510 



NATURE 



[Sept. 19, 1889 



Liquids, therefore, are porous bodies whose constituent 

 particles have great freedom of motion. Tt is no wonder, 

 consequently, that two dissimilar liquids, placed in contact with 

 each other, should interpenetrate one another completely, if 

 time enough be allowed ; and this time, as might be expected, 

 is considerably greater than that required for the blending of 

 gases, because of the vastly greater mobility of the particles of 

 the latter. The diffusion of liquids takes place not only when 

 they are in actual contact, but even when they are separated by 

 partitions of a porous nature, such as plaster of Paris, unglazed ; 

 earthenware, vegetable or animal membranes, and colloidal | 

 substances, all of which may be perfectly water-tight in the 

 ordinary sense of the term, but yet powerless to prevent the I 

 particles of liquids making their way through simultaneously in •, 

 both directions. j 



The rate of diffusion increases with the temperature ; but an ' 

 increase of temperature, we know, is synonymous with increased 

 molecular motion of the body, and with increased activity of 

 this kind we would naturally look for more rapid interchanges 

 of the moving atoms. Such phenomena are only conceivable on 

 the supposition that active molecular motion is going on in an 

 apparently still and inert mass. 



When we come to solid substances the same phenomena 

 appear. 



The volumes of solids do not differ greatly from the volumes 

 of the liquids from which they are congealed, and the solid i 

 volumes are generally •greater. The volume of ice, for example, 

 is one tenth greater than that of the water from which it separates, j 

 Solid cast-iron just floats on liquid iron, and most metals behave j 

 in the same way ; consequently, if the liquids be porous the solids 

 formed from them must be so also ; hence, as might be expected, ! 

 solids also occlude gases in a remarkable manner. Platinum ! 

 will take up live and a half times its own volume of hydrogen, 

 palladium nearly 700 times, copper 60 per cent., gold 29 per ' 

 •cent., silver 21 per cent, of hydrogen and 75 percent, of oxygen, 

 iron from eight to twelve and a half times its volume of a gaseous 

 mixture chiefly composed of carbonic oxide. 



Not only are gases occluded, but they are also transpired 

 under favourable conditions of temperature and pressure, and 

 even liquids can make their way through. Red-hot iron tubes 

 will permit the passage of gases through their substance with 

 great readiness, common coal-gas under high pressure transpires 

 through the steel of the containing vessel, and it is well 

 known that mercury will penetrate tin and other metals with 

 great rapidity, completely altering their structure, their properties, 

 and even their chemical compositions. 



The evidence of the mobility of the atoms or molecules of 

 solid bodies is overwhelming. Substances when reduced to 

 powder, may, even at ordinary temperatures, be restored to the 

 homogeneous solid condition by pressure only. Thus, Prof. 

 W. Spring, some ten years ago, produced from the powdered 

 nitrates of potassium and sodium, under a pressure of thirteen 

 tons to the square inch, homogeneous transparent masses of 

 slightly greater specific gravity than the original crystals, but not 

 otherwise to be distinguished from them. More than that, from 

 a mixture of copper filings and sulphur he produced, under a 

 pressure of thirty-four tons per square inch, perfectly homo- 

 geneous cuprous sulphide, CU2S, the atoms of the two elements 

 having been broutrht, by pressure, into so intimate a relation to 

 each other that they were able to arrange themselves into mole- 

 cules of definite proportion; and, still more remarkable, the 

 carefully dried powders of potash, saltpetre, and acetate of soda 

 were, by pressure, caused to ( xchange their metallic bases and 

 form nitrate of soda and acetate of potash. 



The same movements and changes have taken place, and are 

 still going on, in Nature's laboratory. During the countless 

 ages with which geology deals, and under the enormous 

 pressures of superincumbent masses, stratified sedimentary rocks 

 become crystallized and assume the appearance of rocks of 

 igneous origin, and not only so, but rocks of whatever origin, 

 crushed and ground to pieces by irresistible geological disturb- 

 ances, reconstruct themselves into new forms by virtue of the 

 still more irresistible and constant action of molecular forces and 

 movements. Those who had the privilege of hearing Prof. A. 

 Geikie's brilliant lecture at the Royal Institution last session will 

 remember the striking series of microscopic slides which he 

 •exhibited, and by the aid of which he illustrated the changes of 

 structure to which I have alluded. 



At high temperatures the same effects are more easily pro- 

 duced on account of the greater energy of motion of the atoms 



or molecules. In the process of the manufacture of steel by 

 cementation, or in case-hardening, the mere contact of iron with 

 solid substances rich in carbon is sufficient to permit the latter 

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

 tion of malleable cast-iron the carbon makes its way out of the 

 castings with equal facility ; it is a complete case of diffusion of 

 solid substances through each other, but, on account of the 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 pitch at which incipient fluidity is obtained, 

 the particles of two pieces brought into contact will inter- 

 penetrate 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 meihods 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, as many of us witnessed at the 

 hands of Prof. Ayrton in the highly instructive lecture on electricity 

 delivered last year at our Bath meeting. Again, a mere super- 

 ficial 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 tempi-rature than the metal to be 

 soldered or brazed, is applied, and at a comparatively moderate 

 temperature and under very slight pressure the particles inter- 

 penetrate each other ; the two metals unite and form an alloy, 

 by the intervention 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 ordi- 

 nary 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 ulti- 

 mately 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 recognize under the names of radiant heat, light, and 

 electricity. 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 augment the quantity of move- 

 ment in the particles of substances, and generally to cause an 

 enlargement of volume. Conversely, when the particles, by con- 

 tact or by radiation, part with their hent, either to surrounding 

 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 conse- 

 quently 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 photo- 

 graphy the marvellous power which it possesses. Light decomposes 

 the carbonic acid of the atmosphere in the chlorophyll of green 

 leaves, and determines 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 'ight 

 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 mole- 

 cular movements caused by chemical a:tivity, whether directly, 

 as in the primary 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 chenical 

 changes and decompositions, to say nothing of the whole series 



