52: 



NATURE 



[September 22, 1904 



Knots in a wood generally, but not always, give a good 

 picture. Some of the resin in immediate contact with the 

 knot is in some cases but little active. The marked differ- 

 ence in properties of resins from different sources is de- 

 scribed, and it is shown how difficult it is to remove it so 

 that the wood shall be no longer active. Boards that have 

 been e.Kposed to the air for a long time, an oak box a 

 hundred or more years old, rotten wood from the stump 

 of a tree, and even bog wood have all been found to be 

 ^rill .-i.-tivp. 



In addition to woods many different resins and allied 

 bodies can, when used alone, be proved to be very active, 

 some naturally much more so than others. Ordinary resins, 

 Burgundy pitch, gum mastic, are very active, asphaltum, 

 dragon's blood much less so, but true gums such as gum 

 Senegal and gum arable are entirely without action on a 

 photographic plate. • 



In certain cases the picture obtained on the plate does 

 not resemble the markings which are visible on the 

 wood. With some woods this more commonly occurs 



NO. 182 I, VOL. 70] 



than with others. That this picture is persistent 

 in the wood is shown by fresh sections giving the same 

 result. The true bark of a wood is apparently quite with- 

 out action on a photographic plate, so is the internal pith 

 of a plant. 



There is another and a very interesting action which 

 occurs with wood ; it is the great increase of activity which 

 it e.xerts on a photographic plate after it has been e.\posed 

 to a strong light. For instance, if a piece of deal be half 

 covered by black paper or tin foil and be exposed for five 

 to ten minutes to bright sunlight, and 

 then put up in the usual way with a 

 photographic plate, it will give a dark 

 picture where the light has fallen on the 

 wood and only a very faint picture of 

 the part which has been covered. This 

 is shown in Fig. 2. Even com- 

 paratively inactive woods such as elm 

 and ivy after a short exposure to bright 

 light give good and dark pictures. The 

 action is not an indiscriminate darken- 

 ing over the whole wood section, but an 

 intensifying of the parts already active. 

 This increase of activity by the action of 

 light appears to occur with all woods. 

 Artificial light, such as that from the 

 electric arc, or from burning magnesium 

 ribbon, act in the same way, so does 

 even a faint light. A piece of wood put 

 at a window for some hours will give a 

 darker picture than a similar piece left 

 in the middle of the room. This in- 

 crease of power of a wood to produce 

 a picture does not rapidly pass away. 

 After twenty-four hours the action is 

 visibly less, and decreases more rapidly 

 at first than after some days, but it will 

 be a fortnight or may be a month be- 

 fore the wood resumes its former con- 

 dition. This action, like the former one, 

 is entirely stopped by interposing the 

 thinnest piece of glass or mica between 

 the photographic plate and the active 

 body. An inactive card painted with an 

 alcoholic solution of resin acts in the 

 same way, and turpentine which has 

 been exposed to a bright light acts more 

 strongly on a photographic plate than it 

 does when it has not been so exposed. 

 Again, old printing which is now nearly 

 inactive becomes much more active after 

 exposure to sunlight. Bodies other than 

 those which may contain resin or allied 

 substances are not affected in this way 

 by light, for instance, flour, sugar, 

 porcelain ; metals are not rendered active 

 by sunlight. 



The next point was to ascertain which 

 of the constituents of light was most 

 active in producing these effects, and the 

 first e.\periments were made by simply 

 placing strips of different coloured glass 

 on wood sections, exposing them to sun- 

 light and afterwards putting them up 

 with the photographic plate in the usual' 

 way. Pictures of the results are given 

 in the paper. Red glass entirely pre- 

 vented any increase in the activity of the 

 wood, in fact, it acted in the same way 

 as a band of black paper or tin foil would 

 act, and a green glass acted much in the same way, but 

 under a blue glass the activity of the wood was increased 

 to much the same extent as under colourless glass or under 

 no glass. Fig. 3 shows what happens when a red 

 glass and a white glass are placed upon it and is ex- 

 posed to sunlight. On the right of the figure there was 

 no glass. 



Further experiments were made by placing similar pieces 

 of deal in light which had passed through different coloured 

 solutions. Three double-cased bell jars were taken ; one 



