L'.'jS PROFESSOR TAIT ON THE 



numerical problem of determining values of the constants which should adapt my main 

 formula to Andrews' experimental data. I contented myself with obviously (and pro- 

 fessedly) provisional assumptions, which showed that it was well fitted to represent the 

 results ; but I also gave the relations among the constants of the formula and the data 

 as to the mass, and the critical values of the pressure, volume, and temperature of the 

 substance. 



Later, having carefully reduced Andrews' data to true pressures (by the help of 

 Amagat's determinations of the isothermals of air at ordinary temperatures), I proceeded 

 to try various assumptions as to the values of the quantities v, p, a in my formulas, on 

 which (as I = 30° '9 C. was already given by Andrews with great precision) all the constants 

 can be made to depend. I at first endeavoured to adjust these so as to make /3 = 0"0017, 

 in consequence of a statement by Amagat [Ann. de Chimie, 1881, xxii. p. 397) as to 

 the ultimate volume of C0 2 . But I failed to get results giving more than a general 

 accordance with Andrews' experiments ; so that I made further guesses without taking 

 account of this datum. I had, however, become accustomed to the employment of it, as 

 a quantity of the order 10" 3 of the volume of the gas at 0° C. and 1 atm., so that I 

 was much surprised to find that one of my chance assumptions, which gave /3 = 0*00005, 

 led to a formula far more closely agreeing with Andrews than any I had till then 

 met w T ith. The reason for this agreement is now T obvious : — The term — ftp is not the 

 proper expression for the part of the virial which it is intended to represent ; and the 

 true mode of introducing that part is, as pointed out in my Abstract, to alter the 

 value of A from isothermal to isothermal, and from volume to volume. 



In January last I happened to ask M. Amagat if he could give me the value of pv for 

 C0 2 at 0° C. and 1 atm., which is wanting in his remarkable table (in the Ann. de 

 Chimie, above referred to). In reply he kindly furnished me with a new and extremely 

 complete set of determinations of pv, in terms of p, for C0 2 ; the range of pressures being 

 1 to 1000 atm., and of temperature 0° to 100° C, some special isothermals up to 258° being 

 added. My first step on receiving these data was to try how far they agreed with 

 Andrews' results, which I had carefully plotted (to true pressures) from 31°*1 to 41° C, 

 and for volumes from '03 to "002. My object was to discover, if possible, by compari- 

 son of the results of two such exceptionally trustworthy experimenters, whether any 

 modification of the behaviour of C0 2 is (as some theoretical writers have asserted) 

 produced by the molecular forces due to the walls of the very fine tubes in which 

 Andrews' measurements w T ere made. I could find nothing of the sort. The isothermals, 

 plotted from Amagat's numbers (which in no case were for any of Andrews' tempera- 

 tures), took their places in the diagram almost as if they had been an additional part of 

 the work of one experimenter. The slight discrepancies at the smaller volumes were 

 obviously due to the trace (1/500) of air which, as Andrews pointed out, was associated 

 with the carbonic acid in his tubes. 



But, although I have got from them only negative information as to the molecular 

 effects said to be due to glass, Amagat's isothermals are so regularly spread oyer the 



