SOLIDS, LIQUIDS, AND GASES. 367 



SOLIDS, LIQUIDS, AND GASES. 



THE growth of accurate knowledge is continually narrow- 

 ing, and often obliterating, the broad lines of distinction 

 that have been drawn between different classes of things. 

 I well remember when our best naturalists regarded their 

 "species" of plants and animals as fundamental and invio- 

 lable institutions, separated by well-defined boundaries that 

 could not be crossed. Darwin has upset all this, and now 

 we cannot even draw a clear, sharp line between the animal 

 and vegetable kingdoms. The chemist is even crossing the 

 boundary between these and the mineral kingdom, by re- 

 futing the once positive dictum that organic substances 

 (i.e., the compounds ordinarily formed in the course of 

 vegetable or animal growth) cannot be produced directly 

 from dead matter by any chemical device. Many of such 

 organic compounds are now made in the laboratory from 

 mineral materials. 



We all know, broadly, what are the differences between 

 solids, liquids, and gases, and, until lately, they have been 

 very positively described as the three distinct states or 

 modes of existence of matter. Mr. Crookes suggests a 

 fourth. I will not discuss this at present, but merely con- 

 sider the three old-established claimants to distinctive ex- 

 istence. 



A solid is usually defined as a body made up of particles 

 which hold together rigidly or immovably, in contradis- 

 tinction to a fluid, of which the particles move freely over 

 each other. "Fluids" is the general term including both 

 gases and liquids, both being alike as regards the mobility 

 of their particles. At present, let us confine our attention 

 to liquids and solids. 



The theoretical or perfect fluid which is imagined by the 

 mathematician as the basis of certain abstract reasonings 

 has no real existence. He assumes (and the assumption is 

 legitimate and desirable, provided its imaginary character 

 is always remembered) that the supposed particles move 

 upon each other with perfect freedom, without any friction 

 or other impediment ; but, as a mutter of fact, all liquids 



