produced hi) Electro-chemical Agencx) and by Heat, 55 
the poles are brought into too close proximity, brown oxide 
of lead is thrown down in a powdery form. Now it is a well- 
known fact, which may be found in almost every work on 
chemistry, that peroxide of lead is precipitated from solutions 
of that metal by means of a voltaic current, platina forming 
the poles It" is much to be regretted that Professor Nobili 
did not during the long period of his experiments put this 
matter to the test of chemical investigation. 
The circumstance of lead in a high stage of oxidation 
being deposited in aggregated films at the positive pole of a 
voltaic circuit, will bear out an observation of Prof. Schoen- 
bein, made at the last meeting of the British Association at 
Birmingham, that he believed that peroxide of hydrogen was 
sometimes formed during the decomposition of water, for that 
the volume of hydrogen gas eliminated was often more than 
double that of the oxygen. It will also afford a reason why 
iron should assume the inert or inactive state in voltaic com- 
binations, particularly in an experiment exhibited by Prof. 
Schoenbein, of heating one extremity of an iron wire so that 
it was covered with a coating of oxide, in the form of a co- 
loured film, and thus becoming inactive, while the end which 
had not been submitted to a similar operation was attacked 
with energy. 
We now pass on to the consideration of the second division 
of our subject, the nature of the coloured films produced on 
the surface of metals by means of heat. This is a part of 
the subject more familiar to all parties, whether scientific or 
not, for every one must have noticed the beautiful colours 
produced on the polished steel bars of an ordinary stove, or 
on the surface of a copper tea-kettle. In the arts its applica- 
tions are numerous and highly interesting, and the processes 
of tempering steel, the formation of the beautiful bronzing 
powders, of rose copper, and a variety of others, are too well 
known already to need anything more than a slight allusion 
to them. 
Before entering on the analysis of the subject I must put 
you in possession, as briefly as possible, of Professor Nobili’s 
views of the question. At p. 108, he states, As to these co- 
lours, the most generally received opinion is, that they de- 
pend on a principle of oxidation. Berzelius calls the metallic 
* I was not aware until the morning of the day on which this pajDer was 
read before the Mathematical Society, that this matter had been investi- 
gated by Prof. Schoenbein, and published by him in the Bibliotheque Uni- 
verselle for May, 1837, and that this investigation had succeeded a hint to 
that effect fi om Professor Faraday, in volume x. of the Lond. and Edin. 
Philosophical Mag. : for this information I am indebted to Mr. Brayley. 
