DR. TYNDALL ON MOLECULAR INFLUENCES. 
221 
The sole use of the mercury in the chambers m and yri is to secure good and 
equable contact ; when the chambers are filled with pure mercury, and this is allowed 
to remain in them throughout an entire series of experiments, it is certain that the 
conditions of contact are perfectly constant, and thus the most fruitful source of 
doubt and error is effectually excluded. In rough experiments the chambers might 
be dispensed with, and the bent wire itself might be brought into contact with one 
face of the cube, while the other face might immediately press against the bismuth 
and antimony. The result however of many hundreds of experiments made with the 
instrument in this state, has been to prove the impossibility of preserving the condi- 
tions of experiment constant, and to compel me to devise some means of avoiding 
the irregularities which exhibited themselves. The instrument just described meets 
the requirements of the case ; care is necessary in the use of it, but when care is 
taken, an accuracy is attainable by it which, I believe, has been hitherto unequalled. 
The action of mercury upon bismuth, as a solvent, is well known ; an amalgam is 
speedily formed where the two metals come into contact. To preserve our thermo- 
electric couple from this action, their ends are protected by a sheathing of the same 
membrane as that used in front of the chambers mrd. 
Previous to the cube being placed between the two membranes, the latter, by virtue 
of the fluid masses behind them, bulge out a little, thus forming a pair of soft and 
slightly convex cushions. When the cube is placed upon its supports and the slider 
is brought up against it, both cushions are pressed flat, and thus perfect contact is 
secured. The surface of the cube is larger than the surface of the membrane in con- 
tact with it* ; and thus the former is always firmly caught between the opposed rigid 
projections, the slider being held fast in this position by means of the spring r, which 
is then attached to the pin jo. The exact manner of experiment is as follows : — 
Having first seen that the needle of the galvanometer points to zero, when the thermo- 
circuit is complete, the latter is interrupted by means of the break-circuit key U. At 
a certain moment, marked by the seconds-hand of a watch, the voltaic circuit is closed 
by the key k, and the current is permitted to circulate for sixty seconds ; at the six- 
tieth second the voltaic circuit is broken by the left hand at k, while almost at the 
same instant the thermo-circuit is closed by the right hand at A:'. The needle of the 
galvanometer is instantly deflected, and the limit of the first impulsion is noted ; the 
amount of this impulsion depends, of course, upon the quantity of heat which has 
reached the bismuth and antimony junction through the mass of the cube during the 
time of action, and consequently upon the conductive power of the latter. The 
limit of the first impulsion being noted, the cube is instantly removed, and the instru- 
ment is allowed to cool until the needle of the galvanometer returns to zero. To 
expedite the cooling, the metallic surfaces of A and B are to a great extent exposed ; 
the wood is cut away all round them, and from the space between them ; they do not 
rest upon the wood, their sole points of support being the ivory i at one end and the 
* ITie edge of each cube measured O' 3 of an inch. 
2 G 2 
