58 PHYSICAL SCIENCE 



needs the use of powerful engines, of elaborate 

 and costly apparatus. The difference is very- 

 marked. Moreover, it becomes more and more 

 difficult to cool a substance through one degree 

 as we pass down the scale. This fact suggests 

 that there is some lower limit of temperature 

 towards which we may strive, but with the 

 prospect of encountering increasing difficulty as 

 we approach ; it suggests, that is to say, the 

 existence of an absolute zero of temperature. 



Our knowledge of an absolute scale of tempera- 

 ture is due to the genius of Lord Kelvin, who, 

 with Clausius, Rankine, and Helmholtz, may 

 be said to have founded the modern science of 

 thermodynamics about the year 1850. It can 

 be shown that Lord Kelvin's absolute scale of 

 temperature coincides with the scale of an ideal 

 gas — a gas, that is, such as air would be if its 

 molecules exerted no forces on each other, and, 

 consequently, its porous-plug-effect were nil. As 

 a matter of fact, at ordinary temperatures and 

 pressures, such gases as air or hydrogen conform 

 very nearly to these conditions — so nearly that, 

 for all ordinary purposes, their deviations may 

 be neglected. Now, if we keep a gas at constant 

 pressure, its volume changes from i to 1.366 as 

 it is heated from the freezing to the boiling-point 

 of water. Similarly, if it be kept at constant 

 volume, its pressure increases in the same ratio. 

 If we use either of these changes as our thermo- 

 metric property, and divide the interval between 

 the freezing and boiling-points into 100° in the 

 Centigrade manner, there will be a change in 



pressure, for example, of 0.00366 or of the 



