642 Sir J. A. Ewing on Specific Heat of Saturated Vapour 



The curve (fig. 4) changes the sign of its slope at about 

 80°, but the position oE the minimum of <j) s is not well defined. 

 Cazin "*, using large amounts of sudden compression and 

 sudden expansion, found that a mist was produced in benzene 

 vapour by expansion at temperatures below 115° and by 

 compression at temperatures above 130°. He places the 

 point of inversion in the neighbourhood of 120°. The 

 entropy diagram shows clearly that the specific heat of 

 the saturated vapour of benzene is positive throughout a 

 considerable range of temperature. 



In the paper cited, Young gives values of L for ten esters 

 of the fatty acids. Of these esters seven are included 

 among the substances whose specific heat, in the liquid 

 state, has been investigated by SchifF, namely, methyl and 

 ethyl propionate, ethyl and propyl acetate, propyl formate, 

 methyl butyrate, and methyl isobutyrate. From his mea- 

 surements of specific heats in these seven esters (and many 

 more) Schiff has concluded that their specific heat is 

 expressed by the formula f 



0-441,6 + 0-00088*, 



which would make, for all of them, 



<^= 0-4635 log ]0 T + 0-00088 T- 1-3696. 



Using this formula along with the values of L given by 

 Young, I have calculated <j> w and </> s and have drawn an 

 entropy-temperature diagram for each of these substances.. 

 The values so obtained are given in the table below, and one 

 example of the diagrams is reproduced in fig. 5, namely the 

 diagram for ethyl propionate, which in its general character 

 is representative of the others. It closely resembles the 

 diagram for benzene, and indicates a point of inversion 

 somewhere about 40° C. The table shows that all these 

 esters belong to the class of fluids in which the specific heat 

 of the saturated vapour is positive throughout a certain, 

 range of temperature. 



* Loc. cit. p. 306. 



t R. Schiff, he. cit. p. 340. 



