198 ON SUPERHEATED STEAM. 



organic substances under heat and pressure, and 

 further, also, as the pressure of saturated steam 

 requires, to be greater than in the case of sub- 

 saturated steam. 



If the heat be carried too far, a substance like 

 charcoal is produced, very porous ; if the pressure 

 be carried forward uniformly, with greater heat, 

 then a substance like jet is formed; but the clear 

 and distinct change is the evolution of hydrogen ; 

 but in the case of the jet-like substance it is partly 

 'displaced by oxygen. 



If impurities or adulterations form part of the 

 rubber compound, then of course totally different 

 results obtain ; and it is well to state, that there is 

 nothing which will take up such a variety of im- 

 purities, or will admit of so much adulteration, as 

 india-rubber, and yet appear the same to the eye ; 

 but the action of the chemical agents, such as 

 the gastric fluids or the saliva, soon makes them 

 evident. I have learnt, upon the authority of 

 Messrs. Macintosh and Co., confirmed by my own 

 experience, and some careful experiments by Mr. 

 Vasey, that rubber will take up, with but little 

 change of bulk, more than half its weight of im- 

 purities ; if these be oxides, acids or caustic alkalies 

 will readily decompose the compound with heat. 



The various modes of hardening india-rubber 

 may be stated to be three : 1st, by saturated 

 steam heat; 2nd, dry heat; 3rd, sub-saturated 

 iiteam heat. 



