Practical Thermometric Standard. 527 



air-thei-mometer on the Zn B.P., with the express object of 

 settling the old controversy. He obtained the number 

 929 0, 6 as the result of one observation, and remarks that 

 " several absolutely concordant experiments gave an identical 

 result, namely 930° C." He attributes this remarkable 

 coincidence to his success in screening the vapour from 

 superheating. He notes the close agreement with Becquerel's 

 original value 932° C, but omits to mention Becquerel's 

 subsequent experiments with the porcelain bulb, which gave 

 a much lower result. Since that date, however, the value 

 930° has been generally assumed for the Zn B.P., and it has 

 been regarded as the best known fixed point in the neighbour- 

 hood of 1000° 0. There is very strong evidence, however, 

 that Violle's determination is still more than 10° too high. 



Barus (Amer. Geol. Survey) in 1889 made a number of 

 experiments with a Pt-Pt Ir thermocouple which he calibrated 

 with an air-thermometer of ingenious design. His final 

 corrected results were published in the 'American Journal of 

 Science ' in 1894, and are given in the fifth line of Table II. 

 He assumed the B.P. of zinc to be 930°, for the purpose of 

 determining the uncertain stem-exposure correction of his 

 air-thermometer. His results, therefore, do not bear directly 

 on the value of the Zn B.P., but he shows very clearly that, 

 according to his thermocouple, if 930° is taken for the Zu 

 B.P., the F.P. of silver comes out 986°, which is 32° higher 

 than Violle's 954°. If, on the other hand, the air-thermometer 

 is corrected by assuming 954° for the Ag F.P., then 

 according to Barus the B.P. of zinc must be as low as 905° ; 

 i. e., there is a difference of 50° to 56° between these two points 

 as against Violle's 24°. Incidentally we observe that the 

 stem-exposure correction of an air-thermometer at 930° may 

 be so uncertain as to admit a variation of 25° in the result. 



Holborn and Wien at the Reichsanstalt in 1892 calibrated 

 their thermocouples with a porcelain air-thermometer, and 

 determined a number of fusing-points by the thermoelectric 

 method with the results given in line 6 of Table II. They 

 give 968° for the F.P. of silver (by the method of observing 

 the melting of a wire), as the mean of several determinations. 

 This again is higher than Violle's 954° ; but it may be noticed 

 that the value 961°, given by the platinum resistance- thermo- 

 meter, is approximately a mean between the two. 



H. Le Chatelier (Comptes Rendus 1895), assuming that 

 Violle's value 930° for the Zn JB.P. is the best determined 

 high-temperature point, and observing with a thermocouple 

 the difference between this point and the F.P. of gold, 

 concludes that Violle's 1045° for the Au F.P. is too low, 



