Page:Reflections on the Motive Power of Heat.djvu/128

We will examine another case in which vapor of alcohol is acted upon by heat. The reasoning is precisely the same as for the vapor of water. The data alone are changed. Pure alcohol boils under ordinary pressure at 78.7° Centigrade. One kilogram absorbs, according to MM. Delaroche and Bérard, 207 units of heat in undergoing transformation into vapor at this same temperature, 78°.7.

The tension of the vapor of alcohol at one degree below the boiling-point is found to be diminished ${\tfrac {1}{25}}$. It is ${\tfrac {1}{25}}$ less than the atmospheric pressure; at least, this is the result of the experiment of M. Bétancour reported in the second part of l'Architecture hydraulique of M. Prony, pp. 180, 195.^[1]

If we use these data, we find that, in acting upon one kilogram of alcohol at the temperatures of 78°.7 and 77°.7, the motive power developed will be 0.251 units.

This results from the employment of 207 units of heat. For 1000 units the proportion must be

   ${\tfrac {207}{0.254}}$ = ${\tfrac {1000}{x}}$, whence x = 1.230.

estimate leads us to attribute a great advantage to atmospheric air, but it is derived by a method of considering the action of heat which is quite imperfect.

1. Note F, Appendix B.