320 Mr. M. Carey Lea on 



the position assigned to it for the particular sample of glass 

 used in these experiments is, at least, approximately correct, 

 is shown by the fact that the red lithium line of wave-length 

 670 lies within the band, both when the glass is cold and 

 when it is hot. 



These observations and those of Feussner show that the 

 absorption-spectra of some substances vary with the tempera- 

 ture, as indeed might have been anticipated from the behaviour 

 of the blowpipe-beads already referred to. In the case of 

 solutions this may be due to the formation of different hy- 

 drates, or to the partial dissociation of the substances ; but 

 in the case of a solid substance, like cobalt glass, an actual 

 change in the chemical constitution of the glass at a tempe- 

 rature considerably below its fusing-point does not seem very 

 probable, although the well-known effects to light in causing 

 glass which has been decolorized with manganese dioxide to 

 become purple seems to show that such a change is not im- 

 possible. 



XXXYI. On Allotropic Silver. By M. Carey Lea*. 



Part II. — Relations of Allotropic Silver with Silver as it exists 

 in Silver Compounds. 



THE first part of this paper was devoted to the examination 

 of one of the well-marked forms of allotropic silver — the 

 gold-coloured. The blue form in its soluble and insoluble 

 varieties will be more particularly described in a future paper. 

 The subject at present to be considered is the relation existing 

 between the allotropic forms of silver taken generally and 

 silver as it exists in its compounds and more especially in the 

 silver haloids. 



It is a well established law that when a substance is capable 

 of existing in two forms, of which one is a polymer of the 

 other, the polymeric form possesses greater density and less 

 chemical activity. Combination is usually accompanied with 

 loss of activity, and the polymerization of a body consists in its 

 combination with itself. When a substance is capable of exist- 

 ing in two allotropic forms and of being converted from the 

 one to the other by pressure, the body resulting from pressure 

 is always the more dense of the two and is a polymer of the 

 first f. In the case of allotropic silver these laws appear to 



* From an advance proof communicated by the Author, 

 f See examination by Spring of the effect of pressure, Ber. D. Ch. Ges. 

 xvi. p. 1002, 1003. 



