PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 473 



Bant, and, of ccuirso, not to be used if the new theory dispels the idea that 

 because they are vaporized at low temperatures they require little fuel. 

 Skam is cheap, but to superheat it much requires a temperature that is 

 destructive to any apparatus yet constructed. Five hundred deg. F. is the 

 highest temperature that has been recommended; and as this involves GOO 

 deg. or more for the metal, the durability has been much less than that of 

 apparatus in which water is present. Some years ago I had occasion to 

 iiuiuire what economy there might be in superheating. As'suming 500 deg. 

 as the iiighest temperature allowable, and the specific heat of steam to be 

 4T5/ieg., as found by Regnuult, and the dilatation to be l-480lh for 1 deg., 

 I found that the cftect of fuel might be a little more than doubled by super- 

 heating steam of 100 pounds pressure to 500 deg., and that the total heat- 

 ing surface might be a little less than that of a boiler alone for equal power. 

 This seems a contradiction of the new theory. But it must be observed 

 that iT) making steam we change the state of water; and it cannot be said 

 that no power is required t>> change its state. Nor am I aware that it is 

 proved that heat will give the same mechanical power through liquids as 

 through gases. Moreover, if there be such a general law of heat, it changes 

 at the maximum density of water, or at 40 deg.; thus, if we apply heat to 

 water at 32 deg., instead of expanding it and lifting weight, we contract it 

 and the weight sinks, until the temperature has risen to 40 deg., and above 

 that, as the temperature increases, the weight is lifted. And in the change 

 of state from liquid to solid, mechanical power is not given out; on the 

 contrary, there is an expansion that bursts the strongest vessels, and at 

 tlie same time 140 deg. of heat are given out. More investigation is needed 

 as to th(! convertibility of heat into power, into liquids, and tlieir conversion 

 into solids and into vapors. But these familiar facts are sufficient to war- 

 rant us in the application of this law of heat to aeriform fluids, and it is 

 even disputed whether there is not a change of law, similar to the change 

 in water, at 40 deg., in steam a little above the temperature of vaporiza- 

 tion. Fourteen years ago Dr. Haycraft, of GreenwJch, England, reported 

 some results of experiments which he thought indicated the existence of 

 such a law; and the disagreement as to the economy of superheating war- 

 rants a suspicion that tliere is something to qualify the calculations we 

 niakf. What I have said as to the possibility of doubling the effect of fuel 

 by superheating, will, therefore, be taken with this qualification," 



Dr. Rowell explained tiiat, according to Dr. Tyndall, there is in each gas 

 or other fluid a zero point, analogous to the point in water below which 

 heat will contract it, and above which expand it. The meaning of the 

 conversion of heat into power was this: that to change the volume of gas 

 or vapor required more or less heat according to the resistance; 10 grains 

 of fuel will double the pressure of a cubic foot of air, the volume being con- 

 stant; or it will double the volume of it if the space be enlarged by exter- 

 nal force, and yet make the pressure ecjual to what it was at first; but if 

 the air as it is heated has to lift tlie weight that confines it, then it requires 

 14^ grains of fuel; and the temperature of the air, also the pressure, are at 

 tiie end (jf the experiment exactly as they were in the case when only ten 

 grains were burnt. The extra 4i grains arc presumed to have been con- 

 verted into tiie power that raised the weight; and from this it has been 



