EEPULSION EESTJLTINGr EEOM EADIATION. 
521 
Eeceived August 18, — Eead December 11, 1873. 
66. A piece of flat plate-glass, 15 millims. wide and F5 millim. thick, was heated in 
the middle before the blowpipe till quite soft, and then drawn out till a long ribbon 
of glass was produced, the width and thickness of which retained the proportions of the 
original piece. From the middle of this a portion 150 millims. long, 4 millims. wide at 
the ends, and 3 millims. wide in the centre, was cut off. A double-pointed needle (24) 
w r as then secured to the centre by binding with platinum wire and fusing the latter to 
the glass. This little balance was adjusted until it was very delicate, and was then 
enclosed in a tube containing potash at one end and platinum wires sealed in at the 
other end ; it was then arranged, in connexion with the Sprengel pump and carbonic- 
acid apparatus, so as to produce a chemical vacuum (51, 52, 53). 
When the flame of a spirit-lamp was passed under one end of this balance, the appa- 
ratus being full of air at the ordinary density, the result was decided attraction (27, 37, 
38, 40, 41). On exhausting and testing from time to time in the same manner *, I 
found the attraction, or rather the sinking of the heated end towards the spirit-flame, to 
keep at about the same strength until the gauge had risen about 500 millims. After this 
there was a gradual decline in the downward movement of the glass end of the balance 
when heat was applied, until the gauge stood about 100 millims. below the barometer. 
On testing the movement at this pressure I at first thought it was very slight ; but on 
keeping the flame of the lamp for about half a minute below the end of the balance 
the latter commenced to sink, and then the downward movement was almost as great 
as ever. With a difference of 95 millims. the phenomena were similar, the lamp, hoAV- 
ever, requiring to be kept under the balance end longer, and the ultimate movement 
not being so great. At 90 millims. the same thing occurred, the time of heating having 
to be still longer. 
When the gauge was raised to 85 millims. of the barometric height, and the lamp 
was applied, the first movement was one of repulsion. The glass end rose instantly, but 
to a very slight extent. On keeping the lamp under for about a minute, the end of the 
balance slowly came down, until it had sunk a little below its original position. 
At 80 millims. difference between gauge and barometer the effect was almost the same 
as at 85 millims. The preliminary rise was, if any thing, a little more marked. For fear 
of injuring the connexions of the apparatus, I did not like to apply the spirit-flame for 
a longer time than a minute, as the glass then commenced to soften ; I therefore, at 
the higher exhaustions, paid most attention to the initial movement of the glass beam, 
merely keeping the lamp beneath long enough to see if the continued heating drew 
the beam down again. 
At a difference of 45 millims. between the gauge and the barometer, the neutral point 
was about reached. The glass beam was not quite motionless on applying the spirit- 
* It is perhaps scarcely necessary to say that the apparatus was allowed full time to acquire the ordinary 
temperature of the laboratory between each of these experiments. 
