220 Mr. Callendar on Experiments ivith a Platinum 



This is the value for the excess of vapour-pressure in equi- 

 librium with a convex surface that is given in Maxwell's 

 " Heat " as a deduction from Sir W. Thomson's principle. 



The application of this principle may be extended in another 

 direction. When liquid rises in a capillary tube open above, 

 the more attenuated vapour at the upper level is in equilibrium 

 with the concave surface, and the more dense vapour below is 

 in equilibrium with the plane surface ot the liquid. But, as 

 was pointed out in the former paper, the rise of liquid is not 

 limited to the height of the meniscus. Above that point the 

 walls of the tube are coated with a layer of fluid, of gradually 

 diminishing thickness, less than the range of forces, and 

 extending to an immense height. At every point the layer of 

 fluid must be in equilibrium ivith the vapour to be found at the 

 same level. The data scarcely exist for anything like a prer 

 cise estimate of the effect to be expected, but the argument 

 suffices to show that a solid body brought into contact with 

 vapour at a density which may be much below the so-called 

 point of saturation will cover itself with a layer of fluid, and 

 that this layer may be retained in some degree even in what 

 passes for a good vacuum. The fluid composing the layer, 

 though denser than the surrounding atmosphere of vapour, 

 cannot properly be described as either liquid or gaseous. 



In our atmosphere fresh surfaces, e.g. of split mica or oi 

 mercury, attract to themselves at once a coating of moisture. 

 In a few hours this is replaced, or supplemented, by a layer of 

 grease, which gives rise to a large variety of curious pheno- 

 mena. In the case of mica the fresh surface conducts elec- 

 tricity, while an old surface, in which presumably the moisture 

 has been replaced by grease, insulates well. 



XXI. Some Experiments with a Platinum Pyrometer on the 

 Melting-points of Gold and Silver. By H. L. Callendar, 

 M.A., Fellow of Trinity College, Cambridge*. 



[Plate V.] 



IN a paper which appeared in the Philosophical Magazine 

 for July 1891 I alluded to some experiments I had made 

 with a platinum pyrometer on the melting-point of silver, and 

 stated that the readings of these instruments were constant to 

 a tenth of a degree at temperatures above 1000° C. 



Through the kindness of Prof. Eoberts-Austen I have 

 recently had an opportunity of making some further experi- 



* Communicated bv the Author. 



