240 



Mr. William Sutherland on the 



of testing it would be to take Eegnault's formula, with one 

 exponential term, 



logp = a + bu, 



and by a simple recalculation from his values for a, b } and a, 

 to cast it in the form 



logp/p c = e + dy T ' T % 



proving that the constants e, d, and 7 are approximately the 

 same for all compounds. But the objection to this plan soon 

 becomes obvious on trial, as the formula owes its empirical 

 convenience to tte power of adjustment amongst the con- 

 stants ; and the same difficulty would be experienced with 

 any purely empirical equation. 



Accordingly, to test this matter, I have thought it best to 

 compare the pressures of a number of bodies at temperatures 

 which are constant fractions of their critical temperatures, 

 such as *6 T c , *7 T c , and so on. The ratio of the pressure of 

 any substance to the corresponding pressure of ethyl oxide 

 ought to be approximately the same for that substance at all 

 values of the fraction. Great uncertainty attends the mea- 

 surement of critical pressures : an error of 20° in the critical 

 temperature is not a large fraction of its value measured from 

 absolute zero, but it makes a large difference in a saturation- 

 pressure, and the critical pressure is the limiting saturation- 

 pressure. In the subjoined Table the critical-pressure ratios 

 are given for what they are worth in the column T o . 



Tablis XXII. 

 Eatios of Saturation -Pressures at constant fractions of the 

 critical temperature to the Saturation-Pressures of Ethyl 

 Oxide at the same fractions of its critical temperature. 





Tc. 



Fractions of T c . 



•6. 



•65. 



•7. 



•75. 



•8. 



•9. 



10. 



Acetone 



506 

 404 

 428 

 404 

 373 

 305 

 308 

 548 

 556 

 537 

 404 

 456 

 509 

 560 



•87 

 1-7 

 1-7 

 2-6 



3 ; i 



13 

 1-6 



1-7 



^ -A 



19 

 1-3 



•90 

 1-5 

 1-7 

 2-6 



3-6 



2-7 

 1-3 

 15 

 1-5 

 1-4 

 1=5 

 1-3 



•92 

 1-4 

 17 



2-5 

 3-2 



2 ; 3 

 1-3 

 1-5 

 1-4 

 1-3 

 1-6 

 1-2 



•94 

 1-4 

 1-7 

 25 



2-8 



2-1 



1-2 

 1-4 

 1-4 



1 2 

 15 

 1-2 



•95 



1-7 

 25 



2-6 



1-8 

 24 



1-3 



1-2 

 1-4 

 11 



2-4 



1-7 



T8 



1-4 



2-2 

 3-2 

 2-6 

 2-1 

 21 

 2-1 

 1-6 

 1-6 

 21 

 1/5 



1-4 



Methyl oxide 



S0 2 ... 



NH 3 



H 2 S 



co 2 



N 2 



cs 2 



OCL 



CHOI3 



CH3OI 



CH-C1 



C a H 5 Br 



Benzene, 6 H G ... 



