170 REPORT— 1900. 



Of these eight substances, three are stable in the anorthic system, 

 three in the orthorhombic system, and two in the monosymmetric system. 

 Change of the one form into the other has been observed in four cases 

 (ly-, V"., VII., VIII.) on allowing the fused substance to cool on a 

 microscopic slide ; the direction of the change in each case is indicated in 

 the table by an arrow. A labile anorthic crystalline form of dibromo- 

 benzene-sulphobromide (IV.) has been obtained from solution ; and it has 

 been found that each of the four sulphochlorides (V.-VIII.) can be caused 

 to crystallise in the alternative system by admixture with a sulphochloride 

 which usually separates in that system. It has been possible to determine 

 the symmetry of all the forms referred to in the table by crystallographic 

 measui'ement, with the single exception of the labile anorthic form of 

 dibromo-benzene-sulphochloride, but the existence of this form is indicated 

 by the dimorphous change which occurs on cooling from the melting point 

 to the atmospheric temperatui-e. 



The detailed study of such a series of isoraorphs — especially of the 

 melting points of mixtures and of the conditions which determine the 

 separation of the various crystalline forms — will be of importance, as it is 

 likely to furnish informatioia of value in discussing the phenomena pre- 

 sented by igneous rocks containing isomorphous minerals. 



The investigation has been extended by Dr. Jee to the corre- 

 sponding derivatives of the 1:3:5 dihalogen-bens^enesulphonic acids. 

 The results obtained indicate the existence of an isodimorphous series 

 having no apparent similarity v.dth the 1:3:4 series. One of the 

 members of this new series — 1 : 3 : 5 dibromo-benzene-sulphobromide — has 

 been obtained in two distinct crystallographic forms, both belonging to 

 the monosymmetric system. At atmospheric temperature one of these 

 forms is labile and isomorphous with the corresponding sulpho-chloride, 

 and with 1:3:5 bromo-chlorobenzene-sulphochloride. The stal^le form 

 of 1:3:5 dibromo-benzene-sulphobromide, on the other liand, is iso- 

 morphous with 1:3:5 bromo-chlorobenzene-sulphobroraide. The deriva- 

 tives of the symmetrical dichloro-acid have not yet been satisfactorily 

 measured. 



Even in their present incomplete form these results are of considerable 

 importance as showing the manner in which the occurrence of polymor- 

 phism may render obscure otherwise well-marked isomorphous or morpho- 

 tropic relationships. Apparently a substance may crystallise in a whole 

 series of different forms. A, B, C, D, the particular form obtained under 

 ordinary conditions being the form stable at the temperature at which the 

 crystals are grown. Another substance, the immediate homologue of the 

 first in an isomorphous series, can also assume crystalline forms corre- 

 sponding with A, B, C, D, etc., but the particular form stable at ordinary 

 temperatures will not be the same as before, owing to the non-corre- 

 spondence of the transition temperatures. Consequently the first member 

 of an isomorphous series may crystallise in a form of type A, the second 

 member in a form of type B, the third of type C, and so on, the 

 isopolymorphism completely masking the isomorphism. 



