Thus von Groth showed first that benzene, C_{6}H_{6}, crystallises in the rhombic system with axial ratios _a_ : _b_ : _c_ = 0.891 : 1 : 0.977. Next, that when one or two of the hydrogen atoms are replaced by hydroxyl OH groups the substances produced, phenol C_{6}H_{5}.OH and resorcinol C_{6}H_{4}(OH)_{2}, are found also to crystallise in the rhombic system, and in the second case, for which alone the axial ratios could be determined, the ratio _a_ : _b_ proved to be very similar, but the ratio _c_ : _b_ was different, the actual values being _a_ : _b_ : _c_ = 0.910 : 1 : 0.540. Pyrocatechol, the isomer of resorcinol, also crystallises in the rhombic system, but the crystals have not been obtained sufficiently well formed to enable any deductions to be made from any measurements carried out with them. Similarly, the nitro-derivatives of phenol, orthonitrophenol C_{6}H_{4}.OH.NO_{2}, dinitrophenol C_{6}H_{3}.OH.(NO_{2})_{2}, and trinitrophenol C_{6}H_{2}.OH.(NO_{2})_{3}, also crystallise in the rhombic system, and with the following respective axial ratios: 0.873 : 1 : 0.60; 10.933 : 1 : 0.753; 0.937 : 1 : 0.974. Again, the value for the ratio _a_ : _b_ is not very different from that of benzene itself, while the ratio _c_ : _b_ differs considerably in the first two cases. Similar relations were also found to hold good in the cases of meta-dinitrobenzene, C_{6}H_{4}(NO_{2})_{2}, axial ratios 0.943 : 1 : 0.538, and trinitrobenzene, C_{6}H_{3}(NO_{2})_{3}, which possesses the axial ratios 0.954 : 1 : 0.733. The introduction of a chlorine or bromine atom or a CH_{3} group in place of hydrogen was found by von Groth to produce more than the above effect, the symmetry being often lowered to monoclinic, a fact which had also been observed to occur in the cases of certain isomers of the substances quoted above, ortho-dinitrobenzene for instance. But it was nevertheless observed that the angles between the faces in the prism zone remained very similar, the angles between the faces of the primary prism (110) and (1̄10), for instance, only varying in eight such derivatives of all three types, whether rhombic or monoclinic, from 93° 45′ to 98° 51′. The crystallographic relationships of organic substances, however, are very much complicated by the possibilities of isomerism, the ortho, meta, and para compounds—corresponding to the replacement of the two hydrogen atoms attached to two adjacent, alternate, or opposite carbon atoms respectively, of the six forming the benzene ring—generally differing extensively and sometimes completely in crystalline form. Consequently, the phenomenon of morphotropy is best considered quite independently of isomorphism.