into the Cause of Voltaic Electricity. 277 



coating of varnish was too thin to prevent this oxidation, which 

 took place through the pores that the alcohol produced by eva- 

 porating. In short, if a plate of zinc well brightened \decap4\ 

 be employed, it will be seen that whether M. Pfaff's method, 

 or that of M. Becquerel be pursued, it will become tarnished at 

 the end of a certain time ; proving that there has been an oxi- 

 dating action of the air, and the formation of a suboxide. The 

 only means by which I have succeeded in preventing all elec- 

 trical signs from the contact of the two plates is, by covering 

 the plate of zinc with a layer of lac-varnish, so thick as not to 

 be transparent ; in which case the air no longer gains access to 

 the zinc, and consequently cannot oxidate it. 



Sufficient attention has not in general been paid to the for- 

 mation of these coatings of suboxide upon the surface of most 

 of the metals, the existence of which may be easily proved by 

 comparing the parts of a metallic surface recently brightened 

 [decape] with one which has been for some time exposed to 

 the air. This formation is often very rapid, which is the rea- 

 son that much more distinct electric signs are obtained by 

 touching an electroscope with metallic surfaces at the very 

 moment after they have been brightened \decap£\\ this is 

 more especially true when, insulating these surfaces by employ- 

 ing a glass handle instead of the fingers, the oxidating action 

 of the air is the only chemical action to which they are ex- 

 posed. I have also proved the rapidity with which these coat- 

 ings of suboxide are formed, in studying the nature of the 

 electricity which is developed by the well-brightened [decape] 

 surfaces of most of the metals when they are rubbed with 

 wood, cork, or a dry finger, or any conducting body what- 

 ever, not metallic. If these surfaces be rubbed the very mo- 

 ment after they are brightened [decaptf] the electricity ac- 

 quired by the metal is always negative; but if the rubbing 

 be deferred for a longer or shorter time, according to the na- 

 ture of the metal, the electricity, even in dry air, is positive, 

 excepting with the metals which are not oxidable, or which 

 are only so in a very small degree. The cause of this altera- 

 tion is, that in the second case a coating of suboxide has had 

 time to form ; it is removed by the substance with which it is 

 rubbed, to which it adheres, and the friction then occurs be- 

 tween the metal and its oxide ; on which account the first is 

 positive. Lastly, I shall cite the following from the facts 

 which I have collected to prove this rapid oxidation of the 

 surface of most metals : If a metallic plate (not of platina), 

 which after having been brightened [decapf] has remained 

 for some time exposed to the air, be fixed to the negative pole 

 of a pile, and a plate of platina to the positive pole, and the 



