714 THE MOLECULAR STRUCTURE OF MATTER. 



and restricted mobility of the particles at ordinary temperatures, a 

 liigher degree and longer time are needed than with liquids or gases. 

 Again, when by the agency of heat, molecular motion is raised to a 

 pitch at which incipient fluidity is obtained, the particles of two pieces 

 will unite into a homogeneous whole, and we can thus grasp the full 

 meaning of the operation known as " welding." By the ordinary coarse 

 methods but few substances unite in this way, because the nature of 

 the operation prevents, or at any rate hinders, the actual contact of 

 the two substances; but when molecular motion is excited to the proper 

 degree by a current of electricity, the faces to be joined can be brought 

 into actual contact, the presence of foreign substances can be excluded, 

 and many metals not hitherto considered weldable, such as tool steel, 

 copper, and aluminium, are readily welded. 



The movement which we term radiant heat, acting through the instru- 

 mentality of the luminiferous tether (which is believed on the strongest 

 grounds to pervade all space and all matter) is competent to augment 

 the quantity of movement in the particles of substances, and generally 

 to cause an enlargement of volume. Again, energy in the form of light 

 operates changes in the surface of bodies, causing colors to fade, and 

 giving to photography the marvelous power which it possesses ; decom- 

 l)osing the carbonic acid of the atmosphere in the chlorophyl of green 

 leaves, and determining chemical combinations, such as chlorine with 

 hydrogen to form hydrochloric acid, or carbonic oxide with chlorine to 

 form chloro carbonic acid. It is inconceivable that these effects conld 

 be produced unless the undulations of light were competent to modify 

 the molecular motions already existing in the solid, liquid, autl gaseous 

 bodies affected. 



Electricity exerts a similar influence. Generated by the molecular 

 movements caused by chemical activity, whether directly, as in the pri- 

 mary battery, or indirectly, as in the dynamo, it is competent to increase 

 the molecular movements in bodies so as to produce the effects of heat 

 directly applied ; it is capable of setting up motions of such intensity as 

 to produce chemical changes and decompositions, to say nothing of the 

 whole series of phenomena connected with magnetism, with induction, 

 or the action through space and through non-conducting bodies, which, 

 as in the case of radiant heat and light, seems to imply the existence 

 of an interatomic ether. Conversely, changes of molecular equilibrium, 

 brought about by the action of external forces, produce corresponding 

 changes in electrical currents; witness the effects of heat, for example, 

 on conductivity, and the wondrous revelations of molecular change 

 obtained by the aid of Professor Hughes's induction balance. 



The behavior of explosives illustrates also, and in a striking manner, 

 the effects of disturbing molecular equilibrium. An explosive is a sub- 

 stance which contains in itself, in a solid or liquid form, all the ele- 

 ments necessary to produce a chemical change by which it is conveited 

 into the gaseous state. The application of heat, of pressure, or of im- 



