1864] 



Prof. Guthrie on Drops. 



467 



weight varies. Indeed with some metals, such as tin, a smooth and bright 

 surface is scarcely wetted by water. 



As the bodies examined have different coefficients of expansion by heat, 

 and one of them expands on solidification, it was necessary to test the 

 equality of their size and remedy any inequality. This was done by ar- 

 ranging three of them, one at each angle of a small equilateral triangle 

 drawn on a large piece of plate glass. Another piece of plate glass was 

 then placed upon the spheres so as to rest on them all three, and slightly 

 loaded. On passing a gauge between the plates, at their edges, the slightest 

 inequality of the spheres could be detected, because the gauge lifted the 

 plate off the smallest of the three balls, which could then be moved. The 

 larger spheres were then reduced in size by brisk agitation in acid. The 

 sulphur and phosphorus were, for the same purpose, washed in ether. 



Although there is only a slight difference between the consecutive terms 

 of column 3, yet between the extremes of antimony and tin a well-marked 

 difference exists. 



This Table shows that the drop-size stands in no simple relation either to 

 the equivalent density or chemical character of the solid, and establishes 

 the existence of a specific adhesion independent of these. Although the 

 differences of Table IX. may be partly owing to the differences of density 

 of the solids cork, glass, and brass, yet we see from Table X. that there 

 is about half as great a difference between the sizes of drops from antimony 

 and tin as between those from cork and brass, although the difference of 

 density between the first two is small compared with that between the last. 

 Again, sulphur gives rise to drops intermediate between those of antimony 

 and cadmium. Without, therefore, venturing to assert that density is with- 

 out influence on drop-size, it is clearly proved that it does not exert the 

 most powerful influence. 



We have finally to examine the direction and extent of variation in drop- 

 size caused by change of temperature. By altering the density of the 

 liquid, a change in its temperature may affect the drop-weight without 

 altering the drop-size. It may further alter the drop-size by altering the 

 size and therefore the curvature of the solid. Any error introduced by 

 the first of these sources is eliminated by dividing the observed weight by 

 the specific gravity at the proper temperature, as in the case of different 

 liquids at the same temperature. Errors from the second source may be 

 certainly safely neglected, being far within the errors of observation. 



In the place where these experiments were made, the range of natural 

 atmospheric temperature is very small. From the coldest to the hottest 

 season the difference scarcely exceeds 10° C. This circumstance made an 

 extended and minute study of the influence of temperature impossible, by 

 preventing more than one observation at each temperature being made. 



The liquid taken was water, and the solid was glass. The water was heated 

 to the boiling-point and placed in the apparatus (Part I. fig. 3). The 

 sphere from which the water fell was the bulb of the thermometer which 



