162 Prof. G. Wiedemann on the Magnetic 



former professors of the type theory ; and in many quarters it 

 is regarded as the firm and final foundation upon which the 

 edifice of our chemical science has to be built. If in particulars 

 views sometimes differ, yet upon the whole the structure theory 

 is believed to express decisively the constitution of chemical 

 compounds, the placing of their atoms. Nay, some have even 

 gone so far as to wish to render the latter in symbolical repre- 

 sentations ; and in consequence of this, a few individuals have 

 esteemed the schematic notation of the bonds on a plane sur- 

 face not sufficient, the molecule being corporeal — an error 

 which can only have resulted from a strange confusion of 

 symbol with reality, the figurative representation of the sup- 

 posed reciprocal binding of the atoms with the shape of the 

 molecule consisting of them. 



It is frequently held that a purely chemical investigation is 

 sufficient, without the measurement of vapour-density, to 

 establish the relative positions of the atoms in compounds in 

 the sense of the structure theory. Corresponding to this me- 

 thod of investigation, besides the molecular weight, the struc- 

 tural formulae represent, although in a very complete manner, 

 yet only the processes out of which they have resulted ; they 

 mark the groups of atoms which, on the various chemical 

 attacks (heating &c.), separate from the combinations or are 

 exchanged with others. Meanwhile other questions are: — 

 whether, beyond these limits of direct observation, the funda- 

 mental hypotheses of the structure theory (often not a little 

 complicated notwithstanding their apparent simplicity) can be 

 brought into harmony with the principles of general mecha- 

 nics ; and how far it can account for the physical properties 

 of the compounds, which properties must yet result immedi- 

 ately from their chemical constitution. 



This is not the place to enter upon a more special reply to 

 these questions, of which especially the first would require a 

 very detailed discussion. In regard to the second we will only 

 cursorily mention that in fact the structural formulse indicate 

 very well in isolated cases certain regularities in the physical 

 properties. Thus the isomorphism of some substances coin- 

 cides with the analogy of their formulae (KNO3 and CaCOs, 

 &c.). The repeated entry in like manner of CH2 into the for- 

 mulas of the fatty acids, alcohols, aromatic compounds, with 

 the normal twofold binding of the carbon atom of that group 

 to two other carbon atoms, corresponds to a like alteration of 

 the boiling-point, the molecular volume, the refraction-equi- 

 valent : the latter constants are different with isomeric com- 

 pounds of different structure, &c. 



On the contrary, the structural formulas do not in any way 



