Allotropic Silver. 323 



Action of Forms of Energy on Silver Haloids. Parallelism 

 with Allotropic Silver. 



It is a familiar fact that certain forms of energy, light 

 especially, affect the silver haloids. In view of what has been 

 already said as to the action of all forms of energy on allotropic 

 silver, it seemed desirable to make a general examination as to 

 their action on the silver haloids and thus to determine how 

 far a parallelism could be traced. 



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 opposing affi- 

 nities. But in every case action does take place and always in 

 a direction corresponding to the action of that form of energy 

 upon allotropic silver. 



High-tension electricity, it is well known, impresses sensitive 

 films of silver haloids, which on development exhibit remark- 

 able 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 colour 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 

 development, could be impressed on silver iodide. Lines 

 drawn with a glass rod or any other hard, neutral substance 

 were reproduced. 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 con- 

 cordant 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 



