RICHARDS AND WELLS. — TRANSITION TEMPERATURE. 439 



rather than to wait for the ice to supply the needed liquid by melting. 

 Marek* has shown that pure ice after being rinsed and drained may 

 indicate a freezing point as much as 0°.016 too low; and six carefully 

 conducted experiments of our own indicated almost as great an error 

 from this source (0°.008). It is almost needless to state that in the final 

 experiments above the ice was properly mixed with the purest water. 



Tiie third cause of irregularity in the melting point of ice is not so 

 frequently considered. Nichols f has shown that the specific gravity of 

 ice may vary from 0.9161 to 0.9180, according to circumstances ; and it is 

 hardly conceivable that such varieties could give identical melting points. 

 It may be that after a skeleton of freely forming crystals has been built, 

 the interstices between the crystals are filled with a less compact struc- 

 ture.t Tyndall's well known experiment on the " flowers of ice" seems 

 to support this conclusion. Marek has shown that even the strains 

 produced in cracking the ice may cause serious errors, unless much water 

 is present. In our own experiments, as already stated, the ice was 

 made by freezing the purest boiled water in a platinum dish, and was 

 wholly clear and devoid of apparent crystalline structure. The pressure 

 under which the ice was formed was essentially the atmospheric pressure, 

 the layer of liquid being shallow, and the ice was easily broken. 



The transition temperature of sodic sulphate, a quadruple point, com- 

 pares favorably with this long established fixed point, in every respect. 

 The volume changes so little during the transition that even the pressure 

 of an atmosphere produces no perceptible effect on the temperature,§ the 

 purity of the substance is quite as easily obtained as that of water, and 

 the certainty of definite crystalline form and consequent homogeneity is 

 probably greater, because the habit of growth causes many independent 

 freely growing crystals, rather than a commingled heterogeneous mass. 

 The somewhat lower " latent heat of melting " is not a serious drawback 

 if reasonable precautions are taken to prevent the inflow of heat from 

 outside. 



The third constant point, 100°, involves pressure and the purity of 

 substance, like the others ; but since no solid enters into the question, 

 the doubt as to homogeneity is eliminated. To neutralize this advantage, 

 two disadvantages appear ; first the danger of superheating the vapor ; 



* Guillaume, Traite, p. 118. 



t Nichols, riiys. Kev., 8, 21 (1899) ; Z. pliys. Chem., 36, 240. 

 t See also Pernet, Guiilaume's Traite, p. 118. 



§ Dr. G. N. Lewis has found in this Laboratory that the volume change is not 

 over 0.5 per cent. 



