466 
When a fine metallic wire is placed in the circuit of a voltaic 
circle, it is well known that it will become heated, and the 
temperature which it finally acquires (provided the length 
of the wire remain the same, and the action of the battery 
continue constant) will depend upon the cooling power of 
the medium in which the wire is placed. If the current be 
of sufficient intensity to heat the wire to redness in air, the ' 
variations in its appearance, when placed in other gases, 
will exhibit, at a glance, their relative cooling powers. 
But, since the conducting power of wires for electricity di- 
minishes as their temperature rises, a measure of the effect 
may be obtained by ascertaining the changes produced in 
the intensity of the current, which will increase or diminish, 
according to the greater or less cooling power of the me- 
dium in which the wire is placed. 
The battery, employed in the following experiments, 
consisted of four large cells, on Daniell’s construction, 
charged with his standard solutions; and of a small cell, 
composed of an exterior cylinder of amalgamated zinc, and 
an interior plate of platina, the latter being separated from 
the former by a cylindrical membrane, and both immersed in 
dilute sulphuric acid. The hydrogen gas, disengaged from 
the platina plate, was collected in a graduated tube, and its 
volume taken as a measure of the intensity of the current. 
A platina wire, about 2°5 inches long, and 735 in dia- 
meter, was stretched in the middle of a wide glass tube, by 
means of copper pincers, which were connected by thick 
wires of the same metal with the poles of the battery. The 
glass tube was so adjusted as to be easily traversed by a cur- 
rent of gas, which afterwards escaped from beneath a sur- 
face of mercury, and the connecting wires being passed 
through collars of caoutchouc, the whole apparatus was ren- 
dered perfectly air-tight. 
In making the observations, a current of the-gas, care- 
fully dried, was passed in great excess through the appara- 
