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 ah*, 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 T ^ n 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- 



