354 THE BEGlNNINaS OF PHOTOGEAPHY. 



violet than in iiny other ray of the spectrum, follo^ved his method. 

 A strip of strong- white paper was coated very eveidy ])y means of a 

 brush witli precipitated silver chloride iineh' rul)bed down Avith water 

 into a seuiifluid mao-ma. When exposed moist in a dark room to the 

 solar spectrum of a prism, it at once quickly darkened at a considerable 

 distance beyond the outer violet, then in the violet itself, and finally 

 showed the weakest action in that part where the l)lue loses itself in 

 the green. Through the yellow and red and on l)eyond, the chloride 

 remained white, however long it might be exposed. 



This experiment, he says, shows the presence of invisible rays 

 beyond the violet in continuation of the visi])le rays, and possessing 

 the same action, and that, just as is shown at the red end by the ther- 

 mometer, their maxinuun action lies beyond the limit of the visible 

 ra3's and at a considerable distance from them. In the same way as 

 the heating action shown by the thermometer is almost confined to 

 the red end of the spectrum, so the darkening action on the chloride 

 is almost entirely confined to the blue and violet. He remarks that 

 Scheele must have made his observation very casually not to have 

 noticed that in half the spectrum there was no action at all, and it is 

 to be regretted that he overlooked this fact and the physical and 

 chemical phenomena connected with it. 



He then discusses the chemical nature of the change in the horn 

 silver, which consists of nuu'iatic acid and silver oxide (silver and 

 oxygen). By the blackening of this substance the silver loses its oxy- 

 gen, and, since it can not remain coml)ined with the ncid. reverts to the 

 metallic state, appearing black on account of its being finely divided. 

 Consequently what occurred on the blue side of the spectrum was 

 deoxidation, one of the two great processes into which every chemical 

 reaction finally resolves itself. Its opposite is oxidation. Seeing 

 that the deoxidation took place only at the l)lue end of the spectrum, 

 the question A\as whether corresponding oxidation took place at the 

 red end. He set himself to prove it by exposing a strip of paper 

 coated with the chloride, but already evenly darkened in the violet or 

 other part of the blue end, so tliat the red rays fell on the darkened 

 part, comparing it from time to time with a similarly darkened strip. 

 He found that near the red and I^CA'ond it there was a place at which 

 the darkened strip became distinct!}' paler, and it gradually spread 

 till, after a (juarter of an hour, the middle of the spectrum had 

 retained its original tint, Avhich became gradually weaker until it 

 almost disappeared at a point beyond the red and then increased again 

 for a short space. 



From this he argues that the loss of color in the darkened chloride 

 was due to oxidation of the silver, and that therefore the red raj's 

 have oxidizing properties, agreeing very closely in gradation with 

 that of the rise and fall of temperature in HerschePs experiments. 



