62 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



the latter case I concluded that the impurity upon the surface of the platina was of a 

 nature not to be removed by the mere solvent action of the alkali, for when the plates 

 were rubbed with a little emery, and the same solution of alkali (592.), they became 

 active. 



600. The action of acids was far more constant and satisfactory. A platina plate 

 was boiled in dilute nitric acid : being washed and put into mixed oxygen and hy- 

 drogen gases, it acted well. Other plates were boiled in strong nitric acid for periods 

 extending from half a minute to four minutes, and then being washed in distilled 

 water, were found to act very well, condensing one cubic inch and a half of gas in 

 the space of eight or nine minutes, and rendering the tube warm (570.). 



601. Strong sulphuric acid was very effectual in rendering the platina active. A 

 plate (569.) was heated in it for a minute, then washed and put into the mixed oxygen 

 and hydrogen, upon which it acted as well as if it had been made the positive pole of 

 a voltaic pile (570.). 



602. Plates which, after being heated or electrized in alkali, or after other treat- 

 ment, were found inert, immediately received power by being dipped for a minute or 

 two, or even only for an instant, into hot oil of vitriol, and then into water. 



603. When the plate was dipped into the oil of vitriol, taken out, and then heated 

 so as to drive off the acid, it did not act, in consequence of the impurity left by the 

 acid upon its surface. 



604. Vegetable acids, as acetic and tartaric, sometimes rendered inert platina active, 

 at other times not. This, I believe, depended upon the character of the matter previ- 

 ously soiling the plates, and which may easily be supposed to be sometimes of such a 

 nature as to be removed by these acids, and at other times not. Weak sulphuric acid 

 showed the same difference, but strong sulphuric acid (601.) never failed in its action. 



605. The most favourable treatment, excepting that of making the plate a positive 

 pole in strong acid, was as follows. The plate was held over a spirit-lamp flame, and 

 when hot, rubbed with a piece of potassa fusa (caustic potash), which melting, covered 

 the metal with a coat of very strong alkali, and this was retained fused upon the 

 surface for a second or two* : it was then put into water for four or five minutes to 

 wash off the alkali, shaken, and immersed for about a minute in hot strong oil of 

 vitriol ; from this it was removed into distilled water, where it was allowed to remain 

 ten or fifteen minutes to remove the last traces of acid (582.). Being then put into 

 a mixture of oxygen and hydrogen, combination immediately began, and proceeded 

 rapidly ; the tube became warm, the platina became red hot, and the residue of the 

 gases was inflamed. This effect could be repeated at pleasure, and thus the maximum 

 phenomenon could be produced without the aid of the voltaic battery. 



606. When a solution of tartaric or acetic acid was substituted, in this mode of 

 preparation, for the sulphuric acid, still the plate was found to acquire the same power, 



* The heat need not be raised so much as to make the alkali tarnish the platina, although if that effect does 

 take place it does not prevent the ultimate action. 



