370 Prof. H. E. Armstromj on Lo w- Temper atv re Research 



arranged spirally, so that a complex compound such as paraffin wax 

 may be pictured as resembling a curl or helix rather than a straggling 

 chain. The most appropriate model, in fact, that can be constructed 

 to symbolise the functions of a carbon atom is a regular tetrahedron.* 

 And supposing the tetrahedron to be inscribed within a sphere, the 

 four affinities of the atom may be pictured as proceeding from the 

 centre of the sphere to the four solid angles of the tetrahedron ; the 

 angle at which two affinities meet is therefore 109° 28' and while 

 one pair of the affinities is situated in one plane the second pair 

 lies in a plane at right angles to the first pair. 



A skeleton wire model of the carbon affinity system is easily 

 made by bending two pieces of wire each at an angle of 109° 28' and 

 then soldering tlie two pieces together at their angles so that they 

 meet in two planes at right angles to one another ; the four arms 

 represent the affinities and the directions in which they act. Models 

 of paraffins are easily constructed by joining the proper number of 

 such skeleton tetrahedra together by laying an arm of one tetra- 

 hedron against an arm of another and then lashing or soldering 

 the two together. 



The representation of the carbon atoms in the paraffins as united 

 by single affinities is but the expression of the fact that the higher 

 hydrocarbons are prepared from the lower by displacing a single 

 atom of hydrogen — say by a single atom of iodine— in any lower 

 hydrocarbon, then withdrawing the iodine by means of a metal ; the 

 hydrocarbon residue thus formed at once unites with a like group 

 formed from another molecule of the iodide. Thus, when methyl 

 iodide from methane is exposed over mercury to sunlight, it is con- 

 verted into dimethyl or ethane — 



C H3 1 + C H3 1 + H/7 = H3 C - C H3 + H^ I2 



It is possible, however, not only to associate carbon atoms by 

 single affinities in open chains or curls having their two ends free 

 but these ends may be united together so as to form closed chains or 

 rings {cycloids). Such a ring is formed with particular readiness by 

 the union of five carbon atoms : the hydrocarbon of these dimensions 

 is particularly stable and unattractive, like the paraffins. 



When the number of carbon atoms united in a ring is less than 

 five, the compound is no longer unattractive but, on the contrary, 

 is acted upon more or less readily by a variety of agents, the ring 

 being broken in the process. Each of the angles within a regular 

 pentagon being very nearly 109° 28' (the angle at which the affinities 

 meet in the tetrahedron model of the carbon atom constructed in the 

 manner described above) it is a striking fact that when five such 

 tetrahedra are joined together they form practically a complete 



* Such a model may be constructed of four equilateral triangles, cut out- 

 of s^out cardboard, joined together at their edges so as to form a solid figure. 



