THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 

 JUNE 1873. 



LI I. On the Expansion of Superheated Vapours. 

 By Dr. Hermann Herwig*. 



. . . . § L 



IN my earlier investigations on the variability of vapour-den- 

 sities f, among other results it appeared that, under certain 

 feeble pressures, continually heated vapours contained within a 

 space of constant dimensions may have a smaller coefficient of 

 expansion than that which is attributed to perfect gases, viz. 

 0*003663. The numbers then obtained, however, rather pointed 

 towards such a behavionr of the vapours than decidedly proved 

 it, as the differences in this direction of the observed densities 

 amounted to very small values J. On this account I have en- 

 deavoured by another method to elucidate further this still dis- 

 putable point. 



The procedure consists in comparing directly the expansion 

 under pressure of a constant volume of vapour with that of a 

 constant volume of dry air. Superheated vapour and dry air 

 were contained in the two legs of a U-shaped glass tube herme- 

 tically sealed at both ends, and were separated from each other 

 by an extensive thickness of mercury. The preparation of such 

 tubes, in which vapour and air have any pressure we please, is 

 very simple. A wide tube, as nearly as possible cylindrical, is 

 bent into a U-shape ; its two ends are drawn out thin ; and it 

 is filled to fully one half with strongly heated perfectly dry mer- 

 cury. Then a quantity of liquid, the vapour of which is to be 

 investigated, is drawn into one of the legs; and now the tube, 

 with the exception of its capillary ends, is put into a heated 



* Translated from a separate impression, communicated b}^ the Author, 

 from Poggendorff's Annalen, vol. cxlvii. p. 161. 



t Pogg. Ann. vol. cxxxvii. pp. 19 & 592, and vol. cxli. p. S3. 



% Compare the remarks, Pogg. Ann. vol. cxli. p. 89. 

 Phil. Mag. S. 4. Vol. 45. No. 30.2. June 1873. 2 D 



