xx President's Address 



of this nature ; and at the present time new elements, like 

 new asteroids, are periodically turning up. But in addition 

 to the facilities thus afforded by the methods of spectrum 

 analysis, theoretical views — those of Mendelejeff especially, 

 on the periodic law of the elements — arrange the known 

 elements in a methodical series, show where a hiatus occurs, 

 and predict, with considerable accuracy, the chemical and 

 physical properties of the yet-to-be-discovered elementary 

 substances. Gallium comes to fill one of these gaps, realis- 

 ing the expectations of its family likeness to the bodies 

 nearest to it as well as individual characteristics of its own. 

 In the discovery of gallium we are helped to further know- 

 ledge concerning the physical states of matter. This metal 

 has so low a melting-point that it may be questioned 

 whether it is a fluid or a solid at ordinary temperatures. 

 When in the solid state, it will become fluid if warmed in 

 the palm of the hand ; and wheu thus fused it may again 

 become solid as it is cooled, or may retain the liquid state 

 for days, hours, or even weeks. This peculiarity regarded 

 per se might appear anomalous. Faraday, however, many 

 years ago, noticed some droplets of sulphur sublimed into 

 the neck of a flask, which remained in the fluid state long 

 after cooling, but which at once solidified and became 

 opaque if touched by a hard substance. Similarly fluid 

 gallium may be reduced to the solid state by simply touch- 

 ing it with a particle of solid gallium. As affording an 

 illustration of the manner in which the several departments 

 of physico-chemical research assist and illuminate one 

 another, Dr. Van Riimsdijk in his investigation concerning 

 the phenomenon of " flashing" in the gold bullion assay, has 

 traced the absence of this change (for it does not invariably 

 happen) to the presence of small quantities of the metal 

 iridium in the gold assayed, the iridium being infusible and 

 insoluble in the gold-silver alloy at the temperature at 

 which the assay is drawn from the fire. When no solid 

 particles of iridium are present the gold-silver alloy sinks 



