HOLMAN. — THERMO-ELECTRIC FORMULAE. 217 



urements of melting points of metals. Each such determination made 

 upon a reproducible metal of known high purity under proper repro- 

 ducible conditions fixes an enduring and reproducible reference point, 

 a pyrometric *' bench mark." And there are enough inexpensive 

 metals, together with a possible system of simple alloys, to give points 

 of sufficient frequency. These would then afford a convenient means 

 of obtaining accurately known high temperatures for purposes of 

 study of all high temperature phenomena, and particulai'ly for cali- 

 brating thermo-eleetric, electrical resistance, optical, or other second- 

 ary pyrometric interpolation apparatus, — for it must be remembered 

 that all such apparatus is necessarily secondary, the gas thermometer 

 being inevitably the primary. 



On the other hand, comparison with the air thermometer of a 

 thermo-couple, or of a resistance pyrometer, or the study of any pro- 

 gressive thermal phenomenon, while it possibly may result in the educ- 

 tion of a natural law, is very unlikely to lead to anything more than 

 the establishment of an approximate equation with constants charac- 

 teristic only of the individual materials actually employed, and not 

 transferable to other, although similar materials. Such results are 

 obviously of a much more ephemeral character than the melting point 

 measurements. Even when any pyrometer thus tested is applied to 

 the establishment of melting points, it must at best yield results in- 

 ferior to direct application of the gas thermometer, except in cases 

 where the latter is hampered by want of sufficient quantity of the 

 metal to be experimented upon, — a condition which need only affect 

 such costly substances as gold and platinum. 



Stated broadly, the great need of the art of pyrometry is convenient 

 methods of producing, or of recognizing vphen produced, a series of 

 accurately known high temperatures. The analogous problem has 

 been partially solved for thermometry at temperatures up to 300° C. 

 by the investigation of boiling points of certain chemically pure sub- 

 stances under controlled pressures. 



Rogers Laboratory op Physics, 

 Massachusetts Institute of Technology, 

 Boston, September, 1895. 



