6O THE SCIENTIFIC PAPERS OP 



ence, and air had been resorted to, because it was perfectly elastic, 

 and always at hand. In practice, however, the elastic medium 

 employed was a matter of very great importance, and he (Mr. 

 Siemens) had given the decided preference to steam, and for 

 the following reasons : 



1. The co-efficient of expansion of saturated steam by heat 

 exceeded that of air in the proportion of about 3 : 2, but decreased 

 with an increase of temperature. This was not in accordance 

 with the established rule by Gay-Lussac and Dalton, but was the 

 result of his own experiments (described in a paper, " On the expan- 

 sion of isolated steam, and the total heat of steam," communicated 

 to the Institution of Mechanical Engineers, in 1852),* and had 

 been borne out by his practical experience on a large scale. Mr. 

 Siemens had been first induced to undertake these experiments in 

 consequence of an observation by Faraday, that the elastic force 

 of the more permanent vapours gave way rapidly, when by abstrac- 

 tion of heat their points of condensation were nearly attained. 

 He conceived that gases and vapours would expand equally by 

 heat, when compared, not indeed at the same temperature, but at 

 temperatures equally removed from their points of condensa- 

 tion. 



2. When saturated steam was compressed (within the regenera- 

 tive cylinder), its temperature would not rise considerably (as the 

 fire-syringe evinced in respect of air), because, as Regnault had 

 proved, the total heat of steam increased with its density, and 

 consequently the heat generated in compression was required by 

 the denser steam to prevent its actual condensation. Without this 

 fortunate circumstance, the steam would be heated already by 

 compression to such an extent, that it would be difficult indeed 

 to double its elastic force by the further addition of heat in the 

 respirator. 



3. Steam exercised no chemical action upon the metal of the 

 heating vessel and respirator, because the oxygen it contained was 

 engaged by hydrogen, which latter had the stronger affinity for 

 it until a white heat was reached ; whereas the free oxygen of 

 atmospheric air attacked iron and brass at much lower tempera- 

 tures. 



4. The specific gravity of steam was only about one-half that 



* Vide p. 17, ante. 



