142 PHENOMENA, ATOMS, AND MOLECULES 



stances vaporize very rapidly. The flame from a 6o-ampere arc was directed 

 against a large piece of calcium oxide. After considerable shrinkage and 

 cracking a surface of several square centimeters can be maintained in a 

 molten condition (m. p. of CaO, 2850° K.)/'^ but evaporation occurs with 



TABLE III 



Time Required to Melt Ends of 6.3-MM. Iron Rods in Flame 

 FROM 6o-Ampere Arc 



Distance from Electrodes Time 



Mm. Inches Seconds 



6.3 J4 4 



9-5 H 7 



13 ^ 8 



19 54 14 



25 I 18 



38 I'A 27 



51 2 36 



64 2^ 80 



76 3 240 



ACTION OF ATOMIC HYDROGEN ON VARIOUS REFRACTORY 



SUBSTANCES 



remarkable rapidity. The flame becomes very brilliant and of a red color, 

 and a spectroscope shows many hundreds of lines extending over nearly 

 the whole spectrum. An optical pyrometer with a red screen (y = 0.66 ^i) 

 sighted on the fused lime when at its highest temperature showed that the 

 luminous intensity corresponded to that of a black body at 3500° K. A 

 large fraction of this light was undoubtedly emitted by the gas. 



Pure magnesium oxide (m. p. 3070° K.) melts with much greater 

 difificulty. When it reaches high temperatures the arc shows a strong 

 tendency to short-circuit on to the heated mass and rapid evaporation 

 occurs where the arc leaves and enters the oxide. The end of a rod of 

 oxide 4X7 mm. in cross section was heated just to fusion for 1.5 minutes 

 and its weight decreased from 1.74 to i.oi grams. The observed black -body 

 temperature just at fusion was 3070° K., but the apparent temperature 

 could be raised to 3200° K. Relatively little light was emitted by the 

 flame gases. 



Pure thorium oxide, ThOo (m. p. > 3070° K.), when just fused gave 

 a black-body temperature of 3360° K., but the apparent temperature could 

 be raised to 3700° K. at the points where the arc short-circuited on to the 

 hot oxide. Evaporation was very rapid under these conditions. 



With none of these oxides, or with alumina, was there any direct in- 



" These melting points are taken from the International Critical Tables. 



