166 
POPULAR SCIENCE REVIEW. 
results with concentrated solar light acting on a fixed and a 
suspended disc ; Saigey described experiments which appeared 
to prove that marked attraction existed between bodies of 
different temperatures. Forbes and Faye had also contributed 
to the subject. 
The author then describes various forms of apparatus of 
increasing delicacy which enabled him to detect and render 
very sensible an action exerted by heat on gravitating bodies, 
which is not due to air currents or to any other known form of 
force. A balance was made of a straw beam with pith ball 
masses at the ends, enclosed in a glass tube, and connected with 
a Sprengel pump. The tube being full of air, a spirit lamp 
was passed along the lower part of the tube : the pith ball 
descended slightly, and then rose to considerably above its 
original position. It seemed as if the true action of the heat 
was one of attraction, instantly overcome by ascending currents 
of air. A glass ball full of heated water was afterwards substi- 
tuted for the lamp. The pump was then set to work, and the effect 
of the hot body regularly diminished until the gauge was about 
1 2 millims. below the barometer, when it was scarcely notice- 
able ; at 7 mill, it ceased entirely. The exhaustion was, how- 
ever, continued ; and it appeared that at 3 mill, the ascent of 
the pith, when a hot body was placed beneath it, was equal to 
what it had been in air of ordinary density. With the gauge 
and barometer level, the upward movement was not only sharper 
than in air, but took place under the influence of far less heat, 1 
the finger sending the ball up to its fullest extent. A piece of 
ice produced an exactly opposite effect to that of a hot body. 
An incandescent spiral of platinum wire was then placed 
within the tube, and produced exactly the same effects. The 
rays of the sun and different parts of the solar spectrum acted 
so energetically as to endanger the apparatus. Various sub- 
stances were used as gravitating masses — ivory, brass, platinum, 
gilt pith, silver, bismuth, selenium, copper, mica, charcoal, and 
glass, among others. The apparatus was somewhat modified by 
suspending the index from a cocoon fibre in a long tube fur- 
nished with a bulb at the lower end, and the results were still 
identical. After comparing these with Cavendish’s celebrated 
experiments, the paper ends with a purely tentative discussion 
of their cause, in which no particular form of radiation appears 
to be suggested. The next contribution to the subject is a 
paper by Professor Osborne Reynolds, received by the Royal 
Society on May 16, 1874, and read the following month, headed, 
64 On the Forces caused by Evaporation from and Condensation 
at a Surface.” He proceeds to show that the movements de- 
scribed by Mr. Crookes are due to the cause named in the title, 
and that they are valuable evidence of the truth of the Kinetic 
