120 A. L. Day and R. B. Sosman — 



It is evident that even without any correction for the dif- 

 ferent distribution in the two cases, the readings agreed w T ithin 

 0'2-0*8 min , or about O^-O-O , so that the variation between any 

 two arbitrary sets of weights which might be given to the dif- 

 ferent readings must lie well within this limit. 



The Transfer to the Fixed Points. — After the thermoele- 

 ments are removed from the bulb, their E.M.F. at the fixed 

 points must be determined by immersing them in melting or 

 freezing metals or salts. The instrumental corrections to the 

 readings so obtained were the same as in the case of the gas 

 thermometer readings. The error due to contamination was 

 also present above 1100°, just as in the gas thermometer fur- 

 nace, and was a very disturbing factor in determining the melt- 

 ing points of nickel, cobalt and palladium. Itjs source, how- 

 ever, was not usually iridium vapor from the furnace or 

 rhodium from the wire of the element, but was either vapor 

 of the melting metal itself, or (when a hydrogen atmosphere 

 was used) the products of reduction of silica. In the presence 

 of hydrogen, silica rapidly deteriorates platinum wire by 

 reduction and alloying, as has been shown in this laboratory 

 by Shepherd,* and elsewhere by several observers. The con- 

 tamination can be partly prevented by the use of a glazed 

 porcelain tube surrounding the thermoelement, instead of an 

 unglazed magnesia tube ; but an additional uncertainty is 

 thereby introduced through the contamination of the melting 

 metal by the melted glaze on the porcelain. For this reason 

 nickel and cobalt did not prove to be as satisfactory fixed 

 points as had been hoped, since it was necessary to melt them 

 in an atmosphere of hydrogen. Palladium, however, can be 

 melted in the open air and serious contamination by silicon 

 thus be avoided, although the palladium itself gradually con- 

 taminates the wire. 



Above 1100° it is better to make direct comparisons of all 

 the elements with one or two whose fixed points have been 

 determined, rather than to contaminate them all by a direct 

 determination. For making these comparisons, the plan first 

 used was to bring a crucible of molten silver to a constant tem- 

 perature and insert the elements (protected by a glazed Mar- 

 quardt porcelain tube) successively into the silver bath. There 

 is an uncertainty, however, in these measurements of 2 to 3 mv., 

 caused by small differences of temperature within the tube 

 and to the slight cooling produced by introducing cold wires 

 into the furnace. A better method is to join together the two 

 platinum wires and the two alloy wires of the elements to be 

 compared, and determine the small E.M.F. 's of each pair at 

 several temperatures, from which the difference between the 

 * This Journal (4), xxviii, 300, 1909. 



