see copious 

 fumes produced. In each case, at a tempera,ture of 20° C, we shall 

 have disengaged a gaseous body which fumes strongly in the air, is 

 colorless, has the characteristics of an energetic acid, combines in the 

 dry state with ammonia, is very soluble in water, and dissolves in the 

 latter with a great increase in temperature. 



If we give to sodium fluoride, to the binary compound of fluorine and 

 sodium, the formula NaF, that of the acid substance produced by the 

 action of sulphuric acid can only be HF. The two reactions are 

 identical. 



The acid gaseous body formed in this reaction is, therefore, a com- 

 pound of fluorine and hydrogen; a body analogous to hydrochloric 

 acid, and to which the name hydrofluoric acid is given. 



But in the natural sciences analogy is not suflicient; the scientific 

 method can only accept that which is rigorously proved. It is there- 

 fore necessary to demonstrate that hydrofluoric acid is a hydrogen acid. 

 And this will take us back to the beginning of the century. You know 

 well how great was the influence of Lavoisier upon the upward flight of 

 chemistry, and indeed upon all true science. You know how this great 

 genius, by the continual use of the balance in the study of reactions, 

 gave to the science which we follow a mathematical exactness. Struck 

 by the important part which oxygen plays in combustion, he believed 

 that that element was indispensable to the formation of acids. To 

 Lavoisier every acid was an oxygen compound; hydrochloric acid, 

 therefore, according to his theories, was regarded as containing oxygen ; 

 and, by analogy, hydrofluoric acid must contain it also. 



To your great investigator, Humphry Davy, belongs the honor of 

 having proved that hydrofluoric acid contains no oxygen. But allow 

 me, before coming to the beautiful researches of Davy, to recall to you 

 the history of the discovery of hydrofluoric acid. We need not con- 

 sider the investigations of IMargraff, which were published in 1768, 

 but we must not forget that it was Scheele who definitely chara(;terized 

 hydrofluoric acid in 1771, without, however, obtaining it in a state of 

 purity. In 1809 Gay Lussac and Thenard took up the study of the 

 compound, and succeeded in producing an acid sufficiently pure and 

 highly concentrated, although far from being anhydrous. The action 

 of hydrofluoric acid upon silica and the silicates was then perfectly 

 elucidated. 



Let us now come down to about the year 1813, the time when Davy 

 undertook the study of hydrofluoric acid. A little earlier Ampere, in 

 two letters addressed to Humphry Davy, advanced the opinion that 

 hydrofluoric acid might be regarded as formed by the combination of 



