152 Mr. J. W. Clark on the Behaviour of Liquids 



heating is rapid, the depression in a capillary tube the im- 

 mersed half of which has been previously heated and drawn 

 out until the diameter is very small is less than it is in the 

 corresponding tube of uniform diameter throughout. Slow 

 heating reverses this result, as shown in fig. 3. By the mere 

 contraction of the extreme end of the immersed part of a 

 capillary tube the depression is almost uninfluenced. The 

 results above described are not invariable (and some points 

 connected with them are still the subject of inquiry); but 

 under conditions which are apparently the most free from 

 error they become sufficiently so to justify their being regarded 

 as normal. 



It seems probable that the effects of unequal and irregular 

 heating are completely removed from the above experiments; 

 for the liquid is equally depressed in two capillary tubes of 

 equal diameter, one made of very thick, and the other of very 

 thin glass. Rapidly heated, the depression reaches a maximum 

 in the thick glass tube. Of two similar capillary tubes the 

 most deeply immersed always shows the deepest depression; 

 but the length of the tube above the liquid is apparently 

 without influence. This led to the result described to the 

 Society in June 1879, viz. that when the capillary tube does 

 not dip more than a millimetre or two below the surface the 

 liquid disappears at the same level in the capillary and external 

 tubes. 



It has been previously stated that the liquid sometimes 

 appears convex near the critical temperature. With ether in a 

 tube 20 millims. in diameter, this apparent convexity is so well 

 marked that I had at first much difficulty in satisfying myself 

 that such was not the case. By means of a lamp, cylindrical 

 lens, and slit, a bright line of light is easily thrown into the 

 liquid in such a tube heated in a large beaker of oil. The 

 changes in the form of the reflected image were then ob- 

 served ; and from this it was inferred that when the liquid 

 surface appears convex and well-defined, as seen through the 

 horizontal telescope, it is slightly but unmistakably concave, 

 and remains so until it loses the power of reflecting when it is 

 plane. This apparent convexity is caused by the raising of 

 the far edge of the liquid by refraction; or perhaps it may 

 resemble mirage, as I have distinctly seen the surface of the 

 liquid in the horizontal telescope. The surface subsequently 

 becomes black and ill-defined, and, as Professor McLeod has 

 pointed out to me, closely resembles the surface of alcohol and 

 water in a test-tube into which they have been carefully poured 

 so as not to mix. The band of light which leaves the cylin- 

 drical lens, and passes through the vertical tube of ether, gives 



