Metals and other Substances on a Photographic Plate. 431 



becoming most evident on whichever side it may be. Interpose a 

 sheet, for instance, of gold-beater's skin, and still the picture is 

 obtained. The ' Standard ' and ' Daily Graphic ' are also very 

 active, and the ' Times ' only a little less so. The ' Evening News ' 

 is only slightly active, and the ' Morning Post,' ' Pall Mall,' ' Echo/ 

 and ' Daily News ' have not the property of acting in this way ; at 

 least, those copies experimented with had not. An admission ticket 

 to the Society of Arts laid on a photographic plate, the ink away 

 from the plate, also gave a very distinct picture. 



Another singular case of an action of this kind was met with 

 when experimenting with the uranium salts. Not having a sufficient 

 number of small, clear glass bottles for a certain set of experiments, 

 one of the compounds, the black oxide, was placed in a pill-box, 

 believing that the action of the uranium would take place through 

 the bottom of the box, and on developing the plate a dark circular 

 space where it had stood was visible. The experiment was, there- 

 fore, considered very satisfactory, and, with different salts and for 

 different objects, it was several times repeated. Ultimately it 

 seemed evident that the uranium salts acted more strongly when in 

 pill-boxes than in any other way, and on placing a pill-box without 

 any uranium salt in it on a photographic plate it was found that 

 action had occurred, as shown by the dark circular space produced. 



The experiment was repeated over and over again, with the result 

 that most pill-boxes have the power of acting on a photographic 

 plate. Both new and old pill-boxes from different sources were 

 experimented with, and almost all of them found to be active. 

 There are, however, exceptions, and these, it was noticed, were 

 always the more expensive and elaborate boxes. On examining 

 the structure of a pill-box it was found that it is usually made of 

 what is known as strawboard, covered with a thin white paper ; 

 on separately testing these two materials it was apparent that 

 the white paper was without action on the plate, and that the 

 strawboard was very active and produced exactly similar effects to 

 those produced by the active pill-boxes. The inactive ones proved 

 to be made of white cardboard, which is not an active substance. 

 Samples of strawboard from several different sources have been 

 tried, and all found to be active, and when separated from the 

 photographic plate by means of screens, like the copal and the zinc, 

 it gives a clear action. Different substances of a like nature have 

 been tried, such as brown papers, &c. Some of them are more or 

 less active, but none more so than common strawboard. Mr. Bevan 

 was good enough to examine a piece of this active strawboard, but 

 was unable to find any material other than straw present. Writing 

 paper and, as mentioned before, white cardboard have not this power 

 of acting on a photographic plate, but many kinds of brown paper 



