POLYMORPHISM AT HIGH PRESSURES. 155 



KCIO4 is chemically more unstable, however, but shows no new modi- 

 fication under pressure, so the apparent connection between chemical 

 instability and polymorphism turns out after all to have been merely 

 accidental. 



In experimenting with members of this group, the danger of explosion 

 has to be particularly guarded against. One very severe explosion 

 resulted from the combination of NaClOs with the kerosene with which 

 pressure was transmitted. To avoid this possibility, the salts were 

 placed in a nickel steel shell, and pressure transmitted by mercury. 

 There is always the possibility, however, that the shell may split, and 

 the kerosene find its way to the chlorate, as it does for many other 

 substances. In view of this possibility I felt justified in confining my 

 exploration with NaClOs and KCIO4 to only 20°. It remains to this 

 day unexplained why the first three potassium salts did not explode 

 when exposed to direct action of kerosene up to 200° and 12000 kgm. 



Organic Compounds. 



Halogen Compounds with one Carbon Atom. — CCI4*, CBr4*, 

 CHCI3, CHBrs, CHI3. 



The first two of this series have already been discussed. It is 

 unfortunate that CI4 is too unstable to investigate. The other mem- 

 bers of the series are formed by replacing a halogen by an atom of 

 hydrogen. Since this substitution is known to produce important 

 changes in the chemical form, it is perhaps not surprising that poly- 

 morphism disappears under the substitution. It is known, however, 

 that lower members of the series, such as CH2CI2 show polymorphism 

 again; the effect of pressure on these substances should be tried. 



Benzol and Mono-substitution Products. — CgHe*; CeHsOH*; 

 CeHaCl; CeHsBr; C6H5NO2; CeHsNHo; CeHaCOaH monodinic; 

 CeHsNHCoHsO (acetanilid) ; acetophenone. It is strange that so 

 few of the crystalline forms are known in this group. 



In the comparatively complicated molecules of series like this, 

 one would not expect a substitution for a single atom to have so large 

 an effect on polymorphism as in series of simpler molecules, and it is 

 therefore perhaps surprising that polymorphism is not of more fre- 

 quent occurrence. But it is to be remembered that the second modi- 

 fication of benzol is stable only at the extreme pressures of the range. 

 It is possibly worthy of remark that the only other member of this 

 series showing polymorphism is the one nearest it in molecular weight. 



