294 Actinism and Magnetism. [July, 



II. ACTINISM AND MAGNETISM. 



By Mungo Ponton, F.R.S.E. 



OW slow have mankind been in searching for and 

 ascertaining the causes of physical phenomena ! 

 How tardy their efforts to apply their knowledge to 

 practical purposes, even where the ultimate uses have 

 proved to be of the highest importance to the well-being of 

 the human race ! How many ages had elapsed before 

 Franklin discovered the cause of electrical phenomena — 

 before Volta found how electricity might be developed by 

 chemical action, and before Oersted perceived the mutual 

 relations of electricity and magnetism ! Even after the 

 finger had thus been, as it were, pointed to a practical ap- 

 plication, how many years intervened before this last dis- 

 covery ripened into the construction of the electric tele- 

 graph ! 



In like manner, how many ages had elapsed before 

 Scheele discovered the actinic action of light in blackening 

 the chloride of silver ; and what a number of years has it 

 taken to develope that discovery into the art of photography ! 

 It seems wonderful that the attention of mankind was 

 not earlier attracted to the action of the sunbeams in de- 

 veloping or altering colours, and that they were not led to 

 investigate the cause of this curious phenomenon. It 

 might have been supposed that a careful study of Nature 

 would have led them to perceive it to be the energy of solar 

 light that tinges the cheek of the peach with crimson, gives 

 the apricot its flesh-like tint, imparts to the harebell its 

 beautiful blue, paints the pansy with alternating brilliant 

 yellow and deep violet, reddens the rose, and dyes the tulip 

 with its richly varied hues. The first attempts at tracing 

 the operation of the sunbeams in the colouring of flowers 

 were made by screening the petals from the action of the 

 light ; but these experiments went no farther than to show 

 that, in some cases, the petals do not acquire their proper 

 hue when they are thus screened. The subject, indeed, 

 was little studied until after the discovery of the actinic 

 action of solar light on other substances. Even yet, one 

 of the most remarkable cases of what may be termed natural 

 photography is but little known. It is that of the beautiful 

 bell-flower of the Cobcea scandens, which on the first day of 

 its opening is of a pale-greenish white, but after exposure 

 for two or three days to the actinism of solar light acquires 



