430 Dr. Waller's Experiments on the 



action of light, though to a very slight degree, acquiring a 

 trifling accession in their brilliancy. It has already been re- 

 marked that light destroys the adherence of the external in- 

 visible film: the same thing obtains with the second or gold- 

 coloured film, which turns green, but only to a certain depth 

 of the film, as may be proved by slightly rubbing the part 

 thus altered ; the green colour is then seen to disappear, and 

 beneath the pulverulent portion thus removed is found the 

 gold colour, having almost the same appearance as before 

 the plate had been exposed. As this experiment may be re- 

 peated several times with the same results, it shows to how 

 inconceivably small a depth the light has acted to produce 

 this effect. To ascertain what would take place on augment- 

 ing the thickness of the portion turned green, and the ad- 

 herence of which was destroyed, a piece of iodine was placed 

 on the plate so that its vapour, by expanding, might arrive 

 upon the green, at the same time the whole being kept from 

 the light; the result was that the additional film combined with 

 the one already existing, producing a blue, being the colour 

 which would have resulted by the combination of the unal- 

 tered yellow films. I have found no chemical substance pos- 

 sessing the power of arresting, or in anyway influencing these 

 changes of colour ; strong acids, provided they do not attack 

 the silver, — for then, of course, the experiment would be de- 

 stroyed, — and alkalies in concentrated solution, allow the ac- 

 tion of light to go on as usual. The hyposulphite of soda, and 

 ammonia in solution have no longer the power of dissolving 

 the green film as they had before the action of light. 



When the plate is left still longer exposed, after the changes 

 above stated have taken place, the colours become more faint, 

 and within the zone of green a white cloudy film is caused by 

 the light, which, as it increases, veils the spectral colours be- 

 neath. 



The knowledge we at present possess in chemistry of the 

 affinities with which different bodies are endowed for com- 

 bining with each other is but very imperfect, and the causes 

 which complicate most chemical phenomena are so numerous, 

 that it is scarcely possible to compare any two chemical ac- 

 tions to each other. Most of the facts upon which chemical 

 science is founded, are acquired either by bringing the two 

 bodies destined to act on each other into contact by dissolving 

 them in a liquid, or by subjecting them to a temperature more 

 or less elevated. 



In the first of these methods, we are so far from being able 

 to calculate the force of the chemical powers called into play, 

 that Berthollet was induced to deny the existence of chemical 



