DILUTE SOLUTIONS AT THE FREEZING POINT. 327 



plug (g), through which a central platinum tube can freely revolve. The top of this 

 platinum is firmly attached to a brass sheath passing through a steel bearing. The 

 bearing is carried by an ebonite plug, fixed in a horizontal brass arm (e), which is 

 supported from the outside of the apparatus by a rigid vertical rod. The whole 

 central system then can revolve round a vertical axis, the steel ring at the top and 

 the gold plug below acting as bearings. The use of this central tube is twofold. In 

 it a mercury thermometer, graduated to i^-th of a centigrade degree, is placed, 

 contact with the platinum walls being secured by the addition of a few drops of 

 mercury. Secondly, it serves as the shaft of a platinum screw (/), shaped like a 

 ship's propeller, which, when it revolves, causes a downward current of water through 

 the platinum cage, and thus mixes the whole volume of solution. The shaft bears an 

 ebonite wheel at its top, which is turned when necessary by a hand wheel and cord. 



A second opening gives access to the interior of the cell, and a platinum tube (h), 

 about 8 millims. in diameter, springs vertically upwards from it. This tube has an 

 outer brass case, which can be closed by a stopper made of a piece of glass rod covered 

 with india-rubber tubing. When the stopper is in its place, no air can enter the cell 

 by this passage, and the only connection between the interior and the atmosphere is at 

 the top of the central screw shaft. How to close this aperture was a problem of some 

 difficulty. If the shaft were allowed to revolve in a simple hole, it would have been 

 apt to grind away fragments, which would have fallen into the liquid below, while 

 if the opening were left free, moist air would enter and water, condensing inside the 

 cold tube, would run down into the platinum vessel. In order to prevent any such 

 effects, the arrangement represented in the figure was adopted. A brass ring with a 

 rim projecting downwards, is fixed to the central shaft, and revolves with it. The 

 rim fits loosely, without touching, into a groove cut in a brass collar which lines the 

 inside of the brass supporting tube. Any air then has to pass over these cold metal 

 surfaces before it can enter the apparatus, and if any moisture is condensed, it is 

 caught in the groove and can be removed. 



The platinum vessel and the vertical tubes fixed to its roof, are surrounded with a 

 stout brass case, a thin air-space being left between them. In this air-space the 

 cooling apparatus is disposed. It consists of a shallow rectangular box, placed below 

 the platinum vessel, into which a tube is led through the outer brass case. The tube 

 ends in a nozzle opening near the bottom of the case, and through it a small 

 quantity of ether can be introduced. Dry air is then sucked through ; the ether 

 evaporates, and its vapour passes with the current of air along a spiral coil of tubing 

 which closely surrounds the platinum vessel. The evaporation absorbs heat, and the 

 temperature of the whole apparatus slowly falls. 



The outer brass case is fixed by means of three projecting arms in a large copper 

 tank of about 30 litres capacity, which can be filled with broken ice. The tank is 

 placed in a large rectangular box of wood and covered by a lid when measurements 

 are being made. 



