632 



altei'.tioii to the following curious historical peculiarity: TheophrasT 

 (a pupil of Aristotle) says in his book rrfcii rrr^r'^ ; yMxriTs^oi^ ycQ 

 (f((C\ y.ui (i<').i[i(fov r^d^ T«x»*r«t êv rtö TIóvTVi cxilyov ymI y^8i[iüjroq 

 ovToq vsaviyoi^ ya7.Y,ov (U Qay^vca. (It is told that tin and lead 

 melted sometimes in the Pontos when it was very cold in a strong 

 winter and that copper was disintegrated). 



10. The properties of copper a and ^ as well as some problems 

 relating to the technical use of copper will be discussed shortly. 



VAN 't HoFï-Labomtori/. 

 Utredit, December 1913. 



Chemistry. — ''The metastability of the metals in consequence of 

 allotropy and its significance for Chemistry, Physics and 

 Technics.'' By Prof. Ernst Cohen. 



1. The research which 1 have carried out during tliese last few 

 monllis in collaboration with A. L. Th. Moesveld and W. D. Helderman, 

 has proxed that several metals which until now were onlj' known 

 in one modification are ca|)able of existing in two (or more) allotropic 

 forms. The continuation of these investigations will show if all 

 metals have this property, but we may even already conjecture that 

 this will be the case. A great many observations described in the earlier 

 literature afford evidence in this direction. 



2. We wei-e also able to state the fact that the pure metals as we 

 have known them until now are metastahle systems consisting of two 

 (or more) allotropic forms. This is a consequence of the very strongly 

 marked retardation which accompajiies the reversible change of these 

 allotropic modifications both below and aiove their transitionpoints. 

 Employing certain devices (using the metals in a very finely divided 

 state, adding an electrolyte) it is })0ssible to increase the transition- 

 velocity in such a degree, that the change of the metastable to the 

 stable form occurs within a short time. 



As such changes are very often accompanied by marked changes 

 of volume, the material is generally disintegrated. 



3. As until now, chemists and physicists have always dealt with 

 the a- and ji-form togetlier, cdl the physical constants of metals, which 

 have been determined, i-efer to the complicated metastable systems. 

 These are entirely undefined as the quantities of the «- and (?-modifi- 

 cations they contain aie not known. 



Now it is known that a special physical property of any substance 

 at a definite temperature and pressure depends on its allotropic 



