16 LECTURES TO SCIENCE TEACHER*. 



that it rises and gives place to a colder liquid, and conse- 

 quently, in the case of a liquid in an unequally heated 

 space, there is a continual circulation of currents going on, 

 tending to equalise the temperature of the space inclosing 

 the liquid. The same is true with gases. 



Now, the question of conduction is a very important one, 

 both theoretically and practically. The most simple experi- 

 ments show us that different bodies vary enormously in 

 their conducting powers. For example, a spoon placed in 

 a cup of tea will soon become hot throughout, although 

 there is only a small part in the hot tea. If you were to 

 put a stick of glass instead of a metallic spoon into the tea, 

 the other end of the glass would never increase perceptibly 

 in temperature, thus showing that the conductivity of glass 

 is much less than that of the metal. Experience shows 

 invariably that metals are all better conductors than 

 organic or other substances, and the question now arises 

 how we can determine a method by which the conductivity 

 of substances can actually be measured. We must define 

 conductivity, not only in such a way that we can actually 

 give a number to represent the conductivity of each differ- 

 ent substance, but after we have acquired this knowledge 

 of what is the exact conductivity of different substances, 

 we can then apply our knowledge to different purposes, 

 theoretically and practically. For example, there is an 

 enormous loss of heat from the boiler of a steam-engine, a 

 loss of heat which is produced by radiation into space. 

 But if, as is very common now, the boiler is surrounded by 

 a cement which is a bad conductor of heat, then there will 

 not be such a great loss of heat, so great a quantity of heat 

 will not be allowed to pass through this non-conductor, 

 and consequently more is retained for the economical 

 purposes of working the steam-engine. Yarious other 

 applications will occur to you, but in the meantime we 

 must go on to the definition which has been given of 

 conductivity. 



The first man who took up the subject of conductivity in 

 a scientific and mathematical manner was the French 

 physicist, Fourier. He wrote in 1812 a most valuable 

 work entitled The Analytical Theory of Heat, but, owing 

 apparently to the jealousy of other members of the Insti- 

 tute of France, this work was not published until twelve 



