TKANSACTIONS OF SECTION O. 721 



to work its way into the heart of the former, while in the formation of malle- 

 able cast-iron the carbon makes its way out of the castinp-s with equal facility, 

 it is a complete case of ditiiision of solid substances throug-h each other, but, 

 on account of tiie inferior and restricted mobility of the particles at ordinary 

 temperatures, a higher degree of heat and longer time are needed than with liquids 

 or gases. 



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

 wbicli incipient fluidity is obtained, the particles of two pieces brought into 

 contact will interpenetrate or diffuse into each other, the 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 sub- 

 stances 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 bo 

 joined can be broiiglit into actual contact, the presence of foreign substances can 

 be e.x:cluded, and many metals not hitherto considered weldable, such as tool steel, 

 copper, and aluminium, are readily welded, as many of us witnessed at the hands of 

 Professor Ayrton in the highly instructive lecture on electricitv delivered last 

 year at our Bath meeting. Again, a mere superficial union of different metals 

 takes place readily under the influence of high temperature and moderate pressure, 

 as we see in the operations of tinning, soldering, and brazing. The surfaces of 

 the metals must be made as clean as possible ; the solder, which melts at a lower 

 temperature than the metal to be soldered or brazed, is applied, and at a com- 

 paratively moderate temperature and under very slight pressure the particles 

 interpenetrate each other ; the two metals unite and form an alloy, by the inter- 

 vention of which the two surfaces are joined. This effect is very well illustrated 

 by the action which takes place at the surface of contact of two dissimilar liquids. 

 If brine, for example, be placed in the lower part of a glass tube, and ordinary 

 water, coloured in some way, be carefully poured on the top, a sharp plane of 

 demarcation will appear, but in a short time the plane of separation will become 

 blurred, and will ultimately disappear, a local blending of the two waters will take 

 place, and will thus present a case of fluid-welding. 



It seems plain, therefore, that apparently inert solid masses are also built up of 

 moving particles in dynamic equilibrium, for without such an assumption it would 

 be hard to explain the phenomena to which I have alluded. But in addition to 

 this evidence we can adduce the effects of other forms of energy, which we recognise 

 under the names of radiant heat, light, and electricit3^ These we know to be 

 forms of motion which can be communicated and converted from one to the other, 

 from the invisible to the visible. The movement which we term radiant heat, 

 acting through the instrumentality of the luminiferous ether which is believed, on 

 the strongest grounds, to pervade all space and all matter, is competent to aug- 

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

 cause an enlargement of volume ; and conversely, when the particles, by contact or 

 radiation, part with their heat, either to surrounditig objects or to space, the 

 quantity of motion is reduced, the body contracts, and this contraction goes on 

 down to temperatures far below those at which we have to work in practice, and 

 consequently at all ordinary temperatures there must be abundant room for 

 molecular motion. 



Again, energy in the form of light operates changes in the surface of bodies, 

 causing colours to fade, and giving to photography the marvellous power which it 

 possesses; decomposing 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 

 chlorocarbonic acid. It is inconceivable that these effects could be produced unless 

 the undulations of light were competent to modify the molecular motions already 

 existing in the solid liquid and gaseous bodies affected. 



Electricity exerts a similar influence. Generated by the molecular movements 

 caused by chemical activity, whether directly, as in the primary battery, or 

 indirectly, as in the dynamo, it is competent to increase the molecular movements 



1889. ^ . 3 a 



