518 
ME. W. CROOKES ON ATTRACTION AND 
allow a white, cloud-like discharge to pass. I have tried experiments with balances 
sealed up in vacua of both kinds prepared in this manner, and I always find the repul- 
sion on the approach of a hot body is very decided, although I do not think it is more 
energetic than with the same kind of balance enclosed in the best Sprengel vacuum I 
could prepare. These experiments have therefore set at rest the doubts which might 
have arisen had I only worked with the Sprengel vacua, that air-currents were the 
cause of the phenomena. They have decided the important point, that in a chemical 
vacuum which will not carry an induction-current the repulsion by radiation is decided 
and energetic. 
55. Besides the straw and pith balance mentioned above (51, 52), I have prepared 
and sealed up in chemical vacua balances made of a glass beam and platinum terminals, 
and also of glass entirely, the ends being flat glass plates. They act in all respects as 
the straw and pith balance, only being somewhat less sensitive. 
56. The delicacy of one of the straw and pith balances in a good vacuum is very 
great, as may be comprehended by the following experiment : — The balance-tube was 
supported on a stand, and immediately beneath one of the pith balls was placed the 
face of a bismuth-antimony thermopile. On connecting the terminal wires of the pile 
with a single cell-battery, so that the current flowed in one direction, heat was produced, 
and the pith ball was repelled. On reversing the current, cold was produced and the 
pith ball was attracted. 
57. The rays of the sun allowed to shine on the terminal pith ball of one of these 
delicate balances in vacuo repel it strongly. If concentrated by a lens, the focal- point 
beats the ball away, as if it were struck with a material agent. If the sun’s light be 
filtered through coloured glasses before concentrating them on the pith ball, the action 
is slightly diminished. Passing the light through two very clear plates of alum with 
parallel sides, and having an aggregate thickness of 8 millims., had but little action. 
58. A beam of sunlight was passed through a prism a (fig. 6), and the spectrum was 
projected horizontally on to a screen ( b ) 18 feet oflf. In front of the screen, a little 
