at the Royal Institution, 1 900-1 907. 371 



pentagon. Any smaller number cannot be joined together in such a 

 way tiiat the wires representing affinities are brought into parallelism ; 

 they can only be joined by crossing the affinities and binding them 

 together at the junctions. The size of the angle between two 

 affinities may be regarded as the departure from parallelism, l^eing 

 greater the smaller the number of atoms united, indicating in some 

 degree the relative stability of the ring system, the approximation 

 of the affinities beiug inversely proportional to the exterior angle 

 between crossed affinities — thus 



The behaviour of the closed chain 

 hydrocarbons is in striking agreement 

 with the geometrical peculiarities of 

 models constructed in the manner des- 

 cribed, so that however far removed 

 they are from being representations of 

 the manner in which the carbon atoms 

 are actually combined, such models 

 nevertheless serve a most useful pur- 

 pose in indicating the peculiarities of 

 the different types of compound. 



Cycloids such as have been described 

 are all paraffinoid compounds, in the 

 sense that the carbon atoms are indi- 

 vidually united in pairs by single affini- 

 ties, as in the paraffins ; they are also 

 paraffinoid or saturated in the sense that they cannot enter directly 

 into combination with other molecules ; they differ from the paraffins, 

 however, in being more or less attractive of other molecules to 

 a greater extent than are the paraffins, which all but shun other 

 molecules. 



But carbon atoms may be united in pairs by more than single 

 affinities — a fact which is clearly brought out in the tetrahedral 

 model, as two tetrahedra may be united not only by joining an apex 

 of the one to that of another but also by approximating an edge of 

 the one to an edge of the other or even by bringing the triangular 

 base of the one into contact with that of the other. Both these 

 forms of combination are well known : the one occurs in ethene or 

 ethylene (olefiant gas), C0H4, the other in acetylene, C2H0. When 

 models of such compounds are made with skeleton tetrahedra, it is 

 obvious that the affinities cannot be made to overlap and saturate 

 one another as in the paraffins but that they can only be crossed : 

 in other words, they saturate one another imperfectly and therefore 

 such hydrocarbons should be attractive of other molecules, to a 

 greater extent, moreover, than is the case with the cyclo-paraffins. 

 In point of fact, both ethylene and acetylene behave as eminently 

 unsaturated compounds, being, for example, immediately absorbed 



2 B 2 



