Dr. E. J. Mills on Isome?-ism. 7 



unanimous voice chemists have explained isomerism on the prin- 

 ciples of the atomic theory. At first, indeed, it was only natural 

 that the phenomenon should have been considered due to the 

 possibility of arranging a given number of atoms in many differ- 

 ent ways ; for the well-known views of Higgins and Dalton con- 

 stituted at that time the creed of chemistry, and were referred 

 to on every occasion of importance. Nevertheless it was not 

 found necessary in most cases to appeal to ultimate atoms ; it 

 sufficed to group these into compound radicals, which should 

 equally admit of differences in arrangement, and which should 

 be numerous enough to cover the existing variations in a given 

 group. This doctrine, however, intimately connected as it was 

 with the employment of rational (and afterwards of typical) for- 

 mulae, was inapplicable to all cases of isomerism, even when 

 physical instances had been removed from the category. Such 

 a defect was keenly felt on all sides ; and there can be no doubt 

 that the subsidiary theory of atomicities, modified as it has been 

 by Erlenmeyer, Butlerow, and others, has met with a very sin- 

 cere and hearty welcome. 



II. 



If we assume the existence of atoms and the possibility of 

 disposing them in many different ways, it nevertheless does not 

 follow that isomeric bodies should exist. For the action of a 

 group of atoms would be a resultant effect, which no change in 

 arrangement could vary. Hence a group of atoms, however ar- 

 ranged, ought to behave in a constant manner on the application 

 of any force, such as heat, electricity, or chemical energy. 



The theory of atomicities is still less satisfactory. One atom 

 of carbon combines with four of hydrogen and no more. It is 

 then said to be saturated, and its four atomicities (or units of affi- 

 nity) to be neutralized by the four which appertain to the four 

 atoms of hydrogen. Each unit of atomicity, therefore, implies 

 a unit of force ; for saturation and neutralization involve the ex- 

 ertion of force. But how can such a view be maintained in pre- 

 sence of the known facts relative to the heat of chemical action ? 

 Yet the consideration of force seems to enter into the doctrine of 

 atomicity, in an obscure and uncertain fashion, even where such 

 an idea has been disclaimed. If, on the other hand, force be 

 excluded from the question, then we are told no more than for- 

 mulae tell us already. Now it is inexpedient to account for the 

 existence of isomeric substances on the supposition of an altera- 

 tion in the value of one of the atomicities of an atom, when we 

 are not even assured of the meaning of the term " atomicity " 

 itself. 



There is no better evidence of the weakness of a theory than 



