TEMPERATURE. 9 



to the use of the mercury thermometer in both directions. The freezing 

 point of mercury being about -39 C. it cannot be used for lower tempera- 

 tures, and hence, for meteorological purposes, it is usual in cold climates 

 to replace it by alcohol, since alcohol has a much lower freezing point. 

 Nor is it safe to use an ordinary mercury thermometer much above 

 350 0., the pressure of mercury rising from about | atmosphere at 

 300 to 1 atmosphere at 356, the normal boiling-point of mercury, and 

 then increasing still more rapidly to 2 atmospheres at 400 and 4 

 atmospheres at 450. This great increase of internal pressure may 

 very seriously alter the capacity of the bulb. Mercury thermometers 

 made of specially hard glass and containing nitrogen above the mercury 

 are now, however, made with a range up to 500 0., but they are hardly 

 suitable for exact work. 



Scales of Temperature given by Expansion depend on the Sub- 

 stances used. Beginners in the study of heat sometimes suppose that 

 mercury and glass are chosen in the construction of thermometers, because 

 their expansion is regular and equal for each successive degree. But this 

 regularity is simply due to our definition, that equal degrees shall be 

 such equal expansions. Within the short range from 0. to 100 0. 

 most substances which remain otherwise in the same physical condition 

 between these points expand nearly regularly with rise of temperature 

 as indicated by the mercury-glass thermometer. And so most substances, 

 with the same fixed points would give nearly the same scale. Thus, if the 

 expansion of a brass rod between and 100 were used and divided into 

 100 equal steps, each step would have very nearly the same value as the 

 corresponding steps on the mercury-glass scale. But not exactly, for even 

 between and 100 there are measurable deviations from expansion in 

 the same rates, and outside that range the deviations become more con- 

 siderable. As we have seen, even different kinds of glass expand 

 differently, so that two thermometers of different glasses agreeing at 

 and 100 will not agree exactly at all intermediate points. The 

 disagreement fortunately is very small within that range. 



The Work Scale of Temperature. There is one scale of tem- 

 perature, due to Lord Kelvin, which is quite independent of the particular 

 substance used to indicate it. We shall give a full account of this scale 

 in chapter xvii. Here we can only attempt a brief sketch of its nature 

 in order that the reader may know that such a scale exists. 



The work scale depends on the amount of work obtained from a given 

 supply of heat to a heat engine. 



We may roughly describe an ordinary steam engine as a heat engine 

 which takes in heat at the temperature of the boiler, and turns some of 

 this heat into work by the expansion of the steam in the cylinder. 

 Though the steam cools as it expands it does not turn all the heat 

 received into work but retains some of it till it passes into the cooler 

 or condenser where it returns to the liquid form. It can therefore at 

 the very most only convert into work the difference between the heat 

 taken into the boiler and the heat put out in the condenser. 



We can imagine an ideal engine, in which any substance is used, like 

 steam in the ordinary engine, to do work by expansion. The substance 

 works between a source of heat, like the boiler in an ordinary engine, 

 and a cooler receiver like the condenser. The working substance takes 



