18 



THE THERMOMETER. 



solid. For degrees of cold below the freezing point of mercury, 

 we must be guided by the contractions of alcohol, or spirits of 

 wine, a liquid which has not been frozen by any degree of cold 

 we are capable of producing. There is no reason, however, for 

 believing that we have ever descended more than 130 or 140 

 degrees below our zero. 



The zero of these scales has, therefoYe, no relation to the 

 real zero of heat, or point at which bodies have lost all heat. 

 Of this point we know nothing, and there is no reason to 

 suppose that we have ever approached it. The scale of tem- 

 perature may be compared to a chain, extended both upwards 

 and downwards beyond our sight. We fix upon a particular 

 link, and count upwards and downwards from that link, and 

 not from the beginning of the chain. 



The means of producing heat are much more at our com- 

 mand, but we have no measure of it, of easy application and. 

 admitted accuracy, above the boiling point of mercury. Re- 

 course has been had to the expansion of solids at high tem- 

 peratures, and various pyrometers, or " measures of fire/' have 

 been proposed. Professor Daniel's pyrometer is a valuable instru- 



ment of this kind, but it has not yet come into general use. 

 Its indications result from the difference in the expansion by 

 heat of an iron or platinum bar, and a tube of well-baked black- 

 lead ware, in which the bar is contained. The metallic bar a 

 is shorter than the tube, and a short plug of earthenware b is 



