304 Prof. W. J. Sollas. 



centre of each primitive octahedron in the first place, and next that 

 which lies in the middle of the face of the complex octahedron. 



The total change in volume of the crystal would thus depend 

 primarily on the expansion of the atomic volumes, and next on the con- 

 traction which follows from a change of configuration ; and it is from 

 the supposed counteraction of these two opposing factors that we 

 may find an explanation of the extraordinarily small coefficient of 

 expansion which is characteristic of cuprite. Its value as measured 

 by Fizeau is 0*00000093, that of sulphur (an homologue of oxygen) 

 being 0-000064, and that of copper 0-0000169; copper in the free 

 state thus possesses a coefficient eighteen times greater than that of 

 cuprite, while that of sulphur is seventy times greater. 



The variation in the coefficient of expansion was very carefully 

 determined by Fizeau, and found to be 2*18. From this it follows 

 that with increasing rise of temperature the effect of the expansion of 

 atomic volumes gains upon that of the contraction due to change of 

 configuration ; while with fall of temperature the contrary is the case, 

 and a point may be looked for at which further cooling will cease to 

 be accompanied by contraction ; at this point, determined as - 4 C. 

 by Fizeau, cuprite will attain its maximum density. 



If the atoms of copper and oxygen are to remain in contact, so that 

 the six atoms of oxygen about the centre of a crystalline element shall 

 touch each other as well as the atoms of copper with which they are 

 associated, then when the configuration is that of maximum volume, 

 the atoms of oxygen cannot have a less diameter than the atoms of 

 copper. We have already seen, however, that at 15 C. the atoms of 

 oxygen do possess a slightly smaller diameter than that of copper. 

 It is therefore evident that further cooling if it tend towards pro- 

 ducing the configuration of maximum volume will effect a separation 

 of the six central oxygen atoms from each other, and this, though it 

 will not destroy, will certainly diminish the stability of the system ; 

 and consequently we may expect to find, if cooling be continued far 

 below 15 C., that the existing configuration will be overturned, and 

 the compound will acquire the symmetry of another crystalline 

 system. 



The cleavage of cuprite, as observed by Professor Miers, is most 

 perfect parallel to the faces of the cube ; this may possibly be con- 

 nected with the fact that these planes are situated at right angles to the 

 tension which we infer to exist between oxygen and copper along the 

 axes of the octahedron. 



In concluding this account of cuprite attention may be directed to 

 the interesting case of cupric oxide, which has been made the subject of 

 frequent comment by students of atomic volumes, on account of the 

 remarkable relation which seems to exist between the volume of 

 oxygen as it exists in this compound, and in the lower oxide which we 



