720 BEPOET — 1889. 



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 particles of liquids 

 making their way through simultaneously in both directions. 



The rate of diffusion increases with the temperature ; but an increase of tempera- 

 ture, 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 inter- 

 changes of the moving atoms. Such phenomena are only conceivable on the suppo- 

 sition 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 volumes are generally gi-eater. The 

 volume of ice, for example, is one-tenth greater than that of the water from 

 which it separates. Solid cast-iron just floats on liquid iron, and most metals be- 

 have 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 five aud 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 per cent, 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 con- 

 ditions 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. Ordinary coal gas, when under high pressure, is retained with 

 difficulty in steel vessels, and it is well known that mercury will penetrate tin 

 and other metals with great rapidity, completely altering their structure, their 

 properties, and even tlieir chemical compositions. 



The evidence of the mobility of the atoms or molecules of solid bodies is over- 

 whelming. Substances when reduced to powder, may, even at ordinary temperatures, 

 be restored to the homogeneous solid condition by pressure only. Thus, Professor 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 other- 

 wise 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 homogeneous cuprous sulphide, Cu,,S, the atoms of the two elements 

 having been brought, by pressure, into so intimate a relation to each other that they 

 were able to arrange themselves into molecules of definite proportion ; and, still more 

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

 were, by pressure, caused to exchange 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 be- 

 come crystallised 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 disturbances, 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 Professor 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 produced 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 



