136 Prof. R. Clausius on the Theoretic Determination of 



in which a, b, and n are constants which have different values 

 for different substances. 



The question now is, to determine these constants for each 

 substance. 



In the first place, in regard to carbonic acid, in the formula 

 which I constructed specially for it*, I have given the tempe- 

 rature-function © a very simple form, namely that which is 

 obtained from equation (7) by putting in it 6 = and n = 2, 

 by which it is transformed into 



© ~ T 2 ' 

 for which we can also write 



@ = T 2 . const. 

 I added, however, at the time of publishing that formula that 

 I had tried to adjust certain differences still existing between 

 it and Andrews's observations, but had again desisted on 

 account of the uncertainty of the results of those observations. 

 What I said at that time of the then existing results of 

 observation still holds good. In particular it must be remarked 

 that Andrews's observations refer only to temperatures above 

 0°, while the formula must remain valid also below 0° down 

 to the freezing-point of carbonic acid, —57°, and hence 

 requires for the determination of its constants the observed 

 values of a similar range of temperatures. Now we possess, it 

 is true, a series of tensions of saturated carbonic-acid vapour, 

 published by Eegnaultt, which extends to —25°, although 

 not to — 50 ; but in the experiments by means of which 

 Regnault found those numbers considerable sources of error 

 appear to have prevailed. The tensions cited by Regnault for 

 temperatures above 0° deviate considerably from those observed 

 by Andrews}; and it is particularly surprising that Regnault 

 professes to have observed the tensions of saturated vapour of 

 carbonic acid up to temperatures above 42°, while after An- 

 drews's experiments it is now established that from 31° 

 upwards there is no saturated carbonic-acid vapour, because 

 condensation no longer takes place. 



Under these circumstances I think it advisable to retain 

 provisionally my above-mentioned formula for carbonic acid, 

 as an approximately correct one, and to defer undertaking the 

 more exact determination of the constants till reliable data of 

 observation shall be available also for temperatures below 0° 

 down to the freezing-point of carbonic acid. 



* Clausius, Wied. Arm. ix. p. 337 (1880). 



f Regnault, Relation des Experiences, &c. ii. p. 625 (1862). 



X Andrews, Proc. Iioy. Soc. xxiii. p. 516 (1875). 



