128 Mr. R. Phillips on Minium. 



29*1 percent.; and in all these cases the residue remained 

 perfectly unchanged in colour : the quantity of protoxide dis- 

 solved by the fourth portion of acid I did not determine, for 

 the residue had acquired a slight brown tint, showing that a 

 portion of the pure minium had been decomposed. From 

 these experiments I conclude that the maximum of protoxide 

 had been dissolved by the acetic acid, and consequently that 

 the specimen consisted of 



Pure Minium 70*9 



Protoxide 29*1 



100-0 

 It now only remained to determine the composition of the 

 70*9 of pure minium. I therefore treated a portion of it with 

 great excess of diluted nitric acid, by which it was separated 

 into 



Protoxide 66 



Peroxide 34? 



100 



It will be observed that this determination agrees very nearly 

 with that resulting from the action of nitric acid upon red- 

 lead. I have already stated that it gave 25*3 per cent, of 

 peroxide, while, as will appear by a slight calculation, ac- 

 cording to the action of nitric acid upon the pure minium, it 

 should have yielded 24*1 per cent. 



We may consider the constitution of minium under dif- 

 ferent points of view: — first, merely as a compound of lead 

 and oxygen ; in this case 104-, an atom of lead appears to be 

 combined with 9*8 of oxygen, which are proportions irreduci- 

 ble to any probable atomic constitution. Indeed on inspecting 

 M. Dumas' s Table of the composition of eight different kinds 

 of red-lead, it must be admitted that this substance is a mix- 

 ture of true red oxide with variable proportions of prot- 

 oxide. 



The next question is, What is the constitution of the true 

 red oxide? by which I mean that left unchanged by the action 

 of dilute acetic acid upon common red-lead. 



Now I have shown that this is resolvable into 66 of prot- 

 oxide, and 34- of peroxide; and these results agree very closely 

 with those of M. Dumas, who states what he calls pure mi- 

 nium, to be composed of 65*1 protoxide, and 34-9 peroxide, 

 which I agree with him in considering as equivalent to two 

 atoms of the former, and one atom of the latter, or three atoms 

 of lead and four atoms of oxygen. And I have great pleasure 

 in observing that these results are in perfect accordance with 



