LENARD'S RESEARCHES ON PHOSPHORESCENCE 61 



states. In the lower state, besides the rapid emission, which is 

 usually feeble, there is always an invisible storage of light- 

 energy proceeding simultaneously, the which energy is liberated 

 as a strong after-glow when the temperature of the phosphoroid 

 is raised to that of the permanent state without subjecting it to 

 further excitation. The energy thus stored in the lower state, 

 which does not give rise to any luminosity so long as the 

 temperature is below that of the permanent state, can be pre- 

 served during an extraordinarily long time, extending into 

 months. The bands which appear at a given temperature are 

 those which are permanent bands at that temperature. All 

 luminosities which were observed by early experimenters to 

 appear in phosphoroids on heating were due to energy having 

 been stored in this way in the cold state of the given substance : 

 after they had once been made luminous by warming, a fresh 

 excitation was necessary before luminosity could be again so 

 produced. Thus heat cannot act as an exciter of phosphorescence, 

 but only as a liberator of light-energy already supplied and 

 stored during the lower momentary state. During the upper 

 momentary state, there is, however, no storage of energy. The 

 two momentary states constitute what is sometimes referred to 

 as fluorescence, but, as already stated, it is proposed to restrict 

 this term to gases and liquids in which the duration of the after- 

 glow is at least so short that it has never been measured. 



Besides the two momentary and the permanent state, there 

 is a fourth process of lesser importance, on which not much 

 work has been done, to which only passing reference can be 

 made. Lenard found that the shorter ultra-violet rays can 

 excite a luminosity of medium duration falling between that 

 of the momentary and the permanent state ; it is most intense in 

 the extreme ultra-violet, and gradually grows fainter with in- 

 creasing wave-length, becoming unnoticeable in the visible violet. 

 This form of incitation he called the ultra-violet process ; it is of 

 account only if the exciting light be of very short wave-length. 

 It has not the definite excitation distribution of the other 

 processes, but seems to be more nearly allied to the permanent 

 than to the momentary states. 



In Lenard's researches in conjunction with Pauli and Kam- 

 merlingh Onnes at low temperatures, and subsequent work with 

 improved apparatus, the fact has been clearly established that 

 there are different exciting spectra corresponding to the momen- 



