1885.] Condensation of Gases at the Surface of Glass. 15 ( .> 



the glass supports are not insulators of electricity. This film of 

 moisture is removed by exposing the glass stems to heat, or to an arti- 

 ficially dried atmosphere. Some years ago, at the wish of Sir William 

 Thomson, I endeavoured to weigh this film of moisture, but was abso- 

 lutely unsuccessful. The film must be of extreme tenuity. Professor 

 Quincke has, however, made important researches on the " distance 

 of capillary action," and on some of the properties of these very thin 

 films. His results are given in two papers, " Poggendorff's Annalen," 

 108, p. 326, 1859 ; and "Wiedemann's Annalen," vol. ii, 1877, p. 145. 

 He finds their thickness to be comparable with 5 X 10 -5 cm. 



With the view of measuring the quantity of gas condensed upon 

 a given surface of glass, I caused to be prepared in August last a 

 large quantity of fine glass thread. Some of this was of flint glass 

 rod or cane, which was softened in the blowpipe flame, and drawn out 

 on to a wheel. The remainder was of flint glass tubes, drawn out in a 

 similar way. The spun glass was carefully parcelled up in paper, 

 and put aside till I should be ready to use it. 



On January 3rd I put a quantity of the non-tubular glass fibre into 

 a glass tube 2 cm. in diameter and 12 cm. long, and attached it 

 by a glass sealing to a five-fall Giraingham Sprengel pump. The 

 pump, which was in excellent order, was then worked rapidly till I 

 had produced a very good vacuum, which by the McLeod gauge gave 

 me an indication of 0'3 M* pressure. The pump was then left for 

 about an hour, and at the end of that time, passing one more bottle 

 full of mercury through the pump, I ascertained that the vacuum had 

 not sensibly deteriorated, the McLeod gauge giving identically the 

 same reading as before. This exhaustion was performed without the 

 application of any unusual heat to the tube containing the glass 

 fibres. The temperature of the room was about 56° P. 



I now raised the mercury to the upper level, and allowed it to flow 

 through the pump, and the drops fell with the well-known loud 

 hammering noise. While this was going on I applied a Bunsen burner 

 to the tube containing the spun glass. In a few seconds the hammering 

 of the mercury ceased, and on applying the test of the McLeod gauge 

 the pressure within the pump was found to have risen largely. 1 did 

 not, however, obtain a measurement with the gauge corresponding to 

 the maximum pressure of the gas driven off, or to any particular state. 



I now proceeded to pump out all the gas I could, working the 

 pump, and heating the tube containing the glass fibres strongly. The 

 heating was carried on from time to time till the tube, which was of 

 German glass, showed signs of softening and of falling in ; and the 

 glass fibres were likewise, some of them, slightly softened and bent. ( 



The pump was worked for over an hour, the heating being applied, 

 and the gas, which was easily seen being carried down, was collected 

 * 1M standing for one-millionth of one atmo. 



