330 
PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
so that when the instrument is attached to the battery no strain is ever exerted on 
the wires which carry the spiral. The ends of the thick wire to which the spiral is 
attached are also of stout platinum ; for when it was attached to copper wires, unsteadi- 
ness was introduced through oxidation. The heat issues from the incandescent spiral 
by the opening d , which is an inch and a half in diameter. Behind the spiral, finally, 
is a metallic reflector, r, which augments the flux of heat without sensibly changing its 
quality. In the open air the red-hot spiral is a capricious source of heat ; but sur- 
rounded by its glass globe its steadiness is admirable. 
The whole experimental arrangement will be immediately understood from the sketch 
Pig. 3. 
Grove’s battery of five cells. It is necessary that this lamp should remain perfectly 
constant throughout the day ; and to keep it so, a tangent galvanometer and a rheocord 
are introduced into the circuit. 
In front of the spiral, and surrounding the tubulure of its globe, is the tube B with 
an interior reflecting surface, through which the heat passes to the rock-salt cell C. 
This cell is placed on a little stage soldered to the back of the perforated screen S S, so 
that the heat, after having crossed the cell, passes through the hole in the screen, and 
afterwards impinges on the thermo-electric pile P. The pile is placed at some distance 
from the screen S S, so as to render the temperature of the cell C itself of no account. 
C' is the compensating cube, containing water kept boiling by steam from the pipe p. 
Between the cube C' and the pile P is the screen Q, which regulates the amount of 
heat falling on the posterior face of the pile. The whole arrangement is here exposed ; 
