CHAPTER V 



The Structural Features of Cellulose and the Spatial 

 Relationships of the Incrusting Substances 



THE X-RAY diagram of cellulose indicates, as we have seen, that this 

 substance is built up of long molecular chains of anhydroglucose 

 residues at least 600 A. long which, over some part of their length, are 

 arranged regularly in a crystalline lattice. It now becomes necessary 

 first to see how far this picture is in harmony with the behaviour and 

 physical characteristics of cellulose, and this chapter will therefore be 

 devoted to those properties of cellulose which are common to all cellu- 

 loses wherever they occur. 



Perhaps attention may first be called to the ready understanding of 

 the insolubility of cellulose, built up though it is of soluble glucose 

 units, and its lack of a melting point. Here an analogy can be drawn 

 with the homologous series of the paraffins. Methane is a gas which 

 condenses to a liquid only at a very low temperature. Ethane, the next 

 in the series, is also gaseous but can be liquefied at a much higher 

 temperature. Passing through propane and butane up to octane we 

 pass through gases with higher and higher liquefying temperatures to a 

 substance which is liquid at room temperature. Higher still in the series, 

 the substances become solid in the familiar paraffin wax. At first sight 

 this is rather remarkable since each member of the series differs from the 

 last only by the addition of another — CHg group. A moment's thought, 

 however, will make the matter clear. A liquid evaporates in so far as the 

 kinetic energy of the molecules due to their heat movement is higher 

 than the energy binding one molecule to its neighbours. The energy 

 which has to be put mto a molecule to cause the separation from its 

 neighbours, and therefore the boiling point of the liquid, depends there- 

 fore on the bond energy between the molecules. Now in methane any 

 two molecules are held together only by single secondary valences. The 

 energy concerned is of very small magnitude and a very little heat motion 

 suffices to keep the molecules apart. As the series is ascended, however, 

 since the molecule is increasing in length by the addition of further 

 — CHg, each of which will exercise a secondary valence attraction on 

 neighbouring molecules, the bond energy increases until with octane, 



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