720 The Outline of Science 



seems more homogeneous than water, but pure water is never 

 found in nature. There are always impurities in it, and one needs 

 only to bring a glass of cold water into a warmish room to see 

 how multitudinous bubbles of gas form on the inner walls of the 

 vessel. These suggest, at any rate, that there is a good deal of 

 gas mixed up with the water. If this were not the case no animals 

 could breathe under water, for while water is a compound (H^O) 

 of hydrogen and oxygen, it is not possible for animals to separate 

 the two components in the way plants are able to do with the car- 

 bon dioxide (CCh) mixed in the air or the water. Perfectly pure 

 substances, though often advertised, are very difficult to obtain; 

 they are indeed almost ideal, which led a great investigator to say 

 that "chemistry is the science of substances which do not exist." 

 Traces of impurity in a substance are often of great practical im- 

 portance ; they sometimes influence the properties of the substance 

 in a remarkable way. We quote a sentence from Dr. Mellor's 

 admirable textbook Modern Inorganic Chemistry (1920): 



H. Vivian says that T irW part of antimony will convert 

 the best selected copper into the worst conceivable; Lord 

 Kelvin says that the presence of TTr Yii P ar t of bismuth 

 in copper would reduce its electrical conductivity so as to be 

 fatal to the success of the submarine cable; and W. R. 

 Roberts Austin says that 3^ part of bismuth in gold would 

 render gold useless, from the point of view of coinage, be- 

 cause the metal would crumble under pressure in the die. 



Molecules and Atoms 



A mixture can usually be separated into its ingredients by 

 more or less simple mechanical means. A compound cannot be 

 broken up into its components without going beyond simple me- 

 chanical methods. We break a piece of salt into finer and finer 

 powder, but each particle of salt-dust remains salt. If we could 

 get hold of a particle which would cease to be salt when we broke 

 it in pieces, that particle would be a molecule of salt, and we 

 should have divided it into groups of constituent atoms sodium 



