Mr. J. Gill on the Dynamical Theory of Heat. 89 



state. This explanation involves the idea that steam at different 

 densities contains a variable quantity of heat in proportion to 

 the work it can perform ; and this inference obtains some slight 

 support from the results of experiment, showing that the total 

 heat of steam increases with increase of density, but by no means 

 enough to account for the greater amount of work it can perform 

 by expansion. 



One of M. Hirer's experiments with an engine cutting off the 

 steam at j of the stroke showed at the same time a production 

 of work and a creation of heat. In another of his experiments 

 steam was thrown into a sheet-iron vessel immersed in cold water, 

 and it was found that the heat thrown out by the condensing 

 steam was more than that due to its constitution as deduced from 

 calculations of acknowledged correctness. Moreover the fact 

 that steam becomes superheated in the act of free tumultuous 

 expansion is not disputed, though it involves the idea of some 

 other form of energy existing in the steam besides molecular 

 motion. The dynamical theory necessitates the assumption that 

 steam is condensed during expansion in the production of work, 

 as theoretically shown by Clausius and Rankine ; and another 

 of Hirer's experiments would seem to corroborate this assumption. 

 A copper tube upwards of 6 feet long and 6 inches diameter, 

 with strong glass ends, was put in communication, by means of 

 stopcocks, with a steam-boiler at one end and with the atmo- 

 sphere at the other. The tube full of high-pressure steam was 

 perfectly transparent ; but the moment the cock communicating 

 with the atmosphere was opened, and part of the steam rushing 

 out allowed the remainder to expand, a cloud was formed in the 

 tube : the steam became quite opake, showing, as it is supposed, 

 the condensation of the steam which should accompany expansion. 



On this experiment, I would remark that a film of moisture 

 must exist on the interior surface of the tube, though at the 

 same time the contained mass of steam may be dry, or free from 

 particles of water mechanically suspended in it. The tempera- 

 ture of the copper must be somewhat lower than that of the high- 

 pressure steam, but much higher than the temperature of the 

 steam at low pressure; consequently, on the sudden lowering of 

 pressure in the tube when the steam is allowed to blow off, an 

 instantaneous " flashing " into steam of the water on the now 

 comparatively hot metal must ensue, with a large amount of 

 priming, or dispersion of the water in the shape of fine spray, 

 probably quite sufficient to account for the opacity observed. 

 The experiment should be made with the tube enclosed in a case 

 with glass heads, filled with steam of the same temperature as 

 the steam inside, to prevent the deposition of moisture on the 

 interior surface of the tube. This precaution would in a degree 



