^20 dr. TYNDALL ON MOLECULAR INFLUENCES. 
The body to be examined is reduced to the shape of a cube, and is placed, by means 
of a pair of pliers, upon the four supports aicd; the slider S is then drawn upagamst 
the cube and the latter becomes firmly clasped between the projections of the piece 
of ivory ir, on the one side, and those of the slider S, on t e other. The chambers 
mm! being filled with mercury, the membrane in front of each is pressed g y 
rds the cube by the interior fluid mass, and in this way perfect contact is secured, 
fnict the rinciple here applied is the same as that made use of by Foub.bk* in his 
IheLometel of cLtact, although both instruments have nothing else in common. 
TrZblem which Requires solution is the following :-It is required to apply a 
source of heat of a strictly measurable character, and always readih^attmnab e to 
that face of the cube which is in contact with the membrane at the end of the slider, 
and to determine the quantity of this heat which crosses the cube to the opposite 
face in a minute of time. For the solution of this problem two things are reqmied 
-first, the source of heat to be applied to the left hand of the face of the an 
secondly, a means of measuring the amount which has made its appearance at the 
onnosite face at the expiration of a minute. ^ 
To obtain a souree of heat of the nature described the follm™g method was 
adopted — B is a small galvanic battery, from the negative pole of which a cuirent 
proLds to the galvanometer of tangents T ; passes round the ring of the mstiumen , 
deflecting, in its passage, the magnetic needle which hangs m the centie of the iin 
The strength of the current is, as is known, proportional to the tangent of the ang e 
of permanent deflection. From T the current proceeds to ^ 
instrument consists of a cylinder of serpentine stone, round which a Ge.man 
is coiled spirally ; by turning the handle of the instrnment any reqnired quan- 
tity of this powerfully resisting wire is thrown into the circuit, the current being t lus 
regulated al pleasure. The sole use of these two last instruments m the pi esen , , 
of experiments is to keep the current perfectly constant from day to day . F.on the 
rheostat the current proceeds to the cistern c, thence through the bent wire, and back 
to the cistern c', from which it proceeds to the other pole of the battery. 
The bent wire, during the passage of the current, becomes heated ; this hea is 
transmitted through the mercury in the chamber m' to the membrane in front of the 
chamber ; this membrane becomes the proximate source of heat which is applie o 
the left-hand face of the cube. The quantity transmitted from this source, throu^u 
the mass of the cube, to the opposite face, in any given time, will, of course, depen 
on the conductivity of the latter, and its amount may be estimated from the defl - 
tion which it is able to produce upon the needle of a galvanometer connected wit 
the bismuth and antimony pair. G is a galvanometer used for thj^s purpose ; from n 
proceed wires to the mercury cups M M', which, as before remarked, are connected 
L platinum wires with A and B. The galvanometer is a carefully constructed and 
dllLte instrument from the workshop of that skilful mechanic, Kleiner, in Berlin. 
* Annales de Chimie et de Physique, Mai'ch 1828. 
