10 Prof. Marshall Ward, On a Problem 



pestris, the Elm. As Exp. No. 5 shows, the Elm gave the merest 

 traces of activity in 12 days at 20 — 22° C, and Russell had also 

 found the Elm but slightly active ; but the same section when 

 streaked with pyro-gallol and incubated for the same period at the 

 same temperature (Exp. No. 31) gave an extremely good image, 

 particularly black where the pyro-gallol was streaked across it. 



The next substance tested was Xylol. As experiments Nos. 29 

 and 30, with Sophora microphylla and Teak, Tectona grandis, 

 show, Xylol is an effective agent, because the paper into which it 

 soaked gives a dark image ; but since I have not as yet tested 

 these woods alone it is impossible to say whether this agent alone 

 is responsible for the action, or whether the results are due to the 

 co-operation of something in the wood. 



One thing seems fairly clear. I thought it possible that the 

 water used for the solutions of tannin, pyro-gallol &c. might play 

 an important part, because, as Russell had also shown, damp wood 

 acts more rapidly than the same wood when old and dry. But in 

 the cases of xylol and clove-oil this seems impossible : these re- 

 agents were used pure, and the surplus merely soaked off by 

 filter-paper. 



I have also obtained positive results with gallic-acid. Berberis 

 vidgaris, the Barberry, showed the merest traces of activity in 

 8 days at 20 — 22° C, as shown in Exp. No. 2 ; and although the 

 Vine (Vitis vinifera, Exp. No. 6) gave better results in 12 days, 

 the image was too faint to show on a screen. But the same 

 sections after washing in gallic-acid (Experiments Nos. 33 and 34) 

 gave fairly dark images after 12 days at 20—22°, proving that 

 this reagent also is effective. 



Enough has now been stated to show that whatever the activity 

 of wood on the photographic plate may be due to, it is not merely 

 Resin or Resin-like bodies ; but that Tannin and Tannin-like 

 bodies as well as some others may be responsible. 



My knowledge of the chemistry and physics of the subject is 

 too slight to enable me to attack the problem of the mode of 

 action on the plate ; moreover the question is in sufficiently able 

 hands with Dr Russell. 



It seemed, however, a sufficient reason for this sketch, to point 

 out that we have here a Botanical problem of great interest — it 

 may be of fundamental importance. 



It is clear that some body or bodies in the wood cell-walls — 

 i.e. lignified cell-walls — reduce silver-salts in the plate. That 

 such bodies are either shot off, as if volatile, or diffuse readily, 

 seems clear from the want of sharpness in the microscopic 

 details. 



I attribute the blurring of the figures to the facts that the 

 activity is not confined to the actual planes of contact of the cut 



