26 MR. J. H. GRIXDLEY ON AN EXPERIMENTAL INVESTIGATION OF 



not represent the true law of cooling at pressures below those obtained in the 

 experiments), the values of the specific heat at constant pressure deduced by taking 

 the temperatures given by the intersection of these curves of free expansion with the 

 zero pressure line show a continual inci'ease with the temperature, which again 

 requires careful consideration, for if steam at ordinary temperatures (under 400 F.) is 

 ever sensibly a perfect gas, it will be so at very low pressures, and it has been 

 assumed by RANKING that at a pressure of 0'085 Ib. per squai-e inch and temperature 

 32 F. saturated steam is a perfect gas, but in a perfect gas the specific heat at 

 constant pressure is independent of the temperature, so that if steam at very low 

 pressures is a perfect gas, the values of the specific heat at constant pressure given by 

 the various intersections of these curves of known constant total heats with the line 

 of zero pressures should be the same for all temperatures. Hence, either the curves 

 as drawn at pressures below those attained in the experiments are very far from 

 representing the true law of cooling or steam even when indefinitely rarefied at 

 ordinary temperatures is never even approximately a perfect gas. 



The latter deduction would appear to be the most correct as no indication of any 

 change in the curvature of the curves of free expansion is found even at low pressures 

 of 2^ Ibs. per square inch, and the change of curvature would have to be very sudden 

 and very great in order to create anything like a constant value of the specific heat 

 at constant pressure, an examination of the sort of curve required showing such 

 changes to l>e very unreasonable. 



To find the variation in the value of the specific heat at constant pressure with 

 temperature, the curves of free expansion on Diagram 5 are used, the inter- 

 sections of the curves with any line of constant pressure giving a series of 

 temperatures between each pair of which the mean specific heat of the steam may be 

 found in the following manner. If Hj be the total heat of evaporation of saturated 

 steam which, when freely expanded to a pressure p, will be at a tempei'ature T 1( and 

 H.J the total heat of steam, which, when expanded freely to the same pressure p, will 

 be at a temperature T 2 , the value of the specific heat at pressure p between 

 temperatures T x and T 2 is 



for, by equation 5, page 4, if we wiredraw dry steam (S = 1) from a saturated 

 condition having a total heat of evaporation Hj to a temperature Tj and pressure p, 

 at which the temperature of saturation is T 3 and total heat of evaporation H 3 , we 

 have 



H! = H 3 + KCTj - T 8 ). 



Similarly by wiredrawing from a saturated condition represented by a total heat H 2 

 to a temperature T 2 at the same pressure p, we have 



H 2 = H 8 + K(T 2 - T.), 



