Mr. J. Denham Smith on an Acid Oxide of Iron. 41 



surface to another, — that even the most solid bodies may be 

 conceived to emit vapours which after a long space of time may 

 produce that which their surfaces rapidly produce when they 

 are covered over with substances which easily evaporate, — we 

 shall then not be compelled to have recourse to new forces, nor 

 even to the rays of a latent and invisible light in order to ex- 

 plain the ingenious experiments of Moser. We are far from 

 pretending to have given a complete explanation ; we only 

 proposed to show that they might be explained in entire con- 

 formity with the fundamental principle of the logic of experi- 

 mental science: Causas rerum naturalium non plures admitti 

 debere, quam quae et versa sint et earum phaenomenis expli- 

 candis sufficiant. 

 Pisa, March 1843. C. M. 



VIII. Note on the Paper published in the Philosophical Maga- 

 zine for September 1843, " On the Composition of an Acid 

 Oxide of Iron {Ferric Acid)." By J. Denham Smith, Esq. 

 To Richard Phillips, Esq., F.R.S. L. and E. 3 fyc. 

 My dear Sir, 

 T1/"ILL you oblige me by inserting in the Philosophical 

 Magazine the subjoined correction of errors in the paper 

 referred to above ? These errors arose partly from the almost 

 invariable presence of manganese in the oxide of iron (ferri 

 sesquiox. Pharm. Lond.) employed in my experiments, — an 

 impurity neither suspected nor guarded against by me, and 

 which usually occurs in such minute quantities as to render its 

 detection impracticable by the ordinary tests, — and partly from 

 the solubility of oxide of iron in potash, under certain condi- 

 tions, a fact pointed out by M. Chodnew. 



The first error occurs in p. 220, where a solution, which 

 subsequently proved to be permanganate of potash, is described 

 as a permanent solution of the ferrate of that alkali. The 

 second and more serious error is that of the announcement of 

 an oxide of iron possessing acid properties and forming a 

 combination with potash, affording a green solution with water. 

 After a careful examination I find this bright emerald green 

 solution to be manganate of potash, and potash holding ses- 

 quioxide of iron in solution. 



Having, in the paper referred to, satisfied myself that an 

 oxide of iron did form a salt with potash, and also that the 

 green salt contained this metal, I was too hastily led to infer 

 that both these solutions were salts of acid oxides of iron, not 

 suspecting the existence of manganese in the precipitated oxide 

 of iron; this impurity probably arises from the iron turnings 

 generally employed to saturate the excess of acid in the iron 



