xviii President's Address 



with the spectroscope a blende (a sulphide of zinc) from a 

 mine in the P3rrenees. He observed new and hitherto 

 unrecognised lines in the spectrum, which have enabled him 

 to pursue, and eventually to separate, and obtain specimens 

 of, the new metal. The chemical and physical properties of 

 this new substance are in some measure ascertained now 

 that the metal has become tangible ; but the delicacy of the 

 means by which this has been brought about may be 

 estimated from the statement that the earliest experiment 

 in which the nature of the spectrum of this new metal was 

 established was made on a quantity something less than the 

 1 '5,000th part of a grain, dissolved in a very small drop of 

 liquid. The melting point of pure gallium is stated to be 

 so low as to warrant our regarding it as being with mer- 

 cury, in the category of metals^ fluid at ordinary atmospheric 

 temperatures ; nor are its already ascertained chemical rela- 

 tions less interesting. It has been shown that elementary 

 bodies may be arranged, according to their combining equi- 

 valents^ into groups of three, or '' triads," in which the 

 combining equivalent of the middle element is the numerical 

 mean of the two others, but in more than one of these groups 

 the middle term is wanting. From what has been ascer- 

 tained concerning gallium, it appears highly probable that it 

 will be found to fill one of these gaps — that, namely, between 

 aluminium and indium ; and it has been moreover suggested 

 that a wanting element with a combining equivalent, the 

 mean of these of silicon and tin, should be sought in the 

 field of natural combinations respectively of arsenic and 

 titanium. These fore sh ado wings of the existence of elements 

 new to science of definite characters and positions in the 

 great chemical scheme suggest a comparison with discoveries 

 in another domain of human knowledge — with those, namely, 

 which predicted and led to the discovery of the planet 

 Neptune. 



