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 
zine 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 zine, 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 ofa 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 
