154 Dr. Andrews on the Properties of Voltaic Circles. 



not perceptibly affected by altering the extent of the platina surface exposed to 

 the liquid, unless that surface was reduced to less than -^ of the surface of the 

 zinc ; and even when it amounted to ■— of the latter, the deflection of the 

 needle was only one-half less than with equal surfaces. This is very different 

 from the well-known effect of similar changes in the extent of the surface 

 exposed by the electro -negative metal in voltaic circles formed with the dilute 

 acids. As a term of comparison, the platina surfaces used in experiments 

 1 and 4, being connected with similar zinc plates as before, and introduced into 

 a mixture of dilute nitric and sulphuric acids, the deflections were 25°. 5 and 7° 

 respectively. 



Although there was no visible disengagement of sulphurous acid gas from 

 the zinc in the preceding experiments, except in No. 7, yet by comparing these 

 results with those before obtained, it will appear that increasing the platina sur- 

 face tends to arrest more completely the ordinary or local action of the acid on 

 the zinc. 



When a slip of zinc in heated sulphuric acid was made the positive pole of 

 a galvanic battery of twenty pairs of plates in moderate action, sulphurous acid 

 ceased to be evolved from its surface, and the solution of the metal was greatly 

 retarded. 



As a contrast with the preceding results, the influence of mercury, in con- 

 nexion with zinc, upon the solution of the latter metal, may be mentioned. If 

 these metals are heated separately in concentrated sulphuric acid, till a gentle 

 effervescence occurs at the surface of both, and then brought into contact, a very 

 violent chemical reaction instantly occurs, an amalgam appears at first to be 

 formed, and afterwards the zinc dissolves with the utmost degree of violence. 

 It is the most remarkable example of increased chemical action from the forma- 

 tion of a voltaic combination with which I am acquainted! These facts are the 

 more singular, as it thus appears that the influence both of amalgamation and of 

 contact with platina on the solution of zinc is reversed in dilute and concentrated 

 sulphuric acid. 



The general phenomena presented by the other metals capable of decom- 

 posing sulphuric acid were similar to these already described in the case of zinc, 

 but in the details there were some important points of difference. 



