188 



angles, as shown in each system, and the general disagreement in 

 the symbols hitherto assigned to the faces which make with some 

 other face those nearly agreeing angles. 



With regard to these facts he observes that no difference of opinion 

 can arise, unless the sources from which they have been derived are 

 incorrect ; but that differences of opinion may be entertained rela- 

 tive to the interpretation of them. 



The interpretation to which the author inclines is, that the near 

 agreement in angle between two corresponding faces is not simply 

 accidental, but that it is the effect of some natural relation not 

 hitherto noticed, among all the crystals in each respective system ; 

 and hence, that where the angles between particular faces nearly 

 agree, there ought to be a corresponding agreement in the forms of 

 their symbols. 



With this view of the subject in his mind, it occurred to the 

 author that there might be a similar agreement among the whole of 

 the elementary angles in each system, and an examination of the 

 crystals in the pyramidal and rhombohedral systems to ascertain 

 how far this conjecture might be well-founded, has shown that a 

 geometrical isomorphism does exist throughout each of these systems, 

 and that similar relations may therefore be imagined to exist in the 

 other systems. 



The author has also suggested that the oblique and anorthic 

 systems are only hemihedral and tetartohedral varieties of prismatic 

 crystals. 



VI. "On some Compounds of Ethylene." By H. L. BUFF. 

 Communicated by A. W. HOFMANN, Ph.D., F.R.S. Re- 

 ceived June 10, 1856. 



Among the hydrocarbons which are capable of replacing hydro- 

 gen, the radicals of the general formula CH( n +i), i. e. the homo- 

 logues of ethyl, are best examined. There is another class of hydro- 

 carbons which may be represented by the general formula C n H( W _i). 

 The only well-known term of this series is the radical allyl, C 6 Hj, to 

 which the attention of chemists has been especially called of late by 



