THE RADIOMETER. 
169 
stration of the occult force. The radiometric balance, shaped 
like an inverted T? is developed, and the similar behaviour of 
various gases at the minimum tension is established. The effect 
of interposing various screens, such as rock salt, rock crystal, 
smoky talc, plate glass, water, alum, calc spar, and ammonio- 
sulphate of copper of varying opacity, is tried. Experiments 
are made with the electric and solar spectrum passed through a 
quartz train, showing the activity of the ultra-violet rays. 
After some discussion of the neutral point between attraction 
and repulsion, various explanatory theories are adverted to. 
The air-current and electrical theories are considered to be 
abundantly disproved. Professor Reynolds’s evaporation is in- 
cluded in this catalogue, from the impossibility of conceiving 
that sufficient gas or vapour was present, after repeated heating 
of the instrument to redness at the highest attainable exhaus- 
tion, to drive backwards a heavy piece of metal, or generally to 
produce the effects ascribed to it. The writer most explicitly 
guards himself against substituting for these any theory of his 
own. 
A supplement, received on April 20, examines more closely 
radiation from black and white surfaces, the former being the 
most energetically repelled. Here, for the first time, occurs 
the familiar name radiometer, and the substitution of rotation 
for oscillation. Four radial arms carry discs of pith lamp-blacked 
on corresponding sides, mounted on a central pivot, and enclosed 
in a globular vacuum. They revolve under the influence of 
radiation with a rapidity proportional to the intensity of the 
incident rays. Several of these instruments were exhibited at 
the soiree of the Royal Society, and from this period may pro- 
bably be dated the great public interest manifested in the 
phenomena. It is deserving of notice that in this paper occurs 
an allusion to the “ limiting walls of the space ” in which the 
rotation takes place, as though the possibility of the action being 
one of mutual reaction had already presented itself to the 
writer. 
At the anniversary meeting of the Royal Society on Novem- 
ber 30, 1875, the Royal medal was awarded to Mr. Crookes as 
already stated. 
Some experiments on the methods of obtaining perfect vacua 
formed the subject of a paper read before the Royal Society of 
Edinburgh on July 12, 1875. A brief resume is given in 
“ Nature ” of July 15, under the heading “ Charcoal Vacua.” 
It is there stated that the writers, Professors Tait and Dewar, 
had taken advantage of the power charcoal has of condensing 
gases. While the exhaustion by means of a mercury pump is 
going on, the charcoal is kept heated ; when the exhaustion has 
been carried as far as possible, the vessel is sealed, and, as the 
