128 Proceedings of the Royal Society 
research in which I was engaged, and rendered any quantitative 
experiment excessively difficult, because we have no means of 
determining the temperatures of such small masses, and because a 
very slight difference of temperature is enough to produce the ob- 
served effect. 
The statement that when A is warmer than B we have repulsion, 
but that when A is the colder we have attraction, cannot he uni- 
versal, because if A and B were merely to change names the 
enunciation of the law would be reversed. Such a law can only 
hold good between members of two distinct classes, and, so far as 
we have yet seen, this distinction is between solids and fluids. 
Reflection on this matter brought to my mind a phenomenon with 
which I have been familiar for more than half a century, and 
whioh I used to refer to some peculiar variety of what is called 
capillary action . In preparing a small drill, such as is used by 
watchmakers, the little tool is first hardened by being plunged 
while red hot into cold water, and is then tempered or softened to 
the proper degree. This tempering is done by dipping the drill 
in oil or tallow, and then heating the stock end of it in a small 
flame. The oil is seen to gather in a drop, which moves rapidly 
towards the point, and the ebullition of this drop serves to mark 
the proper temperature. 
If we coat a common smooth knitting needle with a film of oil 
so thin that it will not flow, and, holding the needle horizontally, 
bring the middle of it to the edge of a flame, we shall see a bulg- 
ing mass of oil form on each side and move away from the flame, 
gathering bulk as it proceeds. Here we have a variation of the 
very phenomenon seen in the calc-spar ; the fluid is repelled by 
the hotter metal. The experiment may be varied thus. Having 
placed the middle of a cleaned wire in the flame, put a small 
drop of oil on it near to the flame; this drop will be seen to 
move towards the colder part of the wire. Another variation is 
to prepare a thin metallic plate, and to coat its upper surface with 
a film of oil ; when the middle of the plate is set upon a piece of 
hot iron, the oil gathers in a wave all round the hot part, and slowly 
recedes from it. 
After having assisted at these experiments with the oil, Dr 
Hunter made a very beautiful variation, which consisted in direct- 
