402 Mr. H. Jackson on Phosphorescence. 



easterly force is the same all over the earth, while the ampli- 

 tudes of the vertical and northerly components vary respec- 

 tively as the cosine and sine of the latitude. 



5. The above investigation has been confined to the case 

 where the sun acts like a homogeneously magnetic sphere. 

 There is no difficulty in extending the calculations to any 

 magnetization. But it must be remembered that if the 

 magnetic potential of the sun is referred to his axis of rotation 

 and expressed in a series of spherical harmonics, it is only 

 the forces due to the terms giving a uniform magnetization 

 which vary inversely as the cube of the distance, the other 

 terms varying with the higher powers. Forces which depend 

 on the next terms, and vary inversely as the fourth powers, 

 must, if they be of equal magnitude at the distance of the earth 

 with those due to uniform magnetization, have magnitudes at 

 the sun's surface which are more than 200 times greater than 

 these. Hence if there are any appreciable effects at the 

 distance of the earth due to the higher terms, these terms 

 would be paramount at the sun, whose magnetization, there- 

 fore,, would have to be of an entirely different character from 

 that of the earth. 



For the present we are justified in leaving that possibility 

 out of account, and limiting the investigation, as has been 

 done, to the case of a homogeneously magnetized sphere. 



XLI. On Phosphorescence. By Herbert Jackson*. 



IT is not possible in one lecture on phosphorescence to 

 give any historical sketch which shall do justice to tiie 

 work of those who have made a study of the phenomena. 

 In a list of the names of the many who have enriched the 

 subject with facts and with theories, those of Pliny, Albertus 

 Magnus, Robert Boyle, Canton, Becquerel, Stokes, and 

 Crookes stand out most prominently. Any attempt to make 

 a sketch of our knowledge of phosphorescence and fluores- 

 cence must be to a very large extent an adaption of the work 

 and of the views of these masters. 



The phenomena themselves may be divided into two main 

 classes — those in which the evolution of light is associated 

 with chemical change, and those in which there is no evi- 

 dence of such direct alteration. In the first class the 

 commonest instances are connected with the process of 

 oxidation. Examples of this kind are numerous. It is 

 hardly possible to take any very easily oxidizable substance 



""» Communicated by Lord Kelvin; Laving been delivered before the 

 Meeting of the British Association at Bristol, Sept. 12, 1898. 



