522 
ME, W. CEOOKES ON ATTRACTION AND 
lamp : it rose very slightly at first, and then descended to the same distance below the 
original level on continuing the heat. 
At 35 millims. the initial rise was immediate and decided. Continued heat only 
lowered the end a trifle, but did not bring it down to any thing near its original posi- 
tion. 
From this point the attraction towards the heat ceased to be perceptible. The 
upward movement increased in strength and amplitude until the Sprengel vacuum was 
reached *. 
At 10 millims. below the barometer the glass beam was repelled by a glass rod or a 
lump of copper heated to 100° C. ; at a difference of 5 millims. between the gauge and 
the barometer the finger applied below repelled the beam, and at 2 millims. difference 
the beam was repelled when the finger was applied above. 
In the Sprengel vacuum the beam was very sensitive to the approach of a warm body, 
a touch of the finger sending it away to the fullest extent. 
67. I consider that the difference observed between the behaviour of this glass beam, 
the straw beam with brass ends (37 to 40), and the straw beam with pith ends (30, 31, 32) 
is to be accounted for by the different materials forming the balances, the different way 
in which heat was applied in the three cases, and the extent of surface exposed in 
proportion to mass. There is, however, much to be found out in respect to the position 
of the critical point, and I am still at work on the subject. 
Carbonic acid was then let into the apparatus, and it was re-exhausted. Not wishing 
to tax the potash too severely, I did not spend much time during this exhaustion ; 
a few observations were, however, taken at different heights of the gauge, and the 
position of the neutral point was apparently a little raised. The general results were, 
however, the same as with air. In a carbonic-acid Sprengel vacuum the repulsion by 
heat was exactly the same as in an air Sprengel vacuum. 
68. The remainder of the process for producing a chemical vacuum was then gone 
through ; the tube was set aside till the potash had done its work, and experiments 
were then tried with it. To the heat of a spirit-lamp, a warm glass or metal rod, the 
fingers, or the image of a gas-flame concentrated by a lens, the balance was very sensi- 
tive, being instantly repelled to a distance varying with the intensity of the heat. 
Experiments were tried with different rays of the solar spectrum with a similar result 
to that described in par. 58. The vacuum was so nearly perfect that an induction-spark 
would not pass, but preferred to strike across its full distance in air. 
69. If one of the most sensitive balances (straw and pith (24), or the glass one last 
described (66)) in a well-exhausted tube is carefully turned on its side, after a little 
* In these and many of the other observations accurate measurements were taken of the extent of move- 
ment by means of a micrometer-eyepiece, and the time was also accurately noted by a seconds’ watch. As, 
however, the equal and uniform delicacy of the instrument could not be depended upon, I think it best only 
to give the results in general terms rather than to mislead by an affectation of accuracy not justified by the 
instrumental means employed. 
