Two cases of Insensibility of the Eye to Coloiirs, 153 



A great number of minerals are known which contain among 

 the number of their elements a hydrosilicate of the protoxide 

 of iron, but none which contain a silicate of the peroxide with 

 water of crystallization. The Nontronite is the first mineral 

 of this kind. As the silicates of the peroxide of iron have ge- 

 nerally a high colour of either red or brown, we ought not at 

 first sight to conjecture the existence of it in Nontronite. The 

 colour of this mineral depends evidently on the presence of 

 water ; this colour actually disappears by calcination, and we 

 know salts of the peroxide, such as several sulphates, which, 

 when they contain water, are of a pale yellow colour, and 

 sometimes almost colourless. 



I have said that Nontronite strongly calcined in a close ves- 

 sel becomes sensibly magnetic ; the silicates, however, of the 

 peroxide of iron do not act on the magnetic needle. This 

 phenomenon may be thus explained : The peroxide of ii*on is 

 a very weak base ; it cannot be combined with silex in the dry 

 way, without the intermedium of another base ; but as on the 

 contrary silex has a great tendency to unite itself to the pro- 

 toxide of iron, it happens, that when we heat to a temperature 

 sufficiently high this substance with peroxide of iron, a por- 

 tion of this peroxide abandons the oxygen, and transforms it- 

 self into peroxide, or at least to an oxide inferior to the real 

 oxide. The combination thus formed may be regarded as a 

 double silicate of the protoxide and the peroxide, in propor- 

 tions which vary according to circumstances ; but the pre- 

 sence of a small quantity of the protoxide is sufficient to com- 

 municate to a silicate the magnetic virtue, when the silica does 

 not exist in too great a proportion. 



Art. XXV. — Account of two remarkahle Cases of Insensi- 

 bility in the Eye to particular Colours. 



The insensibility of some eyes to particular colours is a much 

 more common defect than is generally believed, and it is a cu- 

 rious circumstance, that three of the most distinguished indi- 

 viduals in Great Britain, Mr Dalton, Mr Troughton, and the 

 late Mr Dugald Stewart, were all incapable of distinguishing 

 particular tints. The case of Mr ' has recently been 



