226 Transactions of the Society. 



(h) Presence of Oxygen. — Atmospheric. 



(c) Presence of Sulphur Compounds, including sulphate of 

 aluminium in practically all papers — certainly in all modern , 

 papers containing dendrites; ultramarine blue and aniline dyes 

 containing sulphur in some papers ; traces of organic sulphides in 

 unbleached wood cellulose papers ; sulphuric acid absorbed from 

 atmosphere, derived from gaseous products of combustion of coal- 

 gas, etc., in very old papers. 



{d) Presence of Chlorides derived from water used in paper 

 manufacture, or from bleaching residues, in some papers. 



{e) Presence of Oxidizahle Organic Matter, including primarily 

 the various kinds of cellulose ; also in many papers gelatine and 

 starch. In this connexion it must be remembered that most 

 papers contain the germs of living matter, moulds and bacteria, 

 ready to spring into active development with favourable conditions 

 of moisture and temperature. 



Given such conditions, the mode of growth producing the 

 dendrite is most probably as follows : — 



Stage I. — Solution of bronze nucleus by sulphuric acid of 

 aluminium sulphate, etc., with formation of copper sulphate. 

 This is essentially a process of oxidation, and is aided or intensified 

 by chlorides. 



Stage II. — The soluble copper sulphate creeps along the moist 

 cellular fibres by capillary force. 



Stage III. — The sulphate of copper is reduced to sulphide of 

 copper by hydrogen generated in Stage I., and by oxidizable organic 

 matter. 



Stage IV. — The sulphide of copper is again oxidized to the 

 soluble sulphate by atmospheric oxygen, and thus by alternate 

 oxidation and reduction insoluble copper compounds may be 

 deposited along the fibres in the form of a dendritic growth 

 containing copper sulphide pseudomorphous after copper sul- 

 phate. 



Stage V. — The final action in very old dendrites is oxidation 

 resulting in the formation, with complete solution of the metallic 

 nucleus, of a basic copper sulphate containing oxide and sulphate 

 of copper. 



Papers toned blue with ultramarine frequently show a bleached 

 zone around dendritic growths, showing that the sulphur of the 

 colour has taken part in the reaction, but in most papers the 

 presence of aluminium sulphate is sufficient. On the other hand, 

 in a very old paper in which alum could not be detected a dendritic 

 growth containing sulphide of copper was observed, arising from a 

 brass pin which had been stuck into the paper as a book-mark. 

 In this case the sulphide was most probably of atmospheric 

 origin. 



