in connexion with Chemical Formula. 369 



Example.— Sulphuric acid, C S vi H 2 O 4 , has two loops, and 



— 2a?+6=2 gives x = 2. 



If r atoms of the &-ad are introduced instead of 1, the suffix of 

 It becomes 2n + 2 — 2x + r(k—2), and the general law is suffi- 

 ciently obvious; but it is obtained more satisfactorily in § 8. 

 Example. — Cyanmethine, CgNgHg, 



12 + 2-2#-f3(5-2)=9gives* = 7; 

 and Frankland gives as its formula 



Me— C=N— N=C— Me 



¥ 



8 



I 



Me 



If no hydrogen is present, but only carbon and oxygen, 

 2rc + 2 — 2.r = 0, or x = n+l. 



So calcic oxalate, having 2 carbons and some dyads, will have 3 

 loops. 



Heterocline, Si ('Mn w 6 O n ) lv , containing 7 tetrads, will have 

 8 loops. 



(3) If the formula of a compound be arranged so that some 

 of the bonds cross each other, it is less easy to see how many 



P— P 

 loops there really are ; thus I \S I is the same compound as 



P— P 



P— P 



\ \ // 



\ P /, and has 3 loops, not 4 as would appear from first 



sight. In fact every crossing of the bonds introduces one spu- 

 rious and unreal loop ; because such a crossing may be regarded 

 as a spurious tetrad atom, and each tetrad atom causes one loop. 

 For instance, fig. 1 has 9 apparent loops ; but as there p io . j 

 are 4 crossings the real number of loops is 5. c £> * ' 



Apropos of this crossing of the bonds, Professor ll^x^J 

 Vernon Harcourt, President of Section B, raised a Jivv^ 

 question as to the advantage or disadvantage of the 

 system of representing agglomerations of atoms in space by 

 a plane diagram. Now it is evident that any configuration in 

 space may be projected on to a plane surface ; and the only dif- 

 ference between such a projection and a diagram drawn directly 



Phil. Mag. S. 4. Vol. 50. No. 332. Nov. 1875. 2 B 



