412 Dr. W. J. Eussell. On Hydrogen Peroxide as the 



On the supposition that hydrogen peroxide is the active agent in the 

 action exerted by the metals, it seemed probable that on supplying 

 to the metal more moisture than it obtained from the air and photo- 

 graphic plate, more action would take place, and this was found to be 

 the case. Two pieces of polished zinc were placed in contact with 

 photographic plates in small iron boxes ; one box was quite dry, and 

 put in a bell-jar over calcium chloride, arid into the other box some 

 damp paper was introduced, and the box was placed with a little water 

 under a bell-jar. On examining the plates after three days it was 

 found that the damp plate had much the darker picture on it. 



With the object of obtaining an increased amount of action, experi- 

 ments were made by passing a current of warm moist air over zinc 

 turnings. A glass tube, 6 feet long and 1 inch in diameter, was packed 

 with zinc turnings, and placed within a large brass tube to which 

 steam could be admitted. The amount of action, if any, was indicated 

 on a plate, placed in a dark box at the end of the tube. Even under 

 the most favourable conditions no very large amount of action took 

 place. When a current of moist warm air was passed through the tube 

 for an hour a fairly dark picture was obtained. If the air was dry no 

 picture appeared. If amalgamated zinc was used in place of pure zinc 

 a darker picture was formed, and, as a check on these results, dry and 

 moist air, both warm and at ordinary temperatures, was passed through 

 the tube, no zinc being present, and then no action took place. Also 

 when ozonised air was passed through the tube there was no action. 

 Passing now from the metals to the organic bodies capable of acting on 

 a photographic plate, it has been found that they belong essentially, if 

 not solely, to that class of bodies known as terpenes, and it is a general 

 property of all this class of bodies when oxidising to give rise to 

 hydrogen peroxide. Thanks to Dr. Tilden, experiments have been 

 made with most of the terpenes, and all were found to be very active 

 bodies; both pinene and limonene were tried, in their dextro- and 

 Isevo-rotatory state, but their activity appeared to be the same. Oxi- 

 dised and other compounds connected with the terpenes, such as 

 terpineol camphor, thymol cymene, have no power of acting on a 

 photographic plate, but ordinary turpentine and terebene are very 

 active bodies. 



Most of the ordinary essential oils, such as bergamot, peppermint, 

 pine, lemons, cajuput, &c., have been experimented with, and, without 

 exception, have been found to be active bodies. It is well known that 

 they all contain terpenes. They are also characterised by a strong 

 odour, and as ordinary scents contain some of these bodies, it follows 

 that almost all vegetable bodies having a strong smell are capable of 

 acting on a photographic plate. Eau de Cologne gives a good picture, 

 so do many wines and brandy, and coffee, guaiacum, cinnamon are also 

 active substances ; thus the photographic plate becomes a very delicate 



