THE MOLECULAR STRUCTURE OF MATTER. 715 



pact, causes chemical union to take place, first at the spot where the 

 equilibrium is disturbed by the application of external force, and after- 

 wards, with great rapidity, through the mass, the disturbance being 

 propagated either by the air surrounding the particles or by the lumi- 

 niferous lether, with all the rapidity of light; the chemical re-action is 

 accelerated by the pressure which may arise, for example, if the explo- 

 sive be confined in the chamber of a gun or in the bore-hole of a blast. 

 High explosives (as they are termed) are coui[>aratively inert to ordi- 

 nary ignition; but when the molecular equilibriuui is suddenly disar- 

 ranged throughout the mass by the detonation of a percussion fuse, com- 

 bination takes place instantly throughout, and violent explosion follows. 

 In a similar manner some gases, such a acetylene, cyanogen, and others, 

 can be decomposed by detonation and reduced to their solid constituents. 

 Professor Thorpe has devised a very beautiful lecture experiment, in 

 which carbon disulphide is caused to fall asunder into carbon and sul- 

 phur by the detonation of fulminate of mercury fired by an electric 

 S])ark. In these cases a reverse action takes phice, but it illustrates 

 equally well the conversion of one form of energy into others, and the 

 consequent disturbance of molecular equilibrium in the substances 

 afiected. It seems to me clear therefore the time has come when the 

 conception of dynamic equilibrium in the ultimate particles of matter 

 in all its forms must take the place of the structural system of inert 

 particles. 



I cannot conceive how the i)henomena which I have enumerated can 

 be explained on the supposition tliat matter is built up of motionless 

 particles; — how for example a stack of red and yellow bricks could 

 ever change the order of arrangement without being com])letely pulled 

 asunder and built up again, in which case an intermediate state of chaos 

 would exist: but I can easily comprehend how a dense crowd of i)eople 

 may ai)pear as a compact mass, streaming it may be in a definite 

 direction, and yet how each member of that mass is endowed with lim- 

 ited motion, by virtue of which he may push his way through without 

 disturbing the general appearance; how the junction of two crowds 

 would form one whole, though perchance altered in character; and 

 how even Professor Spring's exj)eriments may be explained by the sup- 

 position that bystanders on the edge of a crowd would be forced, by 

 external pressure, to form i)art of it and partake of its general move- 

 ments. 



When it is conceded that molecular motion pervades matter in all its 

 forms, and that the solid passes (often insensibly) into the fluid, or even 

 direct into the gaseous, it follows, almost of necessity, that there must 

 be a. borderland, the limits of which are determined by temperature 

 and i^ressure, in which substances are constantly changing from one 

 state to another. This is observable in fusion, but to a marked degree 

 in evaporation, where the particles are being incessantly hiunched into, 

 space as gas and return a? constantly to the liquid state. 



