1856.] Regenerative Steam-Engine. 235 



experiment, and to guard against a sweeping condemnation on that 

 account of some of the means Ericsson had employed. 



According to the dynamic theory of heat, the elastic medium 

 employed in a perfect caloric engine was a matter of indifference, 

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

 always at hand. In practice, however, the elastic medium em- 

 ployed 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 Dal ton, but was the result 

 of his own experiments (described in a paper, '• On the expansion 

 of steam, and the total heat of steam," communicated to the Insti- 

 tution of Mechanical Engineers, in 1850), 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 abstraction of heat 

 their points of condensation was nearly obtained. He conceived 

 that gases and vapours would expand equally by heat, when com- 

 pared, not indeed at the same temperature, but at temperatures 

 equally removed from their points of condensation. 



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

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

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

 that the total heat of steam increased with its density, and conse- 

 quently 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 com- 

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

 double its elastic force by the further addition of heat in the respi- 

 rator. 



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 afllinity for it 

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

 spheric air attacked iron and brass at much lower temperatures. 



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

 atmospheric air at equal temperature and pressure ; moreover it 

 was a far better conductor of heat, and both circumstances qualified 

 it for rapid respirative action. 



5. The fresh steam required for starting and sustaining the 

 power of the engine was generated by heat that would otlierwise be 

 lost. No air-pumps, &c., were required, and the management of 

 the engine became as simple as that of an ordinary high-pressure 

 steam-engine. 



