276 Progress in Science. (April, 
whether it was placed above or below the brass ball. The fingers exerted a 
repellent action, as did also a warm glass rod, a spirit-flame, and a piece of 
hot copper.” In order to decide once for all whether these actions really were 
due to air-currents, a form of apparatus was fitted up, which, whilst it would 
settle the question indisputably, would at the same time be likely to afford 
information of much interest. By chemical means a vacuum was obtained in 
an apparatus so nearly perfect that it weuld not carry a current from a 
Ruhmffork’s coil when connected with platinum wires sealed into the tube. 
In such a vacuum the repulsion by heat is decided and energetic. An experi- 
ment is next described, in which the rays of the sun, and then the different 
portions of the solar spectrum, are projected into the delicately suspended 
pith-ball balance. In vacuo the repulsion is so strong as to cause danger to 
the apparatus, and resembles that which would be produced by the physical 
impact of a material body. Experiments are next described in which various 
substances are used as the gravitating masses. Amongst these are ivory, 
brass, pith, platinum, gilt pith, silver, bismuth, selenium, copper, mica 
(horizontal and vertical), charcoal, &c. The behaviour of a glass beam with 
glass ends in a chemical vacuum, and at lower exhaustion, is next accurately 
examined, when heat is applied in different ways. On suspending the light 
index by means of a cocoon fibre in a long glass tube, furnished with a bulb 
at the end, and exhausting in various ways, the author finds that the attraction 
to a hot body in air, and the repulsion from a hot body in vacuo, are very 
apparent. Speaking of Cavendish’s celebrated experiment, the author says 
that he has experimented for some months on an apparatus of this kind, and 
gives the following outline of one of the results he has obtained :—‘‘ A heavy 
metallic mass, when brought near a delicately suspended light ball, attracts 
or repels it under the following circumstances. 
“© T, When the ball is in air of ordinary density. 
a. If the mass is colder than the ball, it repels the ball. 
b. If the mass is Aotter than the ball, it attracts the ball. 
“TI. When the ball is in a vacuum. 
a. If the mass is colder than the ball, it attracts the ball. 
b. If the mass is hotter than the ball, it repels the ball.” 
The author continues :—‘ The density of the medium surrounding the ball, 
the material of which the ball is made, and a very slight difference between 
the temperatures of the mass and the ball, exert so strong an influence over the 
attraGive and repulsive force, and it has been so difficult for me to eliminate 
all interfering actions of temperature, electricity, &c., that I have not yet been 
able to get distiné& evidence of an independent force (not being of the nature 
of heat) urging the ball and the mass together. ‘‘ Experiment has, however, 
showed me that, whilst the action is in one direction in dense air, and in 
the opposite direG&ion in a vacuum, there is an intermediate pressure at 
which differences of temperature appear to exert little or no interfering 
action. By experimenting at this critical pressure, it would seem that such 
an action as was obtained by Cavendish, Reich, and Bailey, should be 
rendered evident.’’ After discussing the explanations which may be given 
of these actions, and showing that they cannot be due to air-currents, the 
author refers to evidences of this repulsive action of heat, and attractive 
action of cold, in Nature. In that portion of the sun’s radiation which is 
called heat, we have the radial repulsive force possessing successive pro- 
pagation required to explain the phenomena of comets and the shape and 
changes of the nebula. To compare small things with great (to argue 
from pieces of straw up to heavenly bodies), it is not improbable that the 
attraction now shown to exist between a cold and a warm body will equally 
prevail when, for the temperature of melting ice is substituted the cold of 
space, for a pith ball a celestial sphere, and for an artificial vacuum a stellar 
void. In the radiant molecular energy of cosmical masses may at last be 
foundthat ‘‘ agent adiing constantly according to certain laws,” which Newton 
held to be the cause of gravity. 
