( i<^o ) 



get fluid phases coexisting with solid aiilhi'a(|uiiioJic. Above 247° liquids 

 can again exist and the coiiliiiiioiis curve d c, <\ c^ r., r, q (\ e^ (\ e^ e, 

 consisting of two branches then repi-esents the series of li({uids and 

 vapours which coexist with solid anthraquinone above 247°. The 

 point of continence here is ([, in which therefore for tlie .^iecond iivaQ 

 a saturated solution reaches its critical condition. Tiiis occurs with a 

 concentration 0,13, temperature 247^ and pressure 64 atm. 



The liquid branch r„ p of the lirst loop and the liquid branch q c^ d 

 of the second loop are what we are accustomed to call two parts of 

 the solubility curve. As iiowever the two liquid branches pass conti- 

 nuously into their vapourbranches, there is no objection to calling 

 the two continuous loops solubility curve. 



Branch Coj) of the first solubiliy curve and branch dc^^q of the 

 second show here a particularity. The circumstance that these branches 

 pass continuously into the branches eo p and d i\ q and that the point 

 of confluence coincides with the highest pressure involves the pheno- 

 menon of retrograde sohdji/iti/. 



Cop points to retrograde solubility in the liquid branch (cf. also fig. 4) 

 and d e^, q to retrograde solubility in the vapour branch. The extent of 

 these phenomena however surpassed all expectations. It was known 

 that the liquid and the vapour branch of the cui've J c^ </ 6% <:/ from ^ 

 to a higher temperature ha\'e to sei)arate first in order to come 

 together again afterwards, but it was not to be foreseen that the 

 distance would be so large as to make the vapour branch extend to 

 the concentration 0,01. Fi'om this particular situation results the very 

 interesting phenomenon that, after we have reached point p, with 

 a concentration 0,015 or in other words after the saturated solution 

 has reached its critical condition, at a higher temperature there may 

 again occur three phases. The xapour l)ranch q e^ d extends namely 

 as already mentioned, to the concentration 0,01, and the concen- 

 tration of point j> is 0,015 ; therefore we get from point j) at a 

 higher temperature into the region on the right of the vapour 

 branch de^q, in wdiich three phases may occui'. This phenomenon 

 was observed at a tenqjerature nearly 60^ abo\e the plait [wint- 

 temperature of the concentration 0,015 (j>), that is at 260\ After 

 the formation of the three phases, first the solid and then the liquid 

 might be pressed away l)y i-aising the pressure, so that finally only 

 a fluid phase was left. 



Fig. 2 shows further the />-,?'-sections at temperatures above that 

 of point q, beginning at 250^ The />-,i'-section corresponchng with 

 this tenq)erature is separately drawn in figure 2a. The continuous 

 curve Ca k^ e^ which represents the coexisting unsaturated liquids and 



