RICHARDS. — RETENTION AND RELEASE OP GASES. 413 



The Effect of Physical Condition upon the Total Amount 

 OF Gas retained by Zincic Oxide, 



(53) 1 grain of ZnO, prepared from zincic carbonate at 880°, gave 0.00 c.o. gas. 



(54) " " " " O.ob " 



Average 0.00 c.c. gas. 



(55) 1 gram of ZnO, prepared from the nitrate at 280°, not 



ignited, gave 0.01 c.c. gas. 



(56) A similar experiment gave 0.02 " 



Average 0.015 c.c. gas. 



(57) 1 gram of ZnO, prepared from zincic nitrate, but heated to 



880° in a slightly reducing atmosphere, gave .... 0.05 c.c. gas. 



(58) A similar experiment gave 0.15 " 



Average 0.10 c.c. gas. 



(-12) 1 gram of ZnO, prepared by gentle heating of the nitrate 

 at 280°, and subsequent ignition for 30 minutes at 880° 

 in an oxidizing atmosphere 0.192 c.c. gas. 



(43) A similar experiment gave 0.192 " 



Average 0.192 c.c. gas. 



(59) 1 gram of similar ZnO, prepared in the same way, but 



heated to 670° for 8 hours and then preserved two 



weeks before heating ^ hour at 880°, gave .... 0.292 c.c. gas. 



(60) A similar experiment gave 0.260 " 



Average 0.276 c.o. gas. 



(61) 1 gram of more compact ZnO, prepared by heating the 



nitrate rapidly, and having been preserved for two 



weeks before being ignited for ^ hour at 880°, gave 0.387 c.c. gas. 



(02) A similar experiment gave 0.396 " 



Average 0.392 c.c. gas. 



The evidence of these results proves that the total amount of gas 

 retained by zincic oxide varies with every detail of the method of prepa- 

 ration ; hence this amount cannot serve as a sure guide to the tempera- 

 ture of ignition, unless a perfectly uniform sample is used for all trials.* 

 Of course the amounts of nitrogen found in these samples varied widely ; 

 for the gas resulted from the decomposition of zincic nitrate, which was 

 imprisoned in varying quantities according to the physical condition 

 of the zincic oxide. 



* Small quantities of such impurities as alkali and silica do not seem to affect 

 the occlusion, however. See These Proceedings, XXVIII. 207. 



