51 Prof. Potter on the Fourth Law of the Relations of the 



also in this respect important practically ; for considerable ex- 

 penses have been incurred, and may again be incurred, from 

 misapprehensions of these properties. 



That the properties of gases are analogous to those of steam, 

 as related above, there can be no doubt ; that is, on expanding 

 from a state of more to a state of less condensation, the sensible 

 heat is diminished, or, as Dr. Black would say, has become latent 

 heat. The important points to be determined are the amount 

 and the laws of the changes of temperature in respect to sudden 

 changes of density and elastic force, as well as the interval of 

 time in which such changes are completed, since they are evi- 

 dently not instantaneous. 



We are not without popular results which we may treat as 

 preliminary experiments. Some years ago two young gentlemen, 

 brothers, possessing talents, ingenuity, and perseverance, the one 

 a scientific chemist, and the other a civil engineer, undertook, 

 after a mild winter, the experiment of trying to make ice on the 

 large scale artificially and economically, by passing air from a 

 high state of condensation in a strong and large receiver to the 

 atmospheric pressure, through water. To their disappointment, 

 and mistrust of the theory, they found the water cooled only a 

 few degrees of temperature when they expected the formation 

 of ice. On the contrary, we have lately heard that in the rail- 

 way tunnel which the Sardinian government is carrying through 

 the Alps, by using condensed air-engines to work the boring 

 machinery, that a degree of cold is produced which causes water 

 to freeze, by the expansion of the air after escape from the 

 engine, — the moderate condensation being performed at the en- 

 trance of the tunnel, and the condensed air carried in pipes to 

 the air-engine at the workings. We may ask how are these 

 results to be reconciled. Is it not a case analogous to the high- 

 pressure steam -jet, where the change of temperature occupies a 

 short but sensible interval of time ? This seems to me the solu- 

 tion of the question. 



To discuss experiments made with scientific views : we find 

 that when MM. Gay-Lussac and Welter were trying experiments, 

 at the suggestion of M. Laplace, on the sensible heat lost in 

 the sudden rarefaction of air, they found it so great that the 

 ratio of the specific heat under a constant pressure was to that 

 under a constant volume as 1*3748 to 1 ; but when the air was 

 allowed to escape from its state of condensation to atmospheric 

 pressure through an aperture, there was no change of tempera- 

 ture due to the expansion*. This latter experiment was evidently 

 a hasty one ; for the scientific gentlemen attempting to form ice, 



* Herschel < On Heat/ art. 121. . 



