( 328 ) 



phenomena ol' retardation, the experimental investigation of tlie critical 

 phenomena in mixtui-es has led to reliable results. That phenomena 

 of retardation give rise to important deviations is shown by the fact 

 that Cailletet has observed condensation at 2r>°, whereas my expe- 

 riments show that the critical point of contact of his mixture after 

 equilibrium of the phases has been obtained, ought to be found at 

 about 23°. 



Physics. — „Measurements on the change cf pressure by suhstiiution 

 of one component bij the ether in mrxtuies of carbonic acid and 

 hydrogen". By l)r. J. Verschaffelt. (Communication N". 47 

 from the Physical Laboratory at Leiden by Prof. H. Ka- 

 meelingh Onnes. (Continued.)) 



§ L Change of pressure by substitution. 



Tables VI — X contain the results of the determination of isotherraals 

 in mixtures with a still larger quantity of hydrogen than those 

 treated of in the former communications. In these mixtures no conden- 

 sation-phenomena appeared in the area observed ; and so they cannot 

 reveal to us anything more about the fui'ther course of the plaitpoint- 

 curve, but in connection with the results communicated before, they 

 show us in what way, at a given temperature and a given volume, 

 the pressure of the mixture depends on its composition. 



With the aid of all the determinations communicated we have 

 first calculated the coefficients of pressure, the values of which for 

 different compositions and volumes are given in Table XL Then 

 we have calculated for one and the same temperature (18° C.) the 

 isothermals of the different mixtures and represented them in a new 

 diagram, the p-v-a- diagram. On this diagram we have read the 

 pressures belonging to one and the same volume for different mixtures : 

 table XII contains the values read in this way for some volumes. 



As will be explained in § 2 we have chosen the units of volume 

 for the different mixtures so that 1 c.m.^ of each mixture contains 

 the same number of molecules, when the volumes of these mixtures 

 expressed in the units accepted for this purpose, have the same 

 value. Table XII therefore shows the change in pressure when, 

 starting from one of the two substances in pure condition, we gra- 

 dually substitute the molecules of this substance for an equal number 

 of molecules of the other substance. 



