Density, and Temperature of Gases, 163 



Journal in the spring of 1851, I have frequently considered the 

 methods it would be the most desirable to pursue in order to de- 

 termine the heat or cold produced in the sudden condensation or 

 rarefaction respectively of gases, and then the ratio of the specific 

 heat under constant pressure to that for a constant volume 

 becomes known. That many precautions were requisite was 

 generally admitted ; but if the changes of temperature were, as 

 supposed, anywhere near 1° Centigrade for a condensation or 

 rarefaction of T ^th part, good indications of great change of 

 temperature would be easily obtained for a condensation or rare- 

 faction equal to unity. Amongst others, I thought the following 

 method would enable me to arrive at the heat absorbed when gas 

 escaped through a small aperture from a higher to a lower 

 pressure. 



Marcet's boiler, the instrument for determining the relation 

 of the pressure and temperature in high-pressure steam, is fur- 

 nished with an adapter for holding a thermometer, which has 

 the bulb inside and the scale outside the strong spherical boiler ; 

 it has also another adapter with a stuffing-box, which holds ver- 

 tical a long glass tube open at both ends, the lower end being 

 immersed in a known quantity. of mercury which has been poured 

 into the boiler ; the elastic force of the vapour or gas inside the 

 boiler, when greater than that of the atmosphere, is known from 

 the height the mercury rises in the tube. It is also furnished 

 with a pipe and stopcock, to which nozles can be screwed. When 

 steam at high pressure issues from such a nozle, the pheno- 

 menon of the disappearance, at a short distance, of the high 

 temperature which the steam possessed inside the boiler, is very 

 satisfactorily witnessed. 



I thought that if air were condensed into the boiler, proper 

 thermometers being employed, the change of temperature of the 

 air on issuing from the high pressure inside to the pressure of 

 the atmosphere outside might be determined. The following 

 points required attention : first, the aperture through which the 

 air issued should be small compared with the size of the pipe, 

 and have a sharp edge that friction might produce no sensible 

 effect ; secondly, the thermometer to indicate the temperature of 

 the jet of air must give its indication rapidly. The slowness of 

 the action of the common mercurial thermometer, even when 

 made with an elongated bulb, as in the case of those inside and 

 outside the boiler, which I used, required a particular mode of 

 use. It was the conviction that I had found this mode that 

 induced me to prosecute the experiments. I considered that, if 

 a chamber containing the exterior thermometer were prepared, of 

 a slow absorbing and conducting power for heat, and attached 

 air-tight to the nozle with the small aperture, but open to the 



M2 



