produced hy Electro-chemical Agency 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 I'eceived 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 paper was 

 read before the Mathematical Society, that this matter had been investi- 

 gated by Prof, Schcenbein, and published by him in the Bibliotheque Uni- 

 verselle for May, 1837, and that this investigation had succeeded a hint to 

 that effect fiom Professor Faraday, in volume x. of the Lond. and Edin. 

 Philosophical Mag. : for this information I am indebted to Mr. Brayley. 



