1873O Actinism and Magnetism. 303 



the oxygen and iodine resume their original positions and 

 relations, so that all things return to the same condition 

 in which they were before the film was subjected to the 

 actinism of the light. 



When chloride of silver is applied to paper, in con- 

 junction with a little free nitrate for printing purposes, the 

 nitrate greatly quickens the decomposition of the chloride. 

 This acceleration can hardly be explained on any other 

 supposition than that the agitation, established by the 

 actinism, enables the chlorine to migrate from the silver 

 to the constituents of the nitric acid — forming hydrochloric 

 and chloric acids — perhaps also chloride of nitrogen, while 

 part of the oxygen of the acid attaches itself to the silver. 

 Hence the colour produced is no longer the purple of the 

 pure chloride, but there is a large admixture of brown from 

 the oxide. 



In the case of the double salts of chromic acid, the sepa- 

 ration of one of the molecules of chromic acid from the base 

 takes place by the action of the incident light alone, with- 

 out the aid of any developer, other than the organic matter 

 to which the double salt has been applied. It results from 

 the vibratory condition established in the salt by the 

 actinism, which, as it were, shakes free one of the mole- 

 cules of chromic acid, and allows it to combine with the 

 organic matter. There is here no latent image properly so 

 called ; nevertheless in the case of black pigmented gelatine 

 the image is, owing to its blackness, invisible. But it may 

 always be rendered visible by immersing the picture, after 

 exposure, in cold water for an hour or so, when the picture 

 will be seen standing out in relief. The portions of the 

 gelatine, which have not been acted on by the light, swell 

 through imbibing the moisture, and that in exact proportion 

 to the degree in which they were protected from the light's 

 actinism. Those parts which have, by the action of the 

 light, been made to combine with the chromic acid, cease 

 to have the power of imbibing moisture and swelling under 

 its influence. 



Some of the metals, more especially zinc, silver, and 

 magnesium, when used as electrodes, generate ethereal 

 waves lying far beyond the limits of the visible spectrum, 

 yet capable of exerting a powerful actinic action. Thus 

 an object might be photographed by means of actinic 

 waves wholly invisible to the eye. This circumstance tends 

 to establish the supposition already propounded, that the 

 vibrations set up by actinism must be very minute, and 

 such as are likely to take place between the ultimate of 



