( 266 ) 



If / is a chord of W then A degeiienites ijito two surfaces jf' " 

 and a snrface of degree 100. 



If / is a line cntting R^ once then A consists of two parts, which 

 will be successively of degree 94 and 194. 



4. The niunbers rfo=^51() and rf/tz^IöÖ furnish well known 

 j-esnlts. ^) The first tells us that the skew surface ^^^ of the trisecants 

 of U'^ possesses a double curve of degree 255 ; for each plane through 

 a double point of a pair of lines is to be regarded two times as 

 tangent plane. The second number furnislies the proi)erty that the, 

 threefold taiigent planes of the surfaces S^ of a pencil envelo}) a 

 surface of class 55. 



The surface (/-*) contains thus 165 right lines lying three by three 

 in 55 planes. 



The numbers v q and /i o furnish with reference to the plane at 

 iniinity the following jn-operties : 



TJie parabolae of the congruence form a surface of degree 354, 

 their phines envelo p a surfeice of diss 138. 



Each >S'' contains i()8 j)arabolae. '■'). As a delinile N'^ can cut the 

 parabolae oji the other N'' oidy in points of the basecurve 7^'' the 

 locus of the parabolae passes (3 X '^^^ — 2 X ^^B) : 9 = 94 times 

 through R\ 



So through each point of W pass 94 parabolae. 



Chemistry. — ''The nitration of disnijstitated benzenes." By Trof. 



A. F. HOLLEMAN. 



(Communicated in the meeting of September 24, 1904). 



If we introduce into a benzene deri\ative C^H.X a second atom 

 or grou}) this takes up in resj)ect Jo X a position either chiefly 

 meta or para-ortho depend'ug chiefly on the nature of X. The cause 

 of this is as yet obscure. The efforts for elucidating this phenomenon 

 are totally inadequate, first of all because they are too vague, secondly 

 because they do not take into account the relative quantities which 

 are foi-med from the isomers; in fact they could not do so, as these 

 were still luiknown at the time that these "explanations" were 

 given. xA. better insight into this problem can only be rendered possible 

 by the quantitative study of the substitution process, which has 



1) Kluyver, page 152. 



2) J. DE Vries, La configuration formée iKir les ririgt-sept droites (rune surface 

 cubique, Arch. Ncerl., sér. 2, t. YI, p. 148. 



