220 Prof. Cayley on the Theory of Reciprocal Surfaces. [Jan. 14, 



Meunier failed in discovering the occluded hydrogen of meteoric iron, by 

 dissolving the latter in a solution of chloride of mercury ; for the hydrogen 

 would be consumed, like the iron itself, in precipitating mercury. Hy- 

 drogen (associated with palladium) unites with chlorine and iodine in the 

 dark, reduces a persalt of iron to the state of protosalt, converts red prussiate 

 of potash into yellow prussiate, and has considerable deoxidizing powers. 

 It appears to be the active form of hydrogen, as ozone is of oxygen. 



The general conclusions which appear to flow from this inquiry are, that 

 in palladium fully charged with hydrogen, as in the portion of palladium 

 wire now submitted to the Royal Society, there exists a compound of 

 palladium and hydrogen in a proportion which may approach to equal 

 equivalents*. That both substances are solid, metallic, and of a white 

 aspect. That the alloy contains about 20 volumes of palladium united with 

 a volume of hydrogenium ; and that the density of the latter is about 2, a 

 little higher than magnesium to which hydrogenium may be supposed to 

 bear some analogy. That hydrogenium has a certain amount of tenacity, 

 and possesses the electrical conductivity of a metal. And finally, that 

 hydrogenium takes its place among magnetic metals. The latter fact may 

 have its bearing upon the appearance of hydrogenium in meteoric iron, in 

 association with certain other magnetic elements. 



I cannot close this paper without taking the opportunity to return my 

 best thanks to Mr. W. C. Roberts for his valuable cooperation throughout 

 the investigation. 



II. " A Memoir on the Theory of Reciprocal Surfaces." 

 By Professor Cayley, P.R.S. Received November 12, 1868. 



(Abstract.) 



The present Memoir contains some extensions of Dr. Salmon's theory of 

 Reciprocal Surfaces. I wish to put the formulae on record, in order to be 

 able to refer to them in a " Memoir on Cubic Surfaces," but without at 

 present attempting to completely develope the theory. 



Dr. Salmon's fundamental formulas (A), (B) are replaced by 



a(n—2) = k—B+ p + 2c 3 

 b{n— 2) = p+2(3 + 3y + 3t, 

 c(n— 2)=2cr + 4/3+ y + 0, 



a(»-2)(w — 3) = 2(3 — C) + 3(ac— 3<r- x ) + 2(ab-2p - j), 

 b(n-2)(n-3) = 4k+ (ab—2p — j) + 3(bc—3p— y—i) s 

 c(n — 2)(n—3)= 6A+ (ac — 3<r— x ) + 2(bc— 3/3 — y— i), 



where j, d, x , B, C refer to singularities not taken account of in his theory ; 

 viz. j is the number of pinch-points on the nodal curve 6, X> the numbers 

 of certain singular points on the cuspidal curve, C the number of conic 

 nodes, B the number of biplanar nodes : the reciprocal singularities/, 0', Xi 



* Proceedings of the Royal Society, 1868, p. 425. 



