Heat of Iron at High Temperatures, 



135 



rest of the tube was filled with dried " light magnesia.'''' 

 This substance possesses the curious property of offering 

 practically no resistance to the passage through it of any 

 reasonably heavy body, even when the magnesia is in the 



form of a layer of considerable 

 thickness. The hole formed in it by 

 the falling of the specimen fills itself 

 up almost instantaneously, leaving 

 a layer of about 8 to 10 cm. in thick- 

 ness to prevent radiation of heat from 

 the hot specimen upwards. 



A considerable number of experi- 

 ments were tried using this arrange- 

 ment and having the iron specimen 

 protected by a quartz sheath. Ex- 

 periments at low temperatures gave 

 values agreeing within the limits of 

 error with those got by the earlier 

 method. It was found, however, 

 that above about 900° C. there was 

 a fairly rapid action of the quartz 

 on the surface-film of the iron even 

 when great precautions were taken 

 in having no oxide present on the sur- 

 face of the sample on its introduction. 

 Besides, an additional objection to 

 the use of quartz in the experiment 

 in this* form was that the time re- 

 quired for complete equilibrium to 

 be established in the calorimeter after 

 the specimen had fallen was so great, 

 owing to the low heat conductivity 

 of the quartz in the thickness com- 

 patible with strength, that appre- 

 ciable uncertainty was introduced 

 the true amount of heat 



Section of magnesia tube. 



given out m 



into the calculation of 

 an experiment. 



It w r as found, however, that by sealing up the iron in a 

 thin tube of glazed porcelain, the oxidation difficulty could be 

 avoided, and also that this substance did not possess the 

 disadvantages named in the case of quartz. After prolonged 

 use of one specimen at temperatures up to nearly 1200° C, 

 the iron on examination was found to be only slightly 

 tarnished, probably from some action of the vapour of the 

 glaze which at these temperatures is quite soft. 



2 H2 



