Dr. Tyndall on Molecular Influences. 123 



reader with a rehearsal of the long series of trials which have 

 led to the construction of the instrument employed in these 

 researches, but will proceed at once to the description of it. 



QQ' RR', fig. 1, is an oblong piece of mahogany 3*4 inches 

 long, 1*8 inch wide and 05 of an inch deep. A is a bar of an- 

 timony, B is a bar of bismuth, each measuring 1*5 inch in 

 length, 0*07 of an inch in breadth, and 0*3 of an inch in depth. 

 The ends of the two bars are kept in close contact by the ivory 

 jaws I, F, and the other ends are let into a second piece of ivory 

 i, in which they are firmly fixed. Soldered to these ends are 

 two pieces of platinum wire which proceed to the little ivory 

 cups MM', enter through the sides of the cups and communicate 

 with a drop of mercury placed in the interior. The wood is cut 

 away, so that the bars A and B are sunk to a depth which places 

 their upper surfaces a little below the general level of the slab of 

 mahogany. The ivory jaws I, I' are sunk similarly. Two small 

 projections are observed in the figure jutting from I, I' ; across 

 from one projection to the other a fine membrane is drawn, thus 

 enclosing a little chamber m, in front of the wedge-like end of 

 the bismuth and antimony junction ; the chamber has an ivory 

 bottom. S is a wooden slider, which can be moved smoothly 

 back and forward along a bevelled groove, by means of the lever L. 

 This lever turns on a pivot near Q, and fits into a horizontal 

 slit in the slider, to which it is attached by the pin p 1 passing 

 through both ; in the lever an oblong aperture is cut through 

 which p 1 passes, and in which it has a certain amount of lateral 

 play, so as to enable it to push the slider forward in a straight 

 line. A small chamber, m!, is cut out at the end of the slider, 

 and across, from projection to projection, a thin membrane is 

 stretched ; a chamber is thus formed bounded on three sides and 

 the bottom by wood, and in front by the membrane. A thin pla- 

 tinum wire, bent up and down several times, so as to form a kind 

 of micrometer grating, is laid against the back of the chamber 

 and imbedded in the end of the slider by the stroke of a hammer ; 

 the end in which the wire is imbedded is then filed down until 

 about half the latter is removed, and the whole is reduced to a 

 uniform flat surface. Against the common surface of the slider 

 and wire an extremely thin plate of mica is glued, sufficient, 

 simply, to interrupt all contact between the bent wire and a 

 quantity of mercury which the chamber m! is destined to contain : 

 the ends w w' of the bent wire proceed to two small cisterns, 

 c d, hollowed out in a slab of ivory ; they enter through the 

 substance into the cisterns, and come thus into contact with 

 mercury which fills the latter. The end of the slider and its 

 bent wire are shown in fig. 2. The rectangular space efgh, fig. 1, 

 is cut quite through the slab of mahogany, and a brass plate is 

 screwed to the latter underneath ; from this plate (which, for 



