STONEY'S FORCE.

Globe, Volume IX, Issue 1286, 3 May 1878, Page 3

STONEY'S FORCE.

[" Scientific American."]

In a recent lecture on the spheroidal state of liquids, Professor Barratt said—To Mr Stoney is unquestionably due the great honor of having been the first fully to explain the true theory of the radiometer. It was in the course of these investigations that Mr Stoney has quite recently been led to show that the force which is bo active in the high rarefaction (that is necessary for the effective rotation of the radiometer) is also present at ordinary atmospheric tensions. Now it is this force which forms the new explanation of the entire phenomenon of the spheroidal state. Professor Barratt proposed to call it " Stoney's force." In order to understand the action that occurs it must be recollected that, according to calculation, the number of molecules of air that at ordinary pressure occupy the space of a pin's head is 1,000,000,000,000,000,000; when the radiometer globe is exhausted of these molecules of air as far as we can do it by mechanical means, there are still some few millions remaining, and these are in constant motion. Heat makes them move more rapidly, cold more slowly. If we have two surfaces very near each other, one surface hot and the other cold, from the hot surface the molecules will be thrown off with greater rapidity than they reached it; and if the cold surface be near enough, they will "bombard " it. Hence there will be a tendency in the hot and cold surfaces to retreat from one another, and when with one of these, as in the radiometer, this is possible, it ensues. This force would obviously disappear (1) if the residual molecules could be wholly removed or bo lessened in number that their action would be insensible, or (2) if the surfaces v/ere so far apart that the augmented molecular activity had expended itself before reaching the cool Burface. Applying the same kind of reasoning to the spherical state of liquids, we can see that it is only at relatively short distances from the metal the interaction will occur. A number of experiments were in conclusion showD, some with fluids from which there could be no vapor, such as the old theory requires, and others with fluids in which the difference in temperature was slight.