Manchester Memoirs, Vol. li. (1906), No. %. 5 



when exposed to the action of solar or electric light, and 

 retaining their phosphorescence for more or less time after 

 the source of light has been removed. Calcium, strontium, 

 barium, and zinc sulphides possess this property in 

 different degrees, while the radium combinations of the 

 same series are permanently self-luminous. 



Now it is admitted on all hands that elementary 

 metallic calcium, strontium, barium, and zinc do not 

 exhibit the property of phosphorescence of their sulphur 

 compounds, and in this respect behave like ordinary 

 metallic substances. By strict analogy metallic radium 

 would, in like manner, be divested of a'l the extraordinary 

 properties which have been attributed to it, and opened 

 out so wide a field for ultra-scientific hypotheses. 



In my first paper on elementary transformations I 

 directed attention (1) to the strict parallelism existing 

 between the electro-negative halogens, fluorine, chlorine, 

 bromine, iodine, with the electro-positive alkaline metals, 

 sodium, potassium, rubidium, caesium, and the common 

 difference of 4 between the atomic weights of the parallel 

 series, and (2) the like parallelism between the electro- 

 negative elements, oxygen, sulphur, tellurium, selenium, 

 with the electro-positive alkaline-earth metals, magnesium, 

 calcium, strontium, barium, and the common difference of 

 8 between the atomic weights of these series. 



The numerical, chemical, and physical properties of 

 the four series of these elements clearly indicate them to 

 be positive and negative analogues of each other and of 

 hydrogen. 



The intensely electro-negative character of the halo- 

 gen and oxygen series induced me to affirm in my first 

 paper that at an early period of their history the two 

 series existed in a state of isolation from all the others. 

 The subsequent chemical combinations of these eight 



