PHYSICAL SCIENCE. 41 



changes, transient or permanent, may occur within all bodies 

 while retaining what we call their solid state, is one of high 

 interest, and not enough regarded in its application to every 

 part of physics. The familiarity of some of the instances 

 disguises what is most curious and important in their nature. 

 The simple expansion of a metallic bar by heat involves an 

 atomic change through its every part ; less complex, it may 

 be, than those changes of molecular arrangement within 

 crystals. which affect the passage of light through them; but 

 analogous in the main fact of the mobility of atoms, and 

 their power of assuming new and definite position within 

 what we call a solid body. We know from recent experi- 

 ments that an iron bar is sensibly elongated, and the 

 elasticity of iron transiently, of steel permanently, altered by 

 magnetisation. We know further that the capacity of iron 

 to conduct heat is variously modified under the electro- 

 magnetic action. We have the certainty, from the effects 

 manifested at its extremities, that every molecule in the wire 

 of an electric telegraph, whatever its length, undergoes change 

 at the moments of transmission or cessation of the electric 

 force. Without stopping to enquire whether such internal 

 changes may not be interpreted as a tendency to what we 

 term fluidity, we clearly see in them a proof of the indivi- 

 duality of atoms ; and very strong evidence that these mole- 

 cules of matter, minute beyond conception though they be, 

 are endowed individually with axes of motion or polarities, 

 determining their mutual relations, and the changes they 

 undergo when submitted to forces from without. Such con- 

 clusions, forced upon us by the simplest view of the subject, 

 are strikingly corroborated by the whole course of modern 

 enquiry ; and very especially in those sciences which have their 

 foundation in the actions of light, and the electrical and mag- 

 netic forces, upon matter. The time may come when mole- 

 cular forces or affinities, now represented chiefly in chemical 



