158 



zinc is heated to the temperature of 240° cent, in sulphuric 

 acid, of the sp. gr. 1.847, it is dissolved with the rapid dis- 

 engagement of a mixture of hydrogen and sulphurous acid 

 gas ; but when a similar plate, voltaically associated with a 

 platina wire, is introduced into the same acid, its rate of so- 

 lution is reduced to one-third of the other, no gas appears 

 at the zinc, and sulphurous acid, almost perfectly pure, sepa- 

 rates at the platina wire. Similar effects occur at other 

 temperatures, but the proportion between the quantity of 

 zinc dissolved when alone, and when connected with platina, 

 varies with the temperature. A minute investigation is given 

 of the effect of the distance between the metallic surfaces, 

 and of their relative extent upon the solution of the zinc, and 

 the development of the electrical current; from which it 

 appears that, as in common cases, the action on the zinc was 

 increased by diminishing the distance between the zinc and 

 platina in the liquid, but on the contrary, was diminished by 

 increasing the extent of the platina surface. The latter ano- 

 malous result is carefully examined and explained. 



The influence of the contact of platina with the other 

 metals, resembles, in general, its effect upon zinc, except in 

 the cases of mercury and arsenic, in which the solution does 

 not appear to be retarded in this way, nor is there almost 

 any gas evolved from the platina. 



The general conclusion drawn by the author from all 

 his experiments is, that the formation of a voltaic circle gene- 

 rally diminishes, and never increases chemical action, when 

 the liquid conductor is an oxy-acid of such a strength, that 

 the electro-positive metal is oxidized from the decomposi- 

 tions, not of the water, but of the acid itself. 



Professor Mac Cullagh exhibited and described a new op- 

 tical instrument, intended chiefly for the purpose of making 

 experiments on the light reflected by metals. The instru- 

 ment consists of two hollow arms or tubes, moveable about 



