129 
of Edinburgh , Session 1872-73. 
ing a stream of warm air upon the end of the piece of spar under 
the microscope. The fluid recedes from that end, and the action is 
reversed by changing the position of the current. 
On holding horizontally a glass tube, of which the inside has 
been thinly coated with coloured oil, and on heating a part of it, 
the oil is seen to leave the heated part and become heaped up on 
each side. The same thing takes place with water; hut on 
making trial with sulphuric acid, no such effect was perceptible. 
The occurrence of so many analogous phenomena points to some 
general or, at least, comprehensive law ; and the question arises — 
Whether is this motion of the fluid dependent on actual contact, 
and due to the unequal heating of the adjacent solid, or is it a true 
repulsion between the colder fluid and the warmer solid, indepen- 
dent altogether of contact ? 
The instantaneous movement of the fluid when a warm body 
is brought near to without touching the spar, favours the latter 
interpretation of the phenomena, but the former interpretation 
seems to be more in accordance with the other variations of the 
experiments. If, when a warm body is brought near, the action 
be to induce an unequal heating of the containing vessel, and if 
the motions he due to the attractions or repulsions between the 
fluid and the spar, no real repulsion will be shown between the 
warm body and the total mass. But if the motion be due to 
a repulsion between the warm body and the colder fluid, the mass, 
as a whole, will be repelled. Hence, by poising the vessel contain- 
ing the fluid so delicately as to allow of this repulsion being 
exhibited if it exist, we shall be able to determine the true nature 
of the action. In making the arrangements we must eliminate 
the influence of aerial currents caused by the difference of tempe- 
rature. I am in hopes of being able soon to decide the question 
as between the two interpretations by help of an instrument of 
sufficient delicacy. 
In making the experiments with glass tubes, it was noticed 
incidentally that when the glass has been so heated as to drive 
the oil or the water completely from it, the surface has acquired 
the property of not being easily oiled or wet again. I show one 
tube, over the surface of which the oil flowed easily ; it has been 
hermetically closed, and has since that been heated. The oil now 
