Dr. Tyndall on Molecular Influences. 125 



reasons to be explained presently, is cut away, as shown by the 

 dotted lines in the figure) four conical ivory points, abed, project 

 upwards ; though appearing to be upon the same plane as the 

 upper surfaces of the bismuth and antimony bars, the points are 

 in reality 0*3 of an inch below the said surfaces. 



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 sup- 

 ports abed; the slider S is then drawn up against the cube, and 

 the latter becomes firmly clasped between the projections of the 

 piece of ivory IF, on the one side, and those of the slider S, on 

 the other. The chambers m m! being filled with mercury, the 

 membrane in front of each is pressed gently against the cube by 

 the interior fluid mass, and in this way perfect contact is secured. 

 In fact the principle here applied is the same as that made use 

 of by Fourier* in his thermometer of contact, although both 

 instruments have nothing else in common. 



The problem which requires solution is the following : — It is 

 required to apply a source of heat of a strictly measurable cha- 

 racter, and always readily attainable, 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 required : — first, the source of heat 

 to be applied to the left hand of the face of the cube, and 

 secondly, a means of measuring the amount which has made its 

 appearance at the opposite face at the expiration of a minute. 



To obtain a source of heat of the nature described the following 

 method was adopted : — B is a small galvanic battery, from the 

 negative pole of which a current proceeds to the galvanometer 

 of tangents T ; passes round the ring of the instrument, deflecting, 

 in its passage, the magnetic needle which hangs in the centre of 

 the ring. The strength of the current is, as is known, propor- 

 tional to the tangent of the angle of permanent deflection. From 

 T the current proceeds to the rheostat R ; this instrument consists 

 of a cylinder of serpentine stone, round which a German silver 

 wire is coiled spirally; by turning the handle of the instrument 

 any required quantity of this powerfully resisting wire is thrown 

 into the circuit, the current being thus regulated at pleasure. 

 The sole use of these two last instruments in the present series 

 of experiments is to keep the current perfectly constant from 

 day to day. From the rheostat the current proceeds to the cis- 

 tern c, thence through the bent wire, and back to the cistern c 1 , 

 from which it proceeds to the other pole of the battery. 



The bent wire, during the passage of the current, becomes 

 heated ; this heat is transmitted through the mercury in the 

 chamber m 1 to the membrane in front of the chamber ; this mem- 

 * Annates de Chimie et de Physique, March 1828. 



