58 Messrs. H. L. Callendar and E. H. Griffiths. [Dec. 18, 



ion the temperature was kept steady to 0'l for about two hours. 

 Allowing for the difference of the atmospheric pressure, the tempera- 

 ture attained was the same on both days. 



The results of the comparison were in perfect agreement with the 

 experiments of 1887, and showed that the e-coofficient of the wire 

 had not altered appreciably in the interval. 



The apparatus which we have found most convenient for standard- 

 ising platinum thermometers by means of the boiling point of sulphur 

 consists of a wide glass tube, 40 cm. long and 4 cm. in diameter, 

 with a spherical bulb at the erd. Tubes of this kind are com- 

 monly used to heat Victor Meyer's vapour-density apparatus. For 

 brevity we have called it a " Meyer " tube. 



The outside of the tube is thickly padded with asbestos wool, with 

 the exception of the lower half of the bulb, and of a short length of 

 3 5 cm. at the top, which serves as a condenser. The tube is tilled 

 with sulphur to a level of 3 or 4 cm. above the bulb, and is heated by 

 a Bunsen burner. The gas is adjusted so as to keep the level of the 

 vapour near the top of the tube, which is covered with asbestos card 

 to prevent the sulphur catching fire. 



Our experiments have shown that a thermometer inserted in an 

 apparatus of this kind will not attain the actual temperature of the 

 vapour, unless it is protected from radiation to the sides of the tube, 

 and from the condensed liquid which runs down the stem. The 

 lowering of temperature due to radiation, &c., may readily amount 

 to upwards of 2 at the boiling point of sulphur. 



The method which we have adopted for screening the thermometer 

 is to bind an umbrella of asbestos card on to its stem a short distance 

 above the bulb. Two coaxial tubes are hung on to this umbrella to 

 screen the thermometer from radiation. We have found that glass is 

 not sufficiently opaque to heat radiation at this temperature. The 

 inner tube at least should be of metal. 



To avoid superheating of the vapour, it is necessary to make sure 

 that the level of the liquid sulphur stands well above that part of the 

 bulb which is exposed to the flame. 



Using these precautions, we have found that the temperature by 

 normal air thermometer at constant pressure of the saturated vapour 

 sulphur boiling freely under a pressure, of 760 mm. of mercury at 

 C., and g = 980-61 C.G.S. (sea level in lat. 45), is 



t = 444-53 C. 



The value given by Regnanlt* is nearly 4 higher than this ; but 

 in the account which he gives of his experiments he has pointed out 

 several sources of error, and it is evident that he did not place much 

 confidence in his results. 



* ' Memoires de I'lnstitut,' TO!. 26, p. 526. 



