( 667 ) 
d = 1 + 0,9280 (l—r) + 1.8893 (1—r)0.3827 ij 
were used as interpolation formulae. 
In the last formula 0 represents the reduced density and the 
sign + gives the liquid, the sign — the vapourdensity. The first formula 
gives for the region which we want (from t = 0.944 to r = 0.995) zen 
with an accuracy of 0.5%, the second gives over the whole range 
on which Youna observed volumes of saturated liquid and vapour 
(from += 0.6824 to += 0.9963) : 0; +0, accurately to within 
0.2/9, d;—d, to within 1.5%/,, 0, to within 0.50/, ; with the lower z’s 
we find for 0, important deviations, but for the region that we 
want the deviations are not more than 20/,, an accuracy more than 
sufficient for our purpose. In consequence of the value found by Youne2) 
for C, for the substances: pentane, hexane, heptane and octane, I 
put for butane C, = 3.6. By means of these data we can compute 
the composition of the mixture of ethane with (according to KUENEN) | 
40/, butane *). I find from the values given by KuEnen forp, and p; 
the pressure at the beginning and at the end of the condensation 
the following values for z: 
| 
14,95 29.02 32.94 0.0132 33.62 
20.15 33.65 36.86 0.01214 37.73 
24.85 37.84 | 40.62 0.0126 4A „60 
30.35 43.15 45.35 0.0174 | 
It is to be expected that the last value for 2 deviates from the 
preceding, if we bear in mind what has been said in §§ 2 and 4, 
as we are getting very near to the critical temperature of the pure 
substance (32.0° C.) The satisfactory agreement of the other values of z, 
shows that formula (9) represents well the relation of the increase 
of pressure to the temperature. Moreover I should derive from this 
that the mixture had the composition «= 0.0126, and therefore 
contained : 2.3°/, of its mass of butane. 
1) Comp. VERSCHAFFELT, Proc. June 1896. Comm. N°, 28 p, 12 and Comm. N°, 55 
Proc. April 1900, p. 588. According to vaN DER Waals, Thermodynamical theory of 
Capillarity, the index for ¢=1 must approach to 0.5 
2) S. Young, Phil. Mag. Sept. 1900, p. 308. 
*) KvENEN, Proc. Roy. Soc. Edinburgh, 1897, p, 442; Zeitschrift für comprimirte 
und flüssige Gase I p. 157, 
