﻿Coloured Cloudy Condensation, 35 



Explanations of this kind are, however, too hazardous to be 

 satisfactory. I therefore resolved to enter into the question 

 thoroughly by exploring what may be called the thermal 

 topography of the steam-jet, i. e. the longitudinal and radial 

 distribution of temperature throughout the jet for all 

 actuating steam-pressures, and for as many temperatures 

 of the surrounding air as I could obtain. The data * thus 

 laboriously gained did not, however, enable me to fashion as 

 trenchant an argument as I had looked for, chiefly because it 

 is difficult, even with the best thermoelectric means, to measure 

 the temperature of the narrowing jet quite up to the nozzle ; 

 and it is just here that the region of marked supersaturation 

 is located, as is evidenced by the sensitiveness of these parts 

 (Aitken). It seems expedient, therefore, to postpone decision 

 for the present, merely stating that if the degree of super- 

 saturation (measured isothermally as pressure) necessary for 

 spontaneous condensation can be reached, the data, in virtue 

 of the Kelvin formula, must lead to a new method for mea- 

 suring the molecular diameter peculiar to water-vapour — a 

 method, moreover, which need not fail for other vapours. 



In concluding, it is interesting to advert to the cor- 

 responding phenomenon in the case of the solidification of a 

 liquid. I have shown f that in an isothermal march the 

 pressure at which a liquid freezes is apt to be much greater 

 than the pressure at which the same (solid) substance again 

 melts. In case of naphthalene, for instance, the magnitude of 

 the lag may reach 300 to 400 atmospheres. One is therefore 

 tempted to seek an analogous explanation for the stated 

 difference of melting and freezing pressures at the same 

 temperature, in the absence of nuclei, postulating a tendency to 

 liquefy (osmotic pressure ?) even in solid substance J, varying 

 with the curvature of the surface. A difficulty, however, now 

 presents itself, inasmuch as a liquid cooled below its melting- 

 point (unlike the corresponding case for vapours) can be kept 

 in a solid vessel without solidifying. Hence the method for 

 measuring liquid molecular diameters which seems to loom 

 into view is not so near at hand. 



The Smithsonian Institution, 

 Washington, D.C., U.S.A. 



* These are fully given in the Weather Bureau Bulletin cited, which 

 has now been in the press for about a year. 



t Barus, Amer. Journ. of Science, xlii. p. 125, 1891. For a full 

 account, see Bulletin of the U.S. Geological Survey, No. 96, p. 83, 1892. 



\ Evidence in favour of the occurrence of solid surface-tension is given 

 by Auerbach (Wied. Ann. xliii. pp. 61, 94, 1891) and others. 



D2 



