THE RADIOMETER. 
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theory of gases, namely, that gases consist of separate molecules 
moving at great velocities. His apparatus was a light stem of 
glass, with pith balls on its ends, suspended by a silk thread in 
a glass flask, so that the balls were nearly at the same level. 
Some water was then put into the flask, and boiled until all the 
air was driven out of the flask, which was then corked and 
allowed to cool. When cold there was a partial vacuum in it, 
the gauge showing ^ to J inch pressure. It was now found 
that when the flame of a lamp was brought near to the flask, 
the pith ball which was nearest to the flame was driven away, 
and that with a piece of ice it was attracted. The pith balls 
were more sensitive to heat and cold when the flask was cold 
and the tension low, but the effect was perceptible until the 
gauge showed an inch, and even after that the ice would attract 
the ball. 
The reason why the repulsion from heat was not apparent 
at greater tensions was clearly due to convection currents which 
the heat generated within the flask. 
The condition of the pith also perceptibly affected the sen- 
sitiveness of the balls. When a piece of ice was placed against 
the side of the glass, the nearest of the pith balls would be 
drawn towards it, and eventually stop opposite it. If it re- 
mained there for some time, the vapour would condense on the 
ball near the ice, while the other ball would become dry, as was 
shown by the former becoming heavier. When the ice was 
removed, the dry ball was found insensible to the heat, or nearly 
so, while that which had been opposite the ice was more than 
ordinarily sensitive. 
If the flask were dry, and the tension of the vapour reduced 
with the pump until the gauge showed f inch, then the vapour, 
though purely steam, was not in a saturated condition, and the 
pith balls, which were dry, were no longer sensitive to the lamp, 
though they would still approach the ice. The last two facts 
-seemed to show that a certain amount of moisture on the balls 
was necessary to render them sensitive to heat. With a dry 
flask and an air vacuum neither the lamp nor the ice produced 
their effects, even when the gauge was as low as J inch. 
It was argued from these data that the real force in action 
was due to evaporation and condensation, a view which was 
xeduced to greater accuracy by means of mathematical formulae. 
The condensible vapour in Mr. Crookes’s experiments was held 
to be that of mercury ; and the necessity for a higher degree of 
exhaustion to be due to the fact that a larger proportion of air, a 
non-condensible gas, was mixed with the vapour, which also was 
not in a state of saturation. 
Professor Reynolds’s paper presents, in addition, a very re- 
markable appendix. “ Since writing the above,” he says, “ it 
