MOLECULES IN A CRYSTAL. 485 



eight radicles PO(OH) 2 are arranged at the corners of two 

 superposed horizontal squares, and that vertically above or 

 below each of these is attached a radicle OK. 



Tutton (4), in a paper read quite recently before the 

 Chemical Society, has independently adopted precisely the 

 same reasoning, and feels himself justified in supposing that 

 in rubidium sulphate (for example) the crystal elements are 

 situated at the corners of a rectangular rhombic prism of 

 certain dimensions, and that each of them is composed of 

 four chemical molecules arranged in a certain symmetrical 

 manner. 



Yet further : the test by which any theory must stand 

 or fall as a working hypothesis is the extent to which it 

 renders prediction possible ; and the relations between 

 form, structure and composition have at length been so 

 far ascertained, that the pioneers in this study already 

 venture to predict the form of new or undetermined com- 

 pounds. 



Thus Muthmann states that rubidium sulphate will 

 doubtless be found to have a form almost identical with 

 that of thallium sulphate, and within a few months this 

 prediction has been fully verified by Tutton's measure- 

 ments. 



To take another and a somewhat different example, 

 Penfield (5), in a recent paper on the Chondrodite 

 group of minerals, uses the following words: "Thus 

 Mg[Mg(F,OH)] 2 Si0 4 is a possible and most likely com- 

 pound to occur. This should crystallise either orthorhombic 

 or monoclinic with /3 = 90 , and should have the axial ratio 

 a : b : c = i'o86 : 1 : 1 "887." Here we have a prediction 

 of the crystalline form of an absolutely unknown compound, 

 and one in this case not based in the least upon considera- 

 tions of isomorphism. 



We hope to return to these and other instances in a 

 subsequent communication ; for the present, we have to see 

 what is the geometrical reasoning upon which the current 

 theories of crystal structure are founded. 



There is some danger lest assertions like those of Muth- 

 mann should be either accepted without reservation as 



