407 



nowliere appreciably from the tliree-phase pressure of the mixture 

 with great quantity of water. 



In tig. 1 the P-T projection is given of the vapour pressure lines 

 of the components, that of tiie hexane by AB, that of the water 

 b}^ CD. The critical point B of the hexane here happens to lie in 

 the immediate neighbourhood of the waterline, so that the volatility 

 of the two components at the critical point of hexane is equally 

 great. The vapour tension line CD of the water must be thought 

 prolonged towards higher temperature as far as the critical point of 

 water, which according to the latest determinations from the Deutsche 

 Reichsanstalt lies at 374° and 217.5 atm. ^) The three-phase line is 

 indicated by EF; hence it lies appreciably higher than the vapour 

 tension lines of both components. 



On the plaitpoint line BF the critical points are given of eight 

 mixtures; in each of these plaitpoints the liquid and the vapour 

 branch of the sections for constant concentration meet. As a rule 

 only a small number of points of the vapour branch have been 

 determined, because at low temperature the volume would have to 

 be more considerably increased than the test-tube allowed. The 

 further continuation of the vapour branches towards lower tempe- 

 rature might be realized by a slighter filling of the test tube, but this 

 would at the same time increase the error in the concentration of the 

 mixtures. The examined mixtures all contain a great excess of 

 hexane ; the water-content varies between and 27 mol. percentages, 

 but if one considers that the molecular weight of hexane is almost 

 five times as great as that of water, and that hence the greatest 

 quantity of water only amounted to a little more than seven per- 

 centages by weight, and not even so many percentages of volume, 

 because the specific weight of hexane is smaller than water, it 

 appears that if the error in the concentration is to be small, a great 

 tilling is required. In this investigation just as in my investigation 

 concerning the system ether-water, a Cailletet tube with widened 

 upper end has rendered excellent services. The mixture that contained 

 the smallest quantity of water (2,3 mol. percent.), contained 3,4 mgr. 

 of water to about 700 mgr. of hexane ; when a Cailletet tube of 

 the common shape had been used, the total filling could at most 

 have been a fifth part. 



For the mixtures from 5 to 8 (8 inclusive) the phenomenon of 

 the isothermal retrogade condensation could be clearly realized, as 

 in fact will also be clear, from fig. 1. In the neighbourhood of the 



') Landolt BöRNSTEiN-RoTH, Tables p 447. 



