certain High-temperature Boiling-points. 147 



of the instrument against wear and tear, constitute the advan- 

 tages of the galvanometer constructed. 



M. Le Chatelier* recommends the Deprez-D'Arsonval gal- 

 vanometer. 



From what has been stated, if D be the twist corresponding 

 in a given instrument to the temperatures T and t of the 

 junctions of the given thermocouple, D = a (T — t) -\-b (T 2 — fi). 

 Again, \id = a (t-20) + b (* 2 -20 2 ), it follows that 



D 20 =D + ^ = a(T-20) + Z>(T 2 -20 2 ). 



Hence a small table for d is to be computed, for tempera- 

 tures t between 10° and 35°. A short preliminary calibration 

 suffices for the purpose. In this way the observer may at 

 once deduce D 20 from D, where D 20 is a function of T, the 

 temperature of the hot junction only. By aid of known 

 melting-points (Le Chatelier), or boiling-points (water, mer- 

 cury, sulphur, cadmium, zinc, bismuth) , the quantity D 20 may 

 then be graphically constructed as a function of T. Inter- 

 mediate points between two consecutive boiling-points, T\ 

 and T 2 , are filled in by using the constants of the equation 



D I -D,=a(T 1 -TJ+4(T 1 *-T, J ). 



From a chart of this kind, the temperature T corresponding 

 to any twist D is taken with facility. 



Observations. 



6. In the above apparatus the normal boiling-point (76 

 centim.) of mercury is sharply determinable ; but there is 

 considerable difficulty in determining the low-pressure boiling- 

 points. Unless the heat be regulated to a nicety, the- lower 

 layers boil under the pressure of the upper layers. Again, 

 since the agitation of the mercury nearly ceases in approxi- 

 mate vacua, a flame of reasonable intensity presupposed, the 

 rate of evaporation is much decreased. In the case of a 

 liquid of small specific heat, of relatively large cohesion, 

 films of which do not adhere uniformly to glass, it is there- 

 fore to be suspected that both liquid and vapour will be super- 

 heated in relatively very large degree at low pressures. If the 

 flame be intensified so as to produce violent ebullition at low 

 pressure, the liquid cau be superheated to such an extent that 

 its direct radiation on the junction of the thermocouple may 

 increase its temperature as much as 10° above the boiling- 

 point. 



* Le Chatelier, Seiziemc Congres Soc, Industr. tlu yaz, 188S. 



