THE PHYSICS OP A SUNBEAM. 
443 
important part in regulating all the conditions of vegetable 
life. Of many of these, experiments have determined the facts ; 
a large field for inquiry is, however, still open to the industrious 
and patient searcher after hidden truths. The influences of 
Light on the animal economy are rather suspected than known. 
Experiments on animals present many difficulties which have 
not yet been overcome. Everything, however, proves the cor- 
rectness of Lavoisier’s poetic expression, “ The fable of Pro- 
metheus is but the outshadowing of a philosophic truth — where 
there is Light there is organization and life ; but where Light 
cannot penetrate, there Death for ever holds his silent court.” 
Light, Heat, and Actinism, existing in the sunbeam, are 
unceasingly at work. It has been shown by the investigations 
of the elder Niepce, and by Niepce de St. Victor, that it is 
impossible to expose any body, however sohd and persistent it 
may appear, to the influence of sunshine without its undergoing 
a molecular or a chemical change. Glass, stone, wood, or metal, 
are, it can be proved, susceptible of Actinic change. The 
images of objects can be obtained upon them without any 
sensitive preparation ; but they cannot be retained. In dark- 
ness all bodies appear to possess the power of restoring them- 
selves to their normal state. If the solar rays shone without 
interruption on a granite monolith, or a bronze statue, it would 
perish, independently of other influences. Night is, therefore, 
as necessary to secure the permanence of the inorganic world 
as darkness and sleep are to maintain in healthful fife the 
organized creations. 
