Measuring the Surj ace- Tension of Liquids. 391 



a rod of almost any material sufficiently rigid which is wet by 

 the liquid will serve as well. For mercury a steel needle 

 plated with copper and well amalgamated has been -suggested. 

 In my investigations the rod was of glass, and formed part 

 of a glass frame (fig. 1) whose form was varied to suit the 

 circumstances. The essentials of a good frame are the 

 following : — 



(a) It must not yield sensibly under the tension of the 



film. 



(b) The bar m must be thin, and all parts of m and n must 



lie evenly in the vertical plane. 



(c) The frame must be balanced so that m hangs horizon- 



tally both with and without the film. 



(d) The two short sides, nn, must be of uniform and nearly 



equal diameter, must be as nearly as possible parallel, 

 perfectly straight, and make sharp right angles 

 with m. 

 With a little practice and more patience good frames were 

 readily made in the following way. Taking ten centimetres of a 

 small hard-glass tube, I bent it at a right angle near one end, 

 then fused that end until only a minute hole with a very thick 

 margin was left. This was connected by a thin rubber tube to a 

 gas-pipe, and the small jet lighted. It burned with a clear blue 

 flame 10 to 15 millim. long, and quite hot enough to melt 

 glass rods a millimetre in diameter. If the orifice is too large 

 the flame has a white tip and is not so hot. An improved 

 jeweller's blowpipe connected by slender rubber tubes to the 

 gas-supply and to the mouthpiece will do nearly as well. 

 This blowpipe is held in the hand. Glass threads of suitable 

 size are laid upon an asbestos mat, which has a number of 

 pencil lines drawn upon it exactly at right angles, and are 

 there fused together in the required form. For general use 

 a frame like the one shown in the figure may be made of rods 

 ^ to 1 millim. thick (the handle thicker) ; having the bar m 

 10 centim., n 2 centim.,and the handle 15 to 20 centim. long. 

 In some cases the lower horizontal bar may be omitted with 

 advantage. 



The frames were measured along lines parallel to m, be- 

 tween the inner edges of nn, at measured distances from m. 

 If n n were not nearly parallel they were reset with the hand 

 blow r pipe. The measurements were made by comparison with 

 a standard metre bar on the bed of a dividing-engine, where 

 it was easy to get a degree of accuracy in excess of what was 

 required for this investigation. 



Figure 2 is a diagram of the general arrangement for 

 weighing the tension. The balance stood upon a marble slab 

 on a high shelf projecting from a brick partition-wall of the 



