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Mr. J. T. Bottomley. Note on the [Feb. 12, 



February 12, 1885. 



THE TREASURER in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. "Note on the Condensation of Gases at the Surface of 

 Glass." (Preliminary.) By J. T. Bottomley, M.A., 

 F.R.S.E. Commnnicated by Professor Sir William 

 Thomson, F.R.S. Received January 27, 1885. 



It is well known to those who have endeavoured to obtain, in glass 

 vessels, the very perfect vacuums first sought after and obtained by 

 Crookes, and producible by the mercurial pumps, that the operation 

 is much assisted by heating the glass vessels to be exhausted, and 

 even the tubes of the pump, to a high temperature. The difficulty 

 of removing the film of air and moisture adhering to glass tubes is 

 also well known to makers of barometers and thermometers. 



When the Sprengel pump is used for producing a vacuum, and 

 when a tolerably good vacuum has been produced, so that the baro- 

 metric gauge indicates a presence of one millimetre or half a milli- 

 metre of mercury, the drops of mercury falling in the tube of the 

 Sprengel give rise to a loud metallic hammering sound ; and they fall 

 with such unbroken sharpness that those who use this form of pump 

 are often troubled by the " fall- tubes " splitting longitudinally 

 through a length of several inches — a phenomenon in itself very 

 remarkable, considering the strength of the tubes and the smallness 

 of the mercurial drops. If, while this hammering is going on, the 

 glass vessel which is being exhausted and the leading tubes of the 

 Sprengel pump be heated by passing the flame of a spirit-lamp or of a 

 Bunsen burner over them, the hammering immediately ceases, and on 

 looking closely at the fall-tubes it is seen that they are carrying down 

 air which the heat has liberated from the glass walls of the appa- 

 ratus. The ordinary barometer gauge is scarcely sensitive enough to 

 show an increase of pressure, but the McLeod gauge readily shows it. 



There is another well known phenomenon connected with the con- 

 densation of gases and vapours on the surface of glass, viz., the con- 

 densation of a watery film over the glass of electric apparatus, in 

 virtue of which, r/t temperatures considerably above the dew point, 



