376 On Nodes and Loops in connexion with Chemical Formulae. 



satisfied* compounds as hitherto; they apply equally well to ra- 

 dicals with free bonds, every free bond being counted as half" a 

 loop. (This is justified either by experiment with the formulae, 

 or by remarking that 2 free bonds join together to form 1 loop.) 

 A (— l)-ad gives \\ negative loop. The removal of atoms of 

 any atomicity less than 2 increases the number of loops. If we 

 replace m atoms of hydrogen in a paraffin by r atoms of a #-ad, 



we introduce — ^ r loops,, — by the removal of the H, the 



rest by the ordinary action of a #-ad. 



The change of zero seems unnatural if the atoms are consi- 

 dered individually and separate ; but then they do not naturally 

 exist in that condition. If atoms be added to a line diagram 

 (§ 7), the meaning of the change is evidentf : triads require one 

 bond to be drawn, dyads can be put in anywhere ; monads usually 

 necessitate some rubbing out, and always produce an abrupt ter- 

 mination. If I may be allowed to express myself rather wildly, 

 it is as if the normal condition of an atom were to have two at- 

 tractive centres, and as if a monad were in a stunted condition. 

 Might I suggest the term order for non-chemical use, instead of 

 atomicity or valency, which, though doubtless they do very well 

 in chemistry, are not pleasant words ? The change of zero 

 might be made at the same time, and atoms of tetravalent atomi- 

 city be called atoms of the 2nd order, and so on. The word has 

 the advantage of an analogous signification in geometry. 



University College, 

 September 20,. 1875. 



* The only thoroughly saturated compounds are the loopless ones, for 

 wherever there is a loop there is the possibility of its opening out and 

 taking a monad on each end ; thus starch (C 6 H 10 O s ) g has z+1 loops, and 

 might therefore take up 2z+2 monad atoms before being fully saturated. 



t It may be noticed that while dyad atoms can build up only dyad radi- 

 cals, and artiad atoms only artiad radicals, triad atoms can build up radicals 

 of any atomicity whatever, and hence that, if the elements are composed of 

 some fundamental substance, the simplest supposition to make concerning 

 the atoms of that substance is, that they are triads, or atoms of the first 

 order. In the article on Classification in Watts's 'Dictionary of Che- 

 mistry,' Professor Foster has calculated the maximum free atomicity of a 

 compound, and points out that additional dyads do not increase it at all ; 

 triads increase it by 1, tetrads by 2, and so on j which is similar to what I 

 am saying above. 



