126 Mr. R. Phillips on Minium. 



oxide, of lead. Dr. Dalton in his work already alluded to, 

 considers the protoxide as composed of 7 oxygen and 90 lead, 

 and the peroxide of 14 oxygen and the same quantity of 

 metal. Now as these differ but little from the atomic propor- 

 tions usually adopted, I shall consider the protoxide as com- 

 posed of an atom of oxygen = 8 +104 an atom of lead, and 

 the peroxide of 16=2 atoms oxygen -f 104 of lead. 



It is well known that when red-lead is treated with nitric 

 or acetic acid, it is separated into protoxide, which is dis- 

 solved, and peroxide remaining unacted upon ; and Dr. Dal- 

 ton correctly observes, that if minium were a sesqui-oxide, it 

 ought to leave more than 50 per cent, of peroxide when acted 

 upon by either of the above-named acids ; the sample, how- 

 ever, which he examined left only 20 per cent, of peroxide, 

 proving that it was not a sesqui-oxide. 



Dr. Dalton concludes from his experiments that " the mi- 

 nium of commerce is constituted of 1 atom of oxygen, holding 

 6 atoms of yellow oxide together ; or it is composed of 100 lead 

 and 9*07 oxygen" And he observes, that " when it is di- 

 gested in cold acetic acid the residuum constitutes another oxide, 

 consisting of I atom oxygen and 3 of yellow oxide, or 100 

 lead, and 10*4 oxygen, possessing the same colour as the former, 

 but distinguishable by its not being acted on by cold acetic acid, 

 and by its containing twice as much brown oxide and extra oxy- 

 gen as minium." 



These experiments are, I confess, new to me, and certainly 

 have not excited the attention which they merit; and without 

 at present entering further into this part of the subject, I shall 

 just state that I have found acetic acid to separate a large pro- 

 portion of protoxide from minium, without altering its colour 

 at all; and consequently proving that it is not a sesqui-oxide, 

 for if it were, the residue would become brown by the de- 

 velopment of peroxide. If, however, the dilute acetic acid 

 be used in excess, then this change actually occurs, at least so 

 I found it; but this does not appear by Dr. Dalton' s experi- 

 ments. 



M. Dumas in the paper above referred to, does not state 

 the opinions of other chemists in detail, consequently it cannot 

 be ascertained whether he was acquainted or not, with those of 

 Dr. Dalton; he merely says, that previous analysts do not 

 agree as to the nature of minium. The facts stated by 

 M. Dumas indeed prove the existence of several varieties 

 of minium; and according to his experiments, which appear 

 to have been conducted with great care, the increase of 

 oxygen goes on very slowly from 8*26 per cent, in the pro- 

 duct of the first calcination, to 8*79 contained in the minium 

 of the eighth operation. He found also that white lead sub- 



