262 M. C. Lea — Allotropic Silver. 



It is to be observed that the action of different forms of 

 energy on the silver haloids is apt to be partial : the influence 

 seems to be antagonized by opposing and almost equally 

 matched forces. Thus in the case of light, its tendency to 

 condense the atoms of silver to molecules is largely counter- 

 acted by the strong affinity of chlorine for atomic silver. The 

 action of high tension electricity as will presently be seen is 

 similar to that of light in that it produces a visible effect. In 

 the case of heat and of contact action on these silver haloids, it 

 will be shown that there is at first a mere indication of effect, 

 invisible to the eye, but readily brought out by the action of a 

 reducing agent, as described below. The action of each form 

 of energy seems to be almost counteracted by apposing affini- 

 ties But in every case action does take place and alvjays in a 

 direction corresponding to the action of that form of energy 

 ujton allotropic silver. 



High tension electricity it is well known impresses sensitive 

 films of silver haloids, which on development exhibit remarka- 

 ble ramifications When electric sparks are passed through 

 paper on which a coating of silver chloride has been made, the 

 point of passage of each spark is marked by a minute circle of 

 violet color indicating a visible change, probably to a sub- 

 chloride. 



Mechanical force. — More than twenty years ago I noticed 

 that by a slight pressure, an invisible effect, capable of develop- 

 ment, could be impressed on silver iodide. Lines drawn with 

 a glass rod or any other hard, neutral substance were repro- 

 duced. An embossed card pressed gently on the film, gave an 

 image of all its details on development. These experiments 

 were extensively repeated by others with concordant results. 



I have recently repeated them with silver bromide with 

 similar effect. 



Heat. — To determine the effect of heat on silver bromide, 

 pieces of bromide paper were placed in a desiccator (of course 

 using inactive light) and heated to the extent indicated. For 

 each piece so heated a corresponding piece cut from beside it 

 in the same sheet was preserved, and these two pieces, that 

 heated (after complete cooling) and that not heated, were 

 placed side by side in an oxalate developer. Comparison 

 between these developments indicated the effect of the heat. 

 The following results were obtained. 



A piece kept for 3 minutes at 145° C. was strongly affected 

 and blackened quickly in the developer, the companion piece 

 remaining white. 



A piece kept for 15 minutes at a temperature commencing at 

 131° C. and ending at 136° was still more thoroughly affected 

 than the foregoing, the longer exposure more than making up 

 for the lower temperature. Companion piece remained white. 



