Under the circumstances it is 
HAROLD L. ALLING 
others, we have come to possess comprehensive data, more especially 
d to the optical properties. 
the studies of Michel-Lévy, Lacroix, Fouqué, Iddings, and many 
216 
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the relation between these optical and physical properties and their 
rather a surprising fact that very few authors correlate or show 
less-known thermal properties. 
