Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/696

Rh the liquid, and knowing from our experiments that the relation between capillary ascent and temperature is a nearly linear one, the rise in the tube $$T$$ could be calculated approximately enough for our purpose by constructing a diagram in which the rise at atmospheric temperature was made one end of a straight line connecting it with the critical temperature, where the rise is zero. At intermediate temperatures the rise in the tube $$T$$ was read off from the diagram; it may be taken as practically accurate; for any divergence from truth is much less than the limits of error in reading the ascent in the narrow capillary tube. This rise was therefore added to the apparent rise in the capillary tube, and the sum was taken to represent the total rise.

No correction has been made for the increase in internal diameter of the capillary tube due to rise of temperature. The surface-tension is directly proportional to the diameter of the tube; but as the latter is increased by only 0.5 per cent, for a rise of temperature of nearly 400°, the correction was neglected.

Nor has any correction been introduced for the form of the meniscus. The total correction should have amounted to 0.05 millim. But readings were taken with the lower edge of the cross-line of the cathetometer on the lower surface of the meniscus, so that the minute error was thereby somewhat compensated.

In calculating the results of experiments so as to ascertain the surface-tension, it has been assumed that there is no angle of contact between the liquid and the glass. The reasons for this assumption are as follows:—

A tube of capillary bore (0.65 millim.) contained ether free from air. It was connected with the pressure-apparatus, and its temperature was raised. Bubbles were made to appear in the tube, and it was found possible, at any given temperature, to keep them stationary; they did not ascend, or more correctly, the rate of ascent was exceedingly slow, so that a bubble might be kept for an hour, without rising a millimetre. A bubble in such a capillary tube is confined on its lower and upper surfaces by cups of liquid; the walls of the tube between these cups is wet, it is true, but by an exceedingly thin film of liquid; so thin that the transference of liquid from the upper to the lower part of the tube is very slow, and hence the almost stationary position of the bubble.

By decreasing the volume of the bubble on screwing in the plunger, the cups approached each other, the lower cup rising nearer to the dome confining the bubble of vapour on its upper side. The bubble began to ascend rapidly only when its vertical diameter had been made equal to its horizontal diameter; i.e., when it was approximately spherical. Above 160° $$\mathrm{C}$$. a bubble of ether refused to remain stationary in the tube, but at once ascended; and with ethyl alcohol, the ascent commenced above 220°. These temperatures are evidently conditioned by the stability of the MDCCCXCIII.—A.