1905.] Glass ly Natural Solar and other Radiations. 525 



different pieces, which may be attributable to the varied length of the 

 exposure of each to the action of the sun's rays. It seems to me that 

 some of the pieces have lost somewhat of their depth of colour since I 

 picked them up, but this may be an impression only. The two pieces 

 forming together the bottom of a broken tumbler, and which have 

 a deeper tint than any of the rest, were found about twenty paces 

 apart in an old Oficina that had been uninhabited for 27 years. It is 

 impossible to give any idea of the length of exposure of the remaining 

 pieces to the sun's rays, as I have obtained them from all parts of the 

 Pampa over an extent of nearly 100 miles. The samples I send you 

 were originally white glass, and although an abundance of glass of 

 various colours is to be found, yet I send you none, as it would not 

 be easy to say what the original colours had been previous to 

 exposure." 



The pieces of glass referred to above are of all depths of tint, from 

 deep violet, almost black in thick pieces, to pale amethyst. Analysis 

 shows the glass to contain manganese. Heating the glass in a covered 

 crucible to its softening point, discharges the colour, leaving the glass 

 white and transparent. 



The colouration is not superficial. On immersing a piece of the 

 coloured glass in a liquid of about the same refractive index as itself, 

 the colour is seen to have penetrated throughout the mass. 



At first sight the explanation of this phenomenon would seem to be 

 that it is produced by the action of light, the intense radiation 

 occasioning a re-arrangement of the oxygen molecules in the glass, the 

 ferric salt becoming ferrous, and the manganous salt changing to a 

 manganic compound.* The change of colour might then be expected 

 to be noticed in any part of the world where broken glass is thrown 

 about and the sun's rays are very intense. In the Transvaal, where 

 both these conditions are well fulfilled, I have neither heard of nor 

 noticed any such colouration, and it would be interesting to hear if 

 travellers in other tropical countries have observed any such change of 

 colour of glass. 



Probably height above sea level has much to do with the phenomenon. 

 At a height of 4000 metres nearly half the atmosphere is beneath one's 

 feet, and that which remains will allow rays of shorter wave-length to 

 pass through than the atmosphere at sea level will transmit. 



For this reason it is not necessary to invoke another mode of 

 explanation that might possibly suggest itself. It now has been well 

 established that many natural bodies, water from great depths, some 

 samples of earth and rock, air from underground sources, together with 

 some minerals, are more or less radio-active. Radium, acting for a few 



* In this connection it may be of interest to recall the fact that in the early days 

 of photographic research the ultra-violet rays of the spectrum were called the 

 " deoxidising rays." 



