248 Prof. S. U. Pickering on a New Form 



error by starting with the two liquids in the same vessel : the 

 other objects to be considered are, however, numerous, and 

 create considerable difficulty in obtaining a satisfactory appa- 

 ratus. The volumes of the liquids to be mixed are generally 

 required to be equal, and should not exceed 500 cub. centim. in 

 each case : this gives very little spare room, when each liquid 

 contains a thermometer with a large double bulb, and stirrer. 

 No part of the apparatus must be removed, nor must any- 

 thing be introduced into the liquids to effect the mixing. 

 Whatever operation is necessary to mix the liquids must be 

 performed quickly, so as to avoid the disturbing influence of 

 the experimenter ; and must be consistent with the calori- 

 meter not being held in its position, but with its resting on 

 pointed wooden supports. Finally, the mixing of the liquids 

 must not occupy more than one minute. 



The apparatus which I ultimately adopted, after many un- 

 successful trials, consisted of an oblong platinum vessel (figs. 

 1 and 2, Plate VII.), measuring 14 by 8 centim. and 10 centim. 

 deep, with a frame, F, in the middle, against which small 

 folding-doors, D, close, and divide the vessel into two equal 

 compartments. A thin slip of sheet indiarubber, I, is sewn on 

 to the edges of these doors, and a small beading, B, running- 

 round the doors and round the partition against which they 

 close, affords a good bedding for the indiarubber, and prevents 

 the leakage, which it was found impossible to obviate when 

 the surfaces of the doors and frame were flat. Each door is 

 kept closed by the pressure of two pairs of bow-shaped pieces 

 of iridio-platinum, S, which act as springs. One of these is 

 at the bottom of the doors, and is attached to a rod, R, of the 

 same alloy, rising vertically above the top of the calorimeter ; 

 the other is at the top of the doors, and is worked by being 

 attached to a metal tube, T, through which the above- 

 mentioned rod passes. The rod and tube each terminate in 

 ebonite knobs, E ; and, when these knobs are pressed together, 

 the lower spring is lowered below the door, while the upper 

 one is raised above it, and the latter consequently opens, and 

 permits the liquids to mix. To allow for the removal of the 

 lower spring in this manner, the doors do not come within 

 5 or 6 millim. of the bottom of the vessel, and this also allows 

 the stirrer to work freely at the bottom of the liquid. 



Under the bottom of the vessel there are four small cup- 

 shaped projections which help to steady it on the pointed 

 supports on which it rests. 



The various parts of the apparatus are all distinct, and are 

 easily unscrewed and removed for the purpose of cleaning and 

 drying. 



