THE MOLECULAR STRUCTURE OF MATTER. 713 



the particles of liquids inakiuj? their way through siinnltaneously in 

 both directions. 



When we come to solid substances the same ])lienomena appear. 

 The volumes of solids do not differ greatly from the volumes of the 

 liquids from whiiih tliey are congealed, and the solid volumes are gen- 

 erally greater. The volume of ice (for example) is one tenth greater 

 than the water from which it separates. Solid cast-iron just floats on 

 liquid iron, and most metals behave in the same way; consequently, 

 if the li<iuids 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 and a half times its own volume 

 of hydrogen, palladium nearly 700 times; copper,, CO per cent.; gold, 29 

 per cent,; silver, 21 per cont. of hydrogen, and 75 ]ier cent, of oxygen ; 

 iron from 8 to 12.4 times its volume of a gaseous mixture chiefly com- 

 po.sed of carbonic oxide. Not only are gases occluded, but they are 

 also transpired under favorable conditions of temi)eratnre and pressure, 

 and even liquids can nuike their way through. Red-hot iron tubes will 

 permit the passage of gases through their substance with great readi- 

 ness. Ordinary coal gas — when under high pressure — is retained with 

 difficulty in steel vessels, and it is well known that mercury will pene- 

 trate tin and otiier metals with great rapidity, comifletely altering their 

 structure, their properties, and even their chemical compositious. 



The evidence of the mobility of the atoms or moleculesof 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 Professor W. Spring some ten years ago pro- 

 duced 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 press- 

 ure of thirty-four tons per square inch— perfectly homogeneous cuprous 

 suli)hide (Cua 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 potasii, saltpeter, 

 ami acetate of soda, were by pressure caused to exchange their metallic 

 bases, and form nitrate of soda and acetate of potash. 



At high temperatures the effects are more easily produced on account 

 of the greater energy of motion of the atoms or moleiuiles. 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 sufii- 

 cient to permit the latter to work its way into the heart of the former, 

 while in the formation of malleable cast-iron the carbon makes its way 

 out of the castings with equal facility ; it is a complete (;ase of dittusion 

 of solid substances through each other, but on account of the inferior 



