486 Prof. Silas W. Holman on 



of its great convenience it may find application in industrial 

 pyrometry, as will be elsewhere indicated. Although failing 

 below 300° or 400°, it may probably be applied to the irido- 

 or rhodo-platinum couple between 400° and 1200° C. with a 

 maximum error not exceeding about 5°. If extended to cover 

 400° to 2000° the error might rise to 15° or 20°. 



More in detail it may be briefly noted by way of 

 summary : — 



That the logarithmic equation fits the Barus data between 

 400° and 1250° with scarcely sensible systematic error, and 

 within the limits of variable errors of the data. 



That the exponential equation similarly fits the Holborn 

 and Wien data within the limits 400° to 1445°. 



That when made to coincide with the data at about 450° 

 and 1200° the systematic deviations of the exponential equa- 

 tion from the Barus data, and of the logarithmic equation 

 from the Holborn and Wien data, are in general of opposite 

 sign and of roughly equal magnitude. 



Barus Melting and Boiling-Point Data. 



From the foregoing demonstration of its applicability, it 

 seems proper to apply the logarithmic formula to the Barus 

 thermo-electric data on melting-points*. 



Whether the extrapolation above 1000° by the logarithmic 

 formula is entitled to any great weight may be questioned, 

 but there is no obvious reason why it is not more reliable than 

 by any of the others. I have employed the equation given 

 on page 474, which represents very closely Barus's high 

 temperature air-thermometer comparisons, calculating thence 

 the temperatures t corresponding to the values of %^e given 

 by Barus for the various points, assuming Barus's value 

 2 2 °£ = 150 mv. The results are given in column 3 of 

 Table X. Column 4 quotes the most reliable previous 

 determinations of the same points by other observers. As to 

 which of the two columns of results best represents Barus's 

 work, there can be little doubt from the above evidence that 

 below 1000° it is the second, that is, the one computed from 

 the logarithmic equation. These combine both his own air- 

 thermometer and melting-point work. Above 1000° the 

 logarithmic values are probably slightly too high. 



* Amer. Journ. Sci. xlviii. p. 332 (1894). 



