PHYSICAL FORCES. 45 



incut. Chemistry teaches us that we may, without pro- 

 ducing any disarrangement of the affinities, but by 

 merely setting up molecular disturbance, effect decided 

 changes, as is strikingly shown in the colour of iodide of 

 mercury changing from red to yellow under slight in- 

 fluences of heat, and back again to red by a gentle 

 mechanical disturbance. By a slight change, merely 

 molecular, iron may be made to resemble platinum in 

 its physical properties.* An iron wire plunged into 

 nitric acid is attacked by the acid with violence ; but if 

 one extremity of the wire is heated in the flame of 

 a spirit lamp, such a change of state is produced 

 throughout the entire length of the wire, that if it be 

 now plunged into nitric acid no effect is produced upon 

 it. On studying this question, we find good reason for 

 supposing that bodies which, though physically different, 

 resemble each other in some of their properties, iodine, 

 bromime, &c., are the results of different allotropic con- 

 ditions which have been impressed upon the ultimate 

 atoms, similar to those observed in the substances 

 named. This hypothesis appears to be more in accord- 

 ance with the great principles which we must conceive 



* On this curious subject, and its history, see Bergman's 

 Dissert, de Phlog. quantitate in Metallis, 1764.* Kirvvan, On the 

 Attractive Powers of Mineral Acids : Philosophical Transactions. 

 Kier's Experiments and Observations on the Dissolution of Metals 

 in Acids : Phil. Trans. 1790. 



From these valuable papers it will be seen that the peculiar 

 states of iron had already attracted attention, particularly those 

 "inactive conditions" noticed in a " Note sur la Maniere d'agir 

 de r Acide nitrique sur le Fer, par J. F. W. Herschel" Aug. 1833 ; 

 and previously indicated by M. H. Braconnot, Sur quelques 

 Proprietes de r Acide nitrique, Aimales de Chimie, vol. Hi. p. 54. 

 Reference should also be made to the Memoirs of Sir John 

 Herschel, On the Action of the Rays of the Solar Spectrum on 

 Vegetable Colours, $c. : Phil. Trans, vol. cxxxiii. p. 221 ; and On 

 the Separation of Iron from other Metals: Phil. Trans, vol. cxi. p. 

 293; and several papers by Schoubein, in the Philosophical 

 Magazine, from 1837. 



