813.] Philosophical Transactions^ Part II, 1812. 209 



rejpeated, and measurements should be made at Botany Bay and 

 in Brazil. 



XVIII. u4n Account of some Experiments on different Com-- 

 linations of Fluoric Acid. By John Davy, Esq.] Fluoric acid 

 was originally discovered by S"he?]e, and if we except the pro- 

 perties which he detected, and the observations on the gas by 

 Dr. Priestley, little was known respecting it till the publication of 

 the Reckerdies Physic - ::>riiiCjU(''^ bv Gay Lussac and Thenard. 

 Many new and extraordinary properties of tliis acid were de^ 

 scribed in that' interesting work; and since that time the at- 

 tention of chemists has been very much turned towards it. In 

 its pure state it does not seem capable of assujuing the gaseous 

 form. But when combined with sib'ca or boracic acid, it readily 

 assumes the aerial state. This singular property of fiuoric acid, 

 of combining with a variety of other bodies, and constituting 

 with them new acids, is peculiar to that acid. It has not yet 

 been accounted for in a satisfactory manner. We may soon ex- 

 pect a set of experiments on the subject by Sir Humphry Davy, 

 which will set the subject in a new point of view, and firther 

 develope certain changes which it seems expedient to introduce 

 into the present theory of chemistry. 



Mr. Jolm Davy's paper contains several valuable facts, a sum- 

 mary of which we simll now lay before our readers. We shall 

 at the same time correct some numerical errors in the paper, re- 

 sulting from Mr. Davy having assumed 3 1 grains as the weight 

 of 100 cubic inches of common air at tlie temperature of 60^, 

 and when the barometer stands at 30 inches. I suspect that I 

 have myself in some measure contributed to occasion this mistake^ 

 by stating 31 grains as the weight of 100 cubic inches of com- 

 mon air. I was induced to adopt that number on the authority 

 t)f Mr. Kirvvan ; but the result of Sir Geojrge Shuckburgh 

 Evelyn's experiments, which I consider as the most accurate 

 which have ever been made on the subject (as corrected by Mr, 

 Fletcher. Nicholson's Journal, 4. 35.) is that 100 cubic inches 

 of common air at the temperature of 60°, and when the baro- 

 meter stands at 30 inches^, weighs very nearly 30*5 grains. 



1. What was formerly called fluoric acid gas, Mr. Davy calls 

 silicated fluoric acid gas. Its specific gravity is 2*990, that of 

 air being i-00; and iOO cubic inches of it weigh 108°992 grains. 

 It is, therefore, the heaviest gas known except phosj^ene gas. 

 This gas is composed of 100 fluoric acid, and 165'88 silica. 

 Water absorbs 3f)3 times its bulk of this gas. When absorbed by 

 water, about a third part of the silica is thrown down, and sub- 

 silicated acid remains composed very nearly of equal weights of 

 acid and silica, 



2, Both of these acids combine with ammonia j 100 measures 

 Vol. LNMIL O 



