Dendritic Growths of Copper Oxide in Paper. 549 



metallic nucleus, as catalytic agents in the presence of water and 

 oxygen. The whole process is a chemical one, in which the cellu- 

 lose plays an important part as a physical conveyer of the means 

 of oxidation, and as a physical and chemical carrier of the pro- 

 ducts of oxidation, to and from the metallic nucleus. The superior 

 crystalline energy of cuprous oxide, the porous nature of the 

 cellulose surface, and the presence of minute traces of oxidised 

 copper in the fibres themselves (derived from the milling engines), 

 are most probably all factors in the activity of dendritic growth. 



IV. Microscopical Examination of Dendrites. 



Dendrites in paper may be mounted dry for examination under 

 the Microscope. This is convenient for examination with low 

 powers by reflected light, especially when it is desired to observe 

 their growth, in which case, of course, they must not be sealed up 

 air-tight. For examination with medium and high powers by 

 transmitted light, I have found mounting in a waxy medium an 

 admirable and convenient plan. The refractive index of Canada 

 balsam renders it unsuitable, and glycerin has a solvent action on 

 the dendrite. Pure spermaceti wax, or white paraffin melting at 

 130° to 135° F., may be utilised for the purpose, preferably the 

 former. The fragment of paper containing the dendrite is first 

 treated with hot distilled water and hot alcoholic ether to remove 

 sizing, then dried thoroughly and soaked in the melting wax for a 

 few minutes. It is then mounted on a glass slip with a drop of 

 melted wax, the cover-glass being applied with moderate pressure 

 until perfectly cold. If sufficient pressure be applied to the cover- 

 glass during mounting the waxy medium shows good detail even 

 with fairly high powers, and brings out very well the translucent 

 nature of the red crystalline cuprous oxide. Particles of kaolin 

 stand out very clearly in the paper thus mounted, and it may be 

 observed how the dendritic growth avoids such obstacles. The 

 fibres may be stained before mounting, and aniline green will be 

 found a suitable colour. If a finer mounting medium be desired, 

 refined paraffin oil (sp. gr. * 900) may be used for this purpose. 



In the micro-chemical examination of metallic particles in 

 paper, a delicate method of distinguishing between extremely 

 minute particles of iron, and of copper, is sometimes required. 

 This may be done as follows : — A small fragment of the paper in- 

 cluding the suspected particle, is placed upon a glass slip, covered 

 with a drop of dilute (20 p.c.) hydrochloric acid, and warmed 

 gently over a small flame. After a few minutes a drop of a pure 

 solution of potassium sulpho-cyanide is added. If iron be present 

 the characteristic blood-red colour of ferric thio-cyanate shows at 

 once. If iron be absent, the examination is continued for copper, 



