Dendritic Growths of Copper Oxide in Paper. 547 



knives of the refining engines. In this paper a nucleus of bronze 

 0*8 mm. in length became a dendrite 3 "2 mm. in length, during a 

 period of twelve months, and even then the nucleus was not much 

 reduced, but merely coated over with oxide of copper. 



3. The occurrence of dendrites in certain classes of paper, and 

 their absence in others, admits of a very simple explanation. For 

 example, dendritic growths are more common in certain fine 

 writing-papers, such as ledger-papers, because the stuff from which 

 the latter are prepared undergoes a prolonged treatment in the 

 beating and refining engines, thus presenting greater opportunity 

 for the contamination of the paper-stuff with particles of bronze 

 ground from the bars and blades of the reducing machinery. 

 Many of these particles are caught in the sand-traps of the paper- 

 machine, but the lighter fragments are carried into the paper-web, 

 thus forming the nuclei of future dendrites. Dendrites are also 

 more common in light, porous cartridge-papers, and light printing- 

 papers, in which the pores of the fabric are not filled to excess with 

 loading materials. In short, dendrites occur most frequently in 

 papers which have undergone either prolonged or severe milling, 

 and in papers which present favourable internal circumstances for 

 their growth. 



4. Several reasons are apparent for the less frequent occurrence 

 of dendrites in recently-made papers than in papers made about 

 the middle of last century. It was formerly supposed that their 

 non-appearance in recent papers was due to their slowness of 

 growth, but that is not the case. An evident reason is the substitu- 

 tion of steel for bronze in modern beating-engines. Particles of 

 iron are extremely common in the cheaper papers made to-day, but 

 these never develop into true dendrites; they give rise, on oxida- 

 tion, to mere red stains, having no apparent crystalline structure. 

 Another reason for their less frequent occurrence is that most 

 modern printing-papers are more heavily loaded and filled than 

 formerly, in answer to the demand for a printing-paper having a 

 smooth surface suitable for half-tone illustrations. As already 

 pointed out, dendritic growths do not flourish in a heavily-loaded 

 or coated paper. 



III. The Size and Mode of Growth of Dendrites. 



Dendritic growths in paper vary much in size according to 

 their age and size of nucleus, the ultimate size of a dendrite 

 depending entirely upon the size of the central nucleus from which 

 it grows. From less than 1 mm., I have found them up to 12 mm. 

 in greatest diameter, which in machine-made papers is usually 

 parallel to the "machine-direction" of the paper. Simon records 

 them "up to say 15 mm. in diameter," but dendrites of copper 

 oxide of that size in paper must In- rather rare. 



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