178 
LORD RAYLEIGH AND MRS. SIDGWICK 
The mercury used for all the measurements except 10 and 14 was distilled in vacuo 
with an apparatus fitted up by Mr. Shaw. In order to see whether a different result 
might not be obtained with other mercury, some was procured from the chemical 
laboratory for measurements 10 and 14. For the latter a portion of this mercury was 
treated with nitric acid and distilled at atmospheric pressure. For measurement 10 
it was treated with nitric acid, but not distilled. An accident occurred in carrying out 
this measurement, so that only the resistance of the column was ascertained ; but this 
agrees so well with the resistances found with the same tube for the other mercury, 
that there is no reason to suppose that any discrepancy would have appeared in 
proceeding with the measurement further. 
The glass tubes used were supplied by Cassella, and were selected for uniformity 
of bore, so that the correction for conicality should be small. They were slender and 
easily broken, which made the manipulation of them difficult, and it w T as in fact owing 
to a breakage that the tube called No. I. was used so short. The measurements 
taken with it, at first intended to be preliminary, were, however, made with the 
same care as in the case of the other tubes, and the difference of length and resistance 
adds some variety to the data. Tubes II. and III. were cut so that their resistance 
should be as nearly as possible one B. A. unit. The section of tubes I., II., and III., 
was approximately 1 square millimetre. Tube IV. was a much larger one, introduced 
with a view of varying the data as much as could conveniently be done. The diameter 
of its bore was about 2 millims., and its length was nearly 2 metres. It was cut so as 
to save a resistance of about half a B.A. unit. 
The ends of the tubes were ground into a convex form with emery powder on a 
lathe, in order that the length (L) of the bore might be measured accurately. This 
measurement was effected by setting two microscopes, which could be adjusted 
longitudinally to the exact position required by micrometer-screws graduated to tottoo 
inch, so that their cross-wires should coincide with the ends of the tube. Observa¬ 
tions were made in three or four different positions as the tube was turned round its 
axis, and the mean taken. After removal of the tube, a brass measuring rod belonging 
to the British Association was substituted for it, and the number of whole divisions 
corresponding most nearly to the distance between the cross-wires of the two micro¬ 
scopes was read off. The outstanding fraction of a millimetre was then ascertained 
by screwing the microscope up to the whole division and reading the difference on the 
screw-head. For the long tube the measuring rod was too short, and a third micro¬ 
scope had to be used to fix an intermediate point as a fresh departure for the scale. 
A thermometer laid beside the tube during the measurement gave the temperature (f) 
at the moment. The brass measuring rod was carefully examined, and its divisions 
were found to agree among themselves. 
The tubes were cleaned by passing through them in succession, by means of a 
suction-pump, sulphuric acid, nitric acid, caustic potash, and distilled water, followed 
by air dried with chloride of calcium. The process with omission of the acids was in 
