380 SCIENTIFIC AMUSEMENTS. 



duced by ordinary printing processes have been introduced, the 

 details of which cannot now be discussed. One simple method 

 consists in transferring a gelatin film, &c., to a wood block, and 

 using this instead of a pencil drawing on the wood, as a guide for 

 the engraver to produce the requisite lines by cutting away the 

 wood. Similarly, combinations of processes, involving treatment 

 with acid, so as to dissolve the metal plate employed, and with 

 ordinary engraving tools to finish and improve the work, are 

 frequently employed, the picture being transferred to the plate or 

 formed thereon by processes akin to those above described. 



Expt. 401. Other kinds of Chemical Action produced by 

 Actinic Bays. Although the preparation of photographs in virtue 

 of chemical changes set up or induced by radiant energy (and 

 more especially by the most refrangible rays) are the best known 

 and most frequently employed phenomena of this class, still many 

 other different, though analogous, cases of chemical action thus 

 induced are well known, e.g., the combination of free hydrogen and 

 chlorine gases determined by the action of light* (Expt. 215). 

 Certain compound vapours are affected by light in such a fashion 

 that if a clear transparent atmosphere of such a vapour be obtained 

 (free from dust or fragments of floating solid matter, and from 

 liquid particles suspended therein), and a powerful beam of light 

 sent through it, chemical change and precipitation of liquid pro- 

 ducts, as finely divided fog or spray in the atmosphere, will be 

 rapidly brought about, the effect of which will be that whereas at 

 first the path of the light was practically invisible, there being no 

 suspended particles to absorb and scatter the light (Pa. 291), in a 

 short time the track of the beam of light will become brightly 

 visible, like a ray of sunshine illuminating the motes' in the air of 

 a dusty room, owing to the development of liquid products of 

 chemical change in the form of innumerable floating vescicles. 

 One of the most important cases where chemical action is set up 

 by actinic rays is afforded by the formation of the green colour- 

 ing matter termed chlorophyll in the vegetable kingdom. This 

 substance appears to be only capable of formation under the 

 influence of sunlight, so that the hearts of tied up lettuces, the 

 stalks of celery (trenched up with earth whilst growing), the inner 

 leaves of cabbages, potato shoots sprouting in a dark cellar, and 



* In similar fashion the gases carbon oxide (Expt. 219) and chlorine do not 

 readily combine together in the dark ; but if exposed to light they will 

 combine tolerably readily, forming a compound gas sometimes termed phosgene 



fas on account of its thus being produced in virtue of the action of light, 

 iaiilarly, chlorine can be easily substituted for hydrogen in many organic 

 compounds containing that element if the action take place in sunlight, but 

 only with great difficulty or not at all in the dark. 



