84 Dr. Meyer Wilder man on the Velocity of 



the temperature of the liquid. Even with such precautions 

 salt crystals easily separate from the solution, and it becomes 

 necessary to redissolve them. Sometimes half a dozen expe- 

 periments must be made, before a solution sufficiently super- 

 saturated (without crystals, or with so few crystals that they 

 may be neglected) is obtained. For this reason it is always 

 absolutely necessary to examine the liquid by illuminating it 

 in the inner chamber from time to time, and to besin the 

 experiment as soon as a few crystals have separated. The 

 crystals seem not to separate so readily from the supersaturated 

 solutions when the inside of the beaker and the stirrer are 

 covered with a thin layer of indiarubber, which was easily 

 done by means of an indiarubber solution. 



b. This difficulty does not exist at any rate for some over- 

 cooled liquids, e. g. water and unsaturated aqueous solutions 

 from which the solvent (e. g. ice) is to he separated. We can 

 easily overcool water or an aqueous solution one degree, or 

 even more, below the freezing-point without any ice separating, 

 if the necessary precautions be taken. On the other hand, it 

 is very difficult, with some solvents like benzene, to get a 

 sufficiently great overcooling. The investigation of such 

 liquids presents great difficulties; the velocity of separation 

 being here very great, so that it can only be carried out 

 between smaller limits of overcooling. The precautions to 

 be taken in overcooling water and aqueous solutions are the 

 following: — The liquid bath in which the liquid is cooled 

 down must not be too cold, and a continuous stirring of the 

 liquid must be kept up to prevent the formation of too cold 

 layers at the sides of the beaker, — the stirrer should not 

 touch the sides of the beaker. The general arrangements of 

 the experiment are those described in the paper of the late 

 P. B. Lewis "On the Freezing-Point Method" (Chem. Soc. 

 Trans. 1894). The number of stirrings was 36 per minute. 

 The convergence-temperature and other constants were deter- 

 mined by the methods given given in the paper already 

 quoted (Phil. Mag. Dec. 1897). 



c. The investigation of the velocity of ice melting was 

 carried out with the arrangements used for ice separation (b). 

 Cubes of ice were used. A cage in the form of a cube was 

 fixed to the stirrer, each side of it consisting of three rods, 

 covered with indiarubber tubes to prevent the formation of 

 grooves in the ice. The ice cube was placed in the cage, 

 closed up in it by the upper lid, and was taken of such a size 

 as not to be able to rotate, and kept under the surface of the 

 water while stirring was going on. They were prepared in 

 the following way: — A cube of ice was cut by a saw from a 

 pure transparent block of ice which had been exposed for some 



