442 Prof. TyndalFs Contributions to Molecular Physics. 



screws a b, which are fixed in the foot of the stand, 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, unsteadiness was introduced 

 through oxidation. The heat from the incandescent spiral issues 

 by the opening d 3 which is an inch and a half in diameter. 

 Behind the spiral, finally, is a metallic reflector, r, which aug- 

 ments the flux of heat without sensibly changing its quality. In 

 the open air the red-hot spiral is a capricious source of heat ; but 

 surrounded by its glass globe its steadiness is admirable. 



The whole experimental arrangement will be immediately un- 

 derstood from the sketch given in fig. 3. A is the platinum 

 lamp just described, heated by a current from a Grove's battery 

 of five cells. It is necessary that this lamp should remain per- 

 fectly 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 pipejo. Between the cube C and the 

 pile P is the screen Q, which regulates the amount of heat fall- 

 ing on the posterior face of the pile. The whole arrangement is 

 here exposed, but in practice the pile P and the cube C are care- 

 fully protected from the capricious action of the surrounding air. 



The experiments are thus performed. The empty rock-salt 

 cell C being placed on its stage, a double silvered screen (not 

 shown in the figure) is first introduced between the end of the 

 tube B and the cell C — the radiation from the spiral being thus 

 totally cut off, and the pile subjected to the action of the cube C 

 alone. By means of the screen Q, the total heat to be adopted 

 throughout the series of experiments is obtained : say that it is 

 sufficient to produce a galvanometric deflection of 50 degrees. 

 The double screen used to intercept the radiation from the spiral 

 is then gradually withdrawn until this radiation completely neu- 

 tralizes that from the cube C', and the needle of the galvano- 

 meter points steadily to zero. The position of the double screen, 

 once fixed, remains subsequently unchanged, the slight and slow 

 alteration of the source being neutralized by the rheocord. Thus 

 the rays in the first instance pass from the spiral through the 



