HOLMAN. — PYROMETRY. 241 



(it should not be unduly superheated), the blast is slightly reduced so 

 that the crucible may cool slowly, and the j^yi'ometer is read con- 

 tinuously. When the solidifying point is reached, the reading will 

 become constant for some minutes (dependent on the rate of cooling) 

 and will then begin to descend. This "steady point" occurs, of 

 course, during the absorption of the latent heat, and is the desired 

 reading corresponding to the solidifying point. The blast is then 

 increased, and the corresponding constant reading in the ascending 

 temperatures gives the melting point. This should, with copper, be 

 sensibly coincident with the solidifying point, and will be found so 

 unless complications are introduced, through too rapid heating or 

 cooling, or through the formation of slag upon the top of the copper. 

 It is well to withdiaw the junction, clean it if necessary, and take 

 check readings. 



The advantages of copper over gold, as demonstrated elsewhere,* 

 are that it is more easily and cheaply obtainable in a state of purity 

 insuring a reliable melting point, and that, by using a large mass, an 

 unmistakable steady reading of considerable duration can be obtained. 

 The latter considei'ation is of special weight where the galvanometer 

 is exposed to jarring, or the observer is not experienced. The 

 operation is no more laborious, and, except possibly for a skilled 

 observer, is less time-consuming, and more satisfactory than the cus- 

 tomary method with gold. 



In Le Chatelier's original method of employing gold (or other 

 metals) this same furnace was used, but with its cover on. The 

 crucible, without a cover, was filled with magnesia or lime, and the 

 thermal wires, passing through a perforation in the furnace cover, 

 terminated with their junction in the centre of the crucible, surrounded 

 by the magnesia. Before its insertion the junction was carefully 

 wrapped witli a small piece of wire or foil of gold, or of any metal 

 under test. This being arranged, the blast was controlled to raise 

 the temperature at a slow and steady rate. The spot of light on the 

 pyrometer scale was then watched closely. It would be seen to ad- 

 vance steadily until a certain point was reached, then to stop abruptly 

 for a moment, then to start upward again almo'st with a bound and 

 go on rising. This temporary stopping was due to the time required 

 for the absorption of the latent heat of fusion. A similar stopping 

 point may then be generally observed on allowing the furnace to cool 

 slowly. These two points, the "melting and freezing points," should 



* See former reference to H., L., and B. 

 VOL. XXXI. (n. s. xxhi.) 16 



