Determination of Temperatures with the Meldometer. 361 



the substance -required enables it to be purified very com- 

 pletely. The current is then put on and increased rapidly 

 until the substance melts, and thus an approximate reading is 

 obtained. This is repeated more cautiously, to obtain an exact 

 reading. Several readings can be taken by remelting the 

 same, and also by using fresh substance, the latter method 

 being usually adopted. 



To translate the readings into temperatures, it is necessary 

 to standardize the instrument by taking the readings with 

 substances of known melting-points. Of these, unfortunately, 

 there are none known with certainty beyond about 350° C. 

 One reading is obtained by taking the temperature of the air, 

 another is the melting-point of potassium nitrate (339°) , and 

 for a third one the melting-point of potassium sulphate (1052°) 

 was adopted, for reasons that will appear. The melting-point 

 of silver is irregular, apparently because of absorption of 

 oxygen, and consequent spitting. Gold can be used, and 

 also palladium. As, however, the expansion of the ribbon is 

 almost a linear function of its temperature, and as the obser- 

 vations hitherto taken did not extend beyond about 1050°, it 

 was considered unnecessary to take readings with palladium, 

 the general character of the expansion of the ribbon being 

 already known from Dr. Joly's observations. The question 

 next arises, what is the melting-point of gold ? There have 

 been two or three determinations of value, which unfortu- 

 nately differ from one another. 



M. Yiolle (C. R. 1879) determined it by a calorimetric 

 method, and obtains as a result 1045° (on the air-thermo- 

 meter). 



Messrs. Holborn and Wien (Wied. Ann. xlvii. and lvi., 

 1892 and 1895), who give 1072°, compared a thermo-element 

 with an air-thermometer, and then used the former for deter- 

 mining the melting-points of silver, gold, and copper. This 

 was done by inserting in a porcelain crucible the thermo- 

 element, and also two platinum wires connected by a wire of 

 the substance whose melting-point was to be taken. The 

 platinum wires formed part of a circuit containing a battery 

 and a galvanometer. When the wire melted, the circuit was 

 broken, and the temperature read at the same moment with 

 the thermo-element. The melting of the substance must in 

 general lag a little behind the thermo-element ; and as no 

 mention is made of the rate at which the temperature was 

 raised, it is difficult to know how far the results can be trusted. 

 They obtain, however, very concordant readings. 



Besides these, there are two determinations with the plati- 

 num pyrometer : one by Professor Callendar (Phil. Mag. Feb. 



