Although the purity of the materials used is of importance, it should be borne in mind that it is the purity of the surface separat ing the juxtaposed ﬂuids that must primarily be considered. This is quite a different thing from the chemical purity of the materials in bulk. Exceedingly small amounts of physical contamination may affect the surface most seriously. Whatever tends to lower the sur face tension will tend to accumulate in the surface. For this reason, other things being the same, the higher values of the surface tension are worthy of the greater confidence.