ON SUPERHEATED STEAM. 197 



of opinion that the whole change in the hardening 

 by heat and sulphur arises from the evolution of 

 hydrogen and the absorption of a small amount of 

 oxygen. As the heating and hardening progress, 

 the substance becomes darker in colour, one 

 evidence of its being richer in carbon; it also 

 gives out at the same time a great deal of sul- 

 phuretted hydrogen, that is, the hydrogen leaves 

 the carbon and takes to the sulphur, forming a 

 gas; and it is the formation of this gas which 

 makes a certain amount of pressure necessary to 

 prevent this gas forming globules in the rubber, 

 which it will without pressure; and hence the 

 porosity which is so often found in sulphur and 

 ru"bber hardened by dry heat. Without pressure,, 

 a greater amount of this sulphuretted gas is 

 formed, and the hardened material is of a darker 

 coloiu' and not so strong, as will be found by 

 making an inch bar of the vulcanite, about a foot 

 long, and subjecting it to a breaking force in the 

 centre, with its ends on supports. I have also 

 found that more sulphuretted hydrogen is evolved 

 with the presence of saturated steam, when under 

 pressure, than with sub-saturated steam, which is 

 accounted for by the rubber compound being 

 slightly porous, and thus enabling a greater 

 amount of moisture to act upon the rubber and 

 carry away more of the sulphur and hydrogen 

 than is necessary ; for water exerts the . power, 

 in an Extraordinary manner, of decomposing all 



