1891.] the Conductivity of Solutions of Sal pit uric Acid. 141 



Additional evidence in favour of the existence of the hydrate 

 H 2 S0 4 . H 2 is afforded by the fact that it can be obtained in the 

 crystalline form. 



The hydrates of S0 3 which have at present been crystallised 

 are: 



melting point 



(1) H 2 S 2 7 



(2) H 2 S0 4 0°C. 



(3) H 2 SG 4 .H 2 7-5° C. 



(4) H 2 S0 4 .4H 2 -25° C. (Pickering, Chem. Netos, 



1889). 



We know from the experiments of W. Kohlrausch upon solu- 

 tions of S0 3 in H 2 S0 4 , and from those of F. Kohlrausch upon 

 aqueous solutions of H 2 S0 4 , that the formation 1 of (2) and (3) is 

 accompanied by an increased electrical resistance. H 2 S0 4 . 4H 2 0, 

 on the other hand, appears to have no effect upon the resistance at 

 ordinary temperatures. What effect the formation of the hydrate 

 H 2 S 2 7 may have upon the resistance has not been ascertained, as 

 the observations of W. Kohlrausch do not extend to solutions of 

 greater concentration than 90'67 °/ S0 3 . 



Judging from these facts it would seem that the formation of 

 a hydrate does not necessarily produce an increased resistance, 

 though in the cases of H 2 S0 4 and H 2 S0 4 . H 2 it appears to do so. 



But we have seen that the hydrate H 2 S0 4 . H 2 dissociates as 

 the temperature rises. It is possible, therefore, that H 2 S0 4 . 4H 2 

 which has a melting point of — 25° C, while that of H 2 S0 4 . H 2 is 

 7'5° C, may be so far dissociated at 18° C. that there is not 

 sufficient of the hydrate present to produce a perceptible effect 

 upon the resistance. 



February 9, 1891. 



Professor G. H. Darwin, President, in the Chair. 



The following Communications were made to the Society : 



(1) On Rectipetality and on a modification of the Klinostat. 

 By Dorothea F. M. Pertz and Francis Darwin, M.A., Christ's 

 College. 



[Abstract; received March 5, 1891.] 



Vochting has shown that geotropically induced curvatures 

 may be removed by subjecting the curved plant to slow rotation 

 on a horizontal klinostat. It is usually assumed that the growing 

 part, being freed from external stimulation, straightens itself by 



1 The point of maximum resistance is between 99"75 / o and 99*9 °/ H 2 S0 4 . 



