( 407 ) 



sition, bill also on llieir eheuiical tstnictiirc in (ho broadest sense 

 this wonhl also be Iho i-ase with the crystalline form. Dr. ,1ak(H''.r 

 has, i^o^^cver, perceived that the relation between (brni and struc- 

 tnre cannot be (jnite so sini[)le and only stands a chance of being- 

 discovered bv a very delicate investigation of |)roperly chosen series 

 of objects and lie jnstities the choice of the six isomeric tribromo- 

 tolnenes used in his i-esearch : by their high moiecnlar weight and 

 the slight ditference in chemical jiroperties, so that practically, only 

 the relative position of (he gronps of atoms in the molecnle causes 

 a ditFerence and because the number of isomers is not too small. 



Dr. Jaeger has j^repared these substances, which were oidy known 

 as tine needles, in a form suited to measurement and an accurate 

 examination of them showed that four of them l)clong to the mono- 

 clinic, one to the rhombic and one to the tetragonal system. The 

 last two are those with the vicinal position of the three bromine 

 atoms. Of the tirst four there are two 'svhich exhibit an isomorphism 

 bordering" on identit}' and which can form mixed crystals in all 

 proportions. 



The densities of the four monoclinic forms do not difïer greatly, 

 (hat of tlie rhombic isomer, however, is smaller and that of the 

 tetragonal form still smaller so that a higher symmetry of form is 

 accompanied by a lesser density. 



He further determined the tifteen possible melting point lines of 

 the binary mixtures of the six isomers and obtained very notable 

 results, some of which have already been mentioiied by Dr. Van Laar 

 at the last meeting. 



From the isomorphism boi-dering on identity of the two monoclinic 

 forms we may certainly, conclude that there is great similarity in 

 the structure of their chemical molecules although these are not 

 expressed in our chemical formulae; tliej" are the compounds 1. 2. 

 3. 5. aiid 1. 2. 4. 6. if CH., stands on i. This similarity appears, 

 however, if we keep in view the analogy which exists, in a certain 

 sense, between the group CH3 and a bromine atom, which has 

 been noticed in a number of cases and which may be referred to 

 a similarity in space relationships, \'olume perhaps. A conformity 

 already ])ointed out by Groth in 1870 and which has been since 

 observed by many chemists, for instance in cases of so-called sterical 

 obstacle. 



The relations found by Dr. Jaeoer between chemical and crystal- 

 lographic symmetry have given rise to a nund)er of problems, which 

 he is now engaged in solving, for instance whether the isomorphism 

 of the two monoclinic tribromotoluenes remains intact when substi- 



