Chemistry and Physics. 237 
the motion must take et ean the parts of the instrument 
itself; or else we are driven to the most improbable alternative 
that lampblack and mica should have such a remarkable selective 
power that the impulses, then supposed in some mysterious way 
to be imparted by the beam of light, could exert a repulsive force 
at one surface and an attractive force at the other. Were there, 
faces were protected or not. It was now an obvious step to heat 
the glass of the bulb pi pees“, of the vanes in order to see if 
under these cireumstances we should not obtain a reverse action. 
This was easily effected “both by ace ng the dase over a lamp 
and also by placing the bulb pase A toward the beam from 
the lantern, so that it heated the glass without striking the vanes 
of a wheel. We thus easily obtained the now familiar reverse 
motion, but which was to us a new Dene very. We weep felt 
bairketees justified in spiking of the radiometer, as we di 
communications referred to, as a heat engine, of which, when x mov- 
ing in the oo eae way, the — surface of the va 8 is 
yeh then the parallel with he steam engine, which we seh Sak 
gested the extension of the dynamical theory of gases to include 
the new phenomena almost in the very words used above. More- 
over, while exhausting the large number of radiometers of which 
we have 5 spoken, we noticed variations in the effects kg ei 
caused by differences in diathermancy between crown and flin 
glass, and were thus led to realize what an important ait this 
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also devised an appé aratus for measuring the force exerted under 
determinate conditions, but our experiments were brought to an 
end by the sudden aak of the only skillful glass blower in our 
nei ipitiochs od. 
