2 THE ZOOLOGIST. 



phosphorescent gases. The photogenic properties of various 

 substances and gases discovered by Sir James Dewar in his 

 remarkable experiments with liquid air still further enlarge the 

 field. Mr. Herbert Jackson, from whose lecture* part of the 

 foregoing is quoted, considers that these phenomena may be 

 looked upon as outward evidences of response on the part of 

 the substances to rapid oscillations, whether these oscillations 

 have their origin in chemical combustion — in what is commonly 

 spoken of as light — or in electrical discharge. The nature of 

 that response may in some cases be of a direct character, but 

 when account is taken of the many degrees of persistence of 

 phosphorescence it seems in many cases first to assume the form 

 of a statical change. The release of this condition of strain 

 is accompanied by oscillations which give rise to the visible 

 undulations of phosphorescent light.! 



Since the discovery of phosphorescence in the garden Nastur- 

 tium by a daughter of Linnaeus — the same who delighted herself, 

 as Arago also did, by setting fire to the inflammable atmosphere 

 surrounding the oil-glands of certain species of Dittany (Fraxin- 

 ella) — this phenomenon has likewise been known to the botanist. 

 Phosphoric light is emitted by various plants, such as the sun- 

 flower, marigold, orange lily, certain Fungi, and Bacteria. 



Placed as we are on the shores of the North Sea, this striking 

 phenomenon must be familiar to most of us — even those who do 

 not go down to the sea in ships — for it is but necessary to stir the 

 stranded seaweeds at night on the east or the west sands after a 

 storm to find every blade sparkling with brilliant points, which 

 glimmer and twinkle like miniature stars. In the sea itself, 

 however, the phenomenon is seen in great beauty, for, leaning 

 over the side of a boat in July or August, the wavelets are seen 

 to gleam with phosphorescent points ; whilst behind a ship the 

 brightly sparkling and seething mass — here and there with 

 circles of fire at the screw— merges into a long trail of luminous 



* Lecture to the British Association, ' Nature,' Oct. 6th, 1898. 



f Physicists state that when the wave-length is greater than 812 millionths 

 of a millimetre no luminous effect is produced on the eye, though the effect 

 on the thermometer may be great. When the length is 650 millionths the 

 ray is visible as red light, and when 500 millionths of a millimetre it is 

 brilliant green, but has much less heating effect than either of the foregoing. 



