194 ox SUPERHEATED STEAM. 



table" (No. II.) of heat and pressure of the sub- 

 saturated steam. All aeriform bodies, according 

 to Dalton, when insulated from liquids, expand in 

 a uniform manner and by the same increase of 

 temperature. Gay-Lussac, whose researches on 

 the subject of vapours, made with very refined and 

 accurate apparatus, states also, that all vapours, 

 when insulated from their liquids, obey the same 

 laws as gases. The four following propositions 

 exhibit at a view the principal facts of the case : 



1. All gases expand alike for equal increments 

 of heat ; and all vapours, when remote from their 

 condensing points, follow the same law. 



2. The rate of expansion is not altered by a 

 change in the state of compression or elastic force 

 of the vapour or steam itself. 



3. The rate of expansion is uniform for all 

 degrees of heat, 



4. The actual amount of expansion is equal to 

 4^^h part of the volume of the vapour at 0° Pahr. 

 for each degree of the same scale. 



The rate of expansion near enough for our pur- 

 pose is shown by table No. II. We thus have in 

 an atmosphere of steam, when being superheated, 

 a safe and manageable medium to deal with, 

 which expands in a known and uniform manner 

 with heat ; and this is a knowledge of great prac- 

 tical importance to the chemist. 



Of the various modes of heating steam, it will 

 be necessary, I think, for the perfect under- 



