FREDERICK GUTHRIE ON THE THERMAL RESISTANCE OF LIQUIDS. 647 
heat of 100° C. Now if 10 millims. of a liquid are athermanous to a heat-difference of, 
say, 80°, may not 3 millims. well be athermanous to a heat-difference of 10° X 
§ 39. Diffusion . — It is well known that when two unequally heated portions of the 
same liquid, or of two miscible ones are mixed, a state of thermal equilibrium between 
the two is established, and a temperature results intermediate between those of the 
mixed liquids, and dependent upon their temperatures, masses, and capacities for heat. 
If such experiments stood alone, it would be difficult, either by the dynamic or the con- 
ditional theory of heat, to disprove the assertion that no interchange of heat takes place 
between particle and particle, — that each particle maintains its original temperature, 
and that, being mutually diluted, the effect (for instance on the mercury of a thermo- 
meter) is the resultant of two unequal but continual efforts. 
§ 40. But the fact that substances in contact establish thermal equilibrium without 
mutual interpenetration, shows that the first method of heat equalization may and does 
take place. The question, however, still remains : — Is the equalization of temperature 
instantaneous when two liquids of unequal temperature are perfectly mixed 1 When a 
vessel of cold water is placed over a flame, may not the water at the bottom which has 
the temperature of the bottom, diffuse into the colder water without immediately losing 
its temperature*] All analogy, indeed, indicates the high improbability of such an 
occurrence, but by no means its impossibility. The question is of importance in our 
present direction of research ; for it may be maintained that the heat which appears at 
the lower cone has been conveyed downwards by the downward diffusion of the warmer 
liquid at the top, instigated by the difference in density of the upper and lower portions. 
§ 41. To set this question at rest, the bottom of the upper cone was painted with a 
soluble aniline dye. Sufficient water was placed upon the lower cone to fill an interval 
of 3 millims., and the upper cone was brought down to that distance. The conditions 
of the experiment were as follows : — 
T=13°-3 C. 
T 1 =23°*3 C. 
AT =10° C. 
^=10 min. (t l is the duration of the experiment) 
£=10 sec. 
At the end of 10 minutes, when nearly the maximum depression had been effected, 
some of the liquid in contact with the lower cone was cautiously withdrawn by means 
of a capillary pipette. The perfect colourlessness of this portion showed that no con- 
vection due to diffusion had taken place ; the only other alternative being the diffusion 
of the water away from the colouring-matter, — a hypothesis sufficiently improbable for 
us to conclude that no diffusion of the kind occurs. 
§ 42. This experiment at the same time furnishes an excellent test of the trust- 
* A giTnilnr suggestion in regard to air, occurs in Maury’s { Physical Geography of the Sea ; ’ the occasion being 
the calm-belts of Cancer and Capricorn. 
4 E 2 
