553 



dW 

 the calculation of tlie differential quotient —- . But this at least may 



cu 



(IW 



be said now that the curve begins with — — = 75 cal., then decreases 



(h 



pretty rapidly, in a way, which corresponds pretty closely with the 



course of this quantity in the heats of mixing (of snlf)huric acid or 



phosphoric acid with water). At /=rü.lO to i=0.15 it begins to 



assume a more or less constant (albeit slowly diminishing) value, 



amounting to about 23 cal., which diminishes again greatly past 



?r=:0.65, and converges to zero. 



It would be very important also to study the volumecontraction 



at the absorption of water; for, where in expansible and in miscible 



substances the relation ( -f^ ) always appeared of the same order 



of magnitude (between 10 and 30 10~*), it would be important 

 to examine what the order of magnitude of this quotient would be 

 in animal carbon. Unfortunately it is not possible to determine these 

 volume contractions, as carbon probably acts as an adsorbent on 

 ei^eri/ pj'^cuoraeter liquid, at least in anhydrous condition. 



The free energy at the sorption can most easily be calculated 

 from the vapour tension of the water at different degrees of sorption. 

 These vapour tensions have not been determined directly, but 

 indirectly by the method of Gay Lussac-van Bemmelen (by bringing 

 the substance into equilibrium with sulphuric acid-water mixtures 

 of known strength till constancy of weight is reached). The 

 absorption and loss of water then appeared to be a phenomenon 

 of equilibrium, which presents liysteresis. This result is in striking 



