52 Chemical and Physical Notes 



transparency of the metal is an obvious disadvantage, and the 

 jacketing is unnecessary, because the latent heat of steam is 

 so great that, with the supply of it which the boiler of 

 Regnault's apparatus can furnish with any efficient lamp, 

 a steam vessel of single envelope, under ordinary conditions, 

 cannot be cooled by even a fraction of a degree below the 

 temperature of saturated steam corresponding to the existing 

 barometric pressure. Any cooling of the outside surface of 

 the envelope is stopped at once and perfectly by the film of 

 water continually descending along its inner surface, while the 

 inner surface of the film is freely exposed to 

 an abundant supply of saturated steam. 



The object of the experiment is to pro- 

 duce an atmosphere of saturated steam, the 

 tension of which is equal to the pressure of 

 the atmosphere, and to have a trustworthy 

 thermometer so immersed in it that it as- 

 sumes, and retains without variation, the 

 temperature of the steam. For this purpose 

 the supply of steam must be ample, while its 

 means of escape into the atmosphere must 

 be so free that no rise of pressure, due to 

 over-supply of steam, can be produced. The 

 steam, which is condensed on the walls of 

 the steam tube, should run freely back into 

 the flask without collecting at the bottom of 

 the wide part of the steam tube. Therefore 

 the tube making the connection with the 

 boiling flask must be pretty wide; and the 

 exit tube from the steam vessel must be a trifle wider still. 

 Of the steam which enters the tube, part is condensed on 

 the walls and keeps them at constant temperature, and the 

 remainder passes away in a stream of good volume through 

 the exit tube. With very little attention to the construction 

 and management of the apparatus, every risk of cooling from 

 without or heating from within is completely avoided. 



Description of the Apparatus. A general view of the 

 apparatus is seen in Fig. i. It consists of four parts : the 



