THE STRUCTURE OF PROTOPLASM 249 



a free carbon bond actually exists; it rather indicates where our 



knowledge ends. The bond is possibly satisfied by a univalent 



(OH) group or attached to an adjoining chain. 



The length of the cellulose chain is not fixed. It is capable, 



stoichiometrically at least, of reaching any length. One cannot, 



therefore, speak of a cellulose molecule in the | 



strict sense if by molecule is meant a unit of /^^\ 



C-C c 



fixed weight and constitution. A length of 40 i i 



glucose residues, or twenty times the length q c 



of the cellobiose molecule (10.3 A. U.), repre- c 



sents a chain length of about 200 A. U. This 9 



is a minimum. Several times this probably ^'\ 



ri HO-C-H H-C-CH,OH 



more closely represents an average, fetau- I I 



dinger has assumed that the macro molecule ho-c-h h o 

 of the cotton fiber, which appears to be the '^ 







I 



0^ 



longest, may be 1,000 A. U., and E. O. Kraemer i 



has shown (from viscosity and ultracentrifugal ^ 



determinations) that the cellulose molecule I | 



may reach the microscopic dimension of 1.7 jj. o^ x 



(the diameter remains the same, viz., that of "^ 



a glucose molecule). Physically, the molecules ^ 



must be regarded as comparatively stiff Fig. 128.— Part of a 



, 1 1 cellulose molecule. 



threads. 



The molecular weight of the average chain molecule has been 

 put at 30,000 to 40,000. Stamm obtained the latter value by 

 centrifuging in a high-speed Svedberg centrifuge (page 477). 

 As the length of the chain varies, the molecular weight will 

 vary; consequently, the much greater length attributed to the 

 cellulose molecule by Kraemer means a greater molecular 

 weight; in fact, the estimated length was based on molecular- 

 weight determinations. The weight now given by Kraemer 

 for the 1.7 M molecule is 500,000. 



There are many polymeric materials which are constituted 

 on the same principle as cellulose, in that their molecules are 

 characterized by a chain of recurring structural units; rubber 

 is such a substance. 



With this information as a starting point — though it was at 

 the time less precise than now — the problem was carried forward 

 by other X ray workers, who included Scherrer and Herzog. 

 Spectrograms (Fig. 129) indicate clearly that the structure of 



