222 Mr, Perkins on the Compressibility of Water, [March, 



endeavoured to render probable, is a prussiate, and not a cyanu- 

 ret. Vauquelin thought, moreover, that he had discovered that 

 those bases capable of decomposing water at the common tem- 

 perature of the atmosphere give hydrocyanates, while the others 

 give onlycyanurets. 



The last work on this subject that has come to my kr.owledge 

 is a note by M. Robiquet on the composition of prussian blue. 

 In it he confirms the results which M. Proust had derived from 

 his experiments on the prussiates, made long since, and among 

 others this, that the white prussiate of iron contains potash. 

 M. Robiquet showed that this prussiate without potash may be 

 obtained in the form of small crystalline grains, of a yellow 

 colour, by exposing prussian blue for a long time to the action of 

 Bulphuretted hydrogen gas. He considers prussian blue as a 

 combination of cyanuret of iron with a prussiate of deutoxide of 

 iron and water ; and he attributes its blue colour to water. He 

 asserts that ferruginous prussiate of potash burned by means of 

 oxide of copper always afforded him the gases in the same pro- 

 portion to one another that Gay-Lussac found them in cyanogen; 

 and he maintains that in this experiment the base retains no 

 carbonic acid, as Mr. Porrett had noticed. 



Results so contradictory, and conjectures so little justified by 

 experiment, are not very well calculated to give us an exact idea 

 of the composition of these salts ; and although the path has 

 been marked out by the labours of Gay-Lussac, we must confess, 

 in spite of what has been done with him, that we are just at the 

 same point at which he left the question. 



M. Proust long ago proved the ferruginous prussiates to con- 

 tain iron, and thatthey must be regarded as salts with double bases, 

 of which the protoxide of iron is always one, exactly as alumina 

 is always one of the bases in the different kinds of alum ; and he 

 showed that prussian blue must be a hydrocyanate in which the 

 deutoxide of iron represents the other bases with the protoxide. 

 Mr. Porrett's idea that the iron is an element of the acid has 

 always appeared to me analogous to that of considering the pot- 

 ash in cream of tartar as an element of the acid in pel de sing- 

 nette, or tartar emetic. 



(To he contiuue4>) 



Article XIV. 



On the Compressibility of Water, By Mr. Perkins. 



(Ta the Editor of the Annals of Philosophy.) 



SSRf, Fr J. 23, 1821. 



In the Annals of Philosophy for February, Dr. Roget has 



discovered a very material error in my computation on the first 



experiment made with the piezometer on the compressibility of 



