TRANSACTIONS OF SECTION A. 483 



uf vibration would difier from that at tlie next instant, if the oscillations were com- 

 pleted. It is manifest that increased amplitude would be piven to the pendulum 

 swings by a series of well-timed blows ditli-rinn- very larjjely in period ; at the same 

 time there would be positions of the jtendulum in which some one series of well- 

 timed blows would produce the greatest etlect. In a somewhat similar manner we 

 should ima;?ino th;it the ethereal waves should produce increased amplitude in the 

 swin;; of the atoms Ijetween very wide limits of period, and, further, that there 

 should be one or more positions in the spectrum when a maximum etlect is pro- 

 duced.' I would here remark that the shape of the curves of sensitiveness, when 

 plotted p^raphically, of the diHerent salts of silver to the spectrum have a marked 

 resemblance to the irraphically drawn curves of the three colour sensations of the 

 normal eye, as determined by Clerk Maxwell. May not the reason for the form of 

 the one be equally applicable for the other!-' I only throw this out as evidence, not 

 conclusive indeed, tliat the colour-sensitiveness of the eye is more probably due to 

 a photofrraphic action on the sensitive retina than to a merely mechanical action. 

 That this is the case I need scarcely say has several times been propounded before. 



The ease with which a silver salt is decomposed is larjrely, if not quite, dependent 

 on the presence of some body which will take up some of the atoms which are 

 thrown off from it. For instance, in chloride of silver we have a beautiful example 

 of the necessity of such a l)ody. In the ordinary atmosphere the chloride is, of 

 course, coloured by the action of light; but if it be carefully dried and purified, 

 and placed in a j^ood vacuum, it will remain uncoloured for years in the strongest 

 sunlight. In tliis case the absence of air and moisture is sutticient to prevent it 

 discolouring. 



If in the vacuum, however, a drop of mercury be introduced the coloration by 

 light is set up. We have the chlorine liberated from the silver and combining with 

 the mercury vapour, and a minute tilm of calomel formed on the sides of the vessel. 



Delicate experiments show that not only is this absorbent almost necessary 

 when the action of liglit is so strong or so prolonged that its effect is visible, but 

 also when the exposure or intensity is so small that the effect is invisible and only 

 to be found by devehipment. The necessity for this absorbent is not far to seek. 

 If, for instance, silver chloride be exposed to light in vacuo, although the chlorine 

 atoms may be swung off from the original molecule, yet they may only be swung off 

 to a neighbouring molecule which has lost one of its chlorine atoms, and an inter- 

 change of atoms merely takes place. If, however, a chlorine absorbent be present 

 which has a greater aflinity for chlorine than has the silver chloride which has 

 lost one of its atoms, then we may consider that the chlorine atoms will be on the 

 average more absorbed by the absorbent than by the subchloride molecules. The 

 distribution of the swung-oft' atoms between the absorbent and the subchloride will 

 doubtless be directly proportitmal to their respective affinities for chlorine, and so 

 for the other salts of silver. If this be so, then it will be seen that the greater the 

 affinity of the absorbent for the halogen the more rapid will be the decoui position 

 of the silver salt. This, then, points to the fact that if any increase in the sensi- 

 tiveness of a silver salt is desired it will probably be brought about by mixing 

 with it some stronger halogen absorbent than has yet been done. 



The question as to what is the exact product of the decom])osition of a silver 

 salt by the action of light is one which has not as yet been fully answered. For 

 my own part, I have my strong beliefs and my disbeliefs. I fully believe the 

 first action of light to be a very simple one, thoi'igh this simple action is masked 

 by other actions taking place, due to the surroundings in which it takes place. 

 The elimination of one atom from a molecule of a silver salt leaves the molecule 

 in an unsatisfied condition, and capal)le of taking up some fresh atom. It is 

 this capacity which seemingly shrouds the first action of light, since when ex- 

 posure is prolonged the molecules take up atoms of oxygen from the air or 

 from the moisture in it. Carey Lea, of Philadelphia, has within the last three 

 years given some interesting experiments on the composition of what he calls the 



' The effect of perfect and nearly perfect .synchronism of one oscillation upon 

 another is al.so to be found exemplified in my Treatise on Photography. Text Book 

 of Science Series. 



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