7J0 SCIENCE PROGRESS 



well be imagined that a slight shear in one direction might 

 determine the passage of one form into the other : 



Fractions of axial Equivalence 



Substance^ Axial ratios. ratios chosen. parameters. 



a : b : c. /3. x : y : z. 



'^ fi-boi : I : 1-141 10529 fa 5736 : 2-389 ^ 2726 



Cl^^ ySOjBrj 1-293 = i = 0-585 io2°3o' - 5 949 : 2-300 : 2-694 



. ^"^ ro-813 : I : o 569 90°o' 3a, 2c 5-731 : 2-349 : 2-675 



/ NsO,Cl|o-835 : i : 0-563 97*27' 3a, 2c 5-872 : 2-343 : 2-639 



CI 



Particularly in the case of the second compound is the 

 similarity between the two modifications brought out. The 

 equivalence parameters of the two forms are almost identical, 

 the only appreciable change involved being the shear of the 

 first orthorhombic form into the monosymmetric form with 



Many of these dimorphous compounds have their transition 

 point very near to, frequently slightly above, common room 

 temperature. In such cases it is possible to observe directly 

 the change from one form into the other. A crystal of the 

 substance is melted upon a microscope slide beneath a cover 

 slip. On cooling, it crystallises first in the form stable at 

 the higher temperature. After some time, however, a change 

 into the form stable at lower temperatures begins to set in, 

 usually starting at one or two points and spreading gradually, 

 becoming complete perhaps in a few minutes, perhaps only 

 after several hours. The change can be very well followed 

 under the microscope or photographs of the specimen can be 

 made at intervals — a method by which very interesting records 

 of the phenomenon can be obtained. 



The morphotropic study of substituted benzenesulphonic 

 acid derivatives has brought to light other relationships of so 

 interesting a character that the writers have been induced to 

 revert to unsubstituted benzenesulphonic acid and accumulate 

 data with respect to derivatives of this acid. Already relationships 

 between derivatives of the substituted and unsubstituted acids 

 are coming to light ; a few of these may be hinted at here. 



The sulphanilides obtained by the interaction of paradi- 

 halogen benzenesulphonic chlorides and aniline are found fre- 

 quently to be dimorphous. The labile form is evidently derived 

 from the rhombic type of benzene, whilst the stable form exhibits 



