132 SCIENCE PROGRESS 



weight of the particles at the corners of a cubic cell and upon 

 the volume of the cell, and will vary directly as the former and 

 inversely as the latter. The former is, of course, given by the 

 molecular weight — the weight of the atoms of which the molecule 

 consists. If, therefore, the two crystals are to have cells of the 

 same volume, the molecular weight divided by the specific 

 gravity {i.e. the molecular volume) must be the same for both. 

 The same principle applies to any two similar structures — not 

 necessarily cubic. The equality of molecular volume accounts, 

 therefore, for the fact that sodium nitrate crystallises in parallel 

 position on calcite alone. That they do possess similar structures 

 is indicated by their similarity of angle, cleavage, and optical 

 and other physical properties. 



If this be a true explanation, it must hold good for other 

 groups. Now Mr. Barker has discovered that the alkaline 

 perchlorates and permanganates have the same structure as 

 the mineral sulphates of lead, strontium, and barium (anglesite, 

 celestine, and barytes), as shown by their similarity of angle, 

 cleavage, and constitution. 



Among the nine salts examined he found only two, namely, 

 KCIO4 and KMn0 4 , which readily yielded parallel growths on 

 barytes BaS0 4 and the other two minerals of the barytes group, 

 and these are precisely the salts which possess nearly the same 

 molecular volumes as the sulphates. 



A yet more extensive series has been studied by Mr. Barker, 

 the alkaline haloids which crystallise in the cubic system. No 

 less than seventeen of these were examined, and among the two 

 hundred and seventy-two possible combinations realised, parallel 

 growths were only obtained between those which possess most 

 nearly the same molecular volumes. 



[Equality of molecular volume in a series of substances having 

 the same crystalline structure may also be expressed as equality 

 of topic axes, a conception which is rapidly becoming familiar 

 since its introduction by Becke, Muthmann, Tutton, and others, 

 and one which enables us to compare not only volumes, but 

 corresponding magnitudes along parallel directions in such a 

 series.] 



In all these groups it seems that when the molecular volumes 

 are almost exactly equal, the parallel growth is so perfect and 

 uniform that it takes place over the whole surface of the crystal, 

 which then appears simply to increase in size by the addition 



