PYROPHORIC METALS. 235 



mercury passes through the hole and runs down the funnel into 

 a bottle, &c., to receive it ; more dirty mercury is poured into the 

 funnel from time to time until all is cleaned, taking care not to 

 allow all to run out before adding more ; as otherwise, the mercury 

 running through may carry some dust with it. 



Another method is to hold a square piece of chamois leather by 

 the four corners, so as to make a sort of loose open bag or saucer, 

 into which the mercury is put; the corners are brought to- 

 gether and twisted round, so as to squeeze up the mercury like a 

 pudding boiled in a cloth ; the mercury is thus forced through the 

 minute crevices of the thin leather, and passes through free from 

 dirt, dropping into a large basin placed underneath to receive it. 



Caution. In all cases where mercury is handled so that it is 

 liable to be spilt and run about, take off all gold and silver orna- 

 ments, watch and chain, rings, &c. ; otherwise they may be spoilt 

 by the mercury getting in contact with them (vide, Expts. 69, 172.) 



Expt. 282. Pyrophoric Lead and other Metals. In Expts. 

 135, 120, and 199 we have seen that gold, silver, and platinum 

 can be obtained by means of suitable chemical actions in a state 

 of fine division, in which case they constitute spongy metals ; these 

 metals belong to the " noble " class because they do not readily 

 rust or oxidise in the air * either at the ordinary temperature or 

 on heating. 



In this spongy condition they powerfully attract oxygen and 

 other gases to their surfaces, and the gas thus attracted or 

 condensed exhibits a higher degree of chemical activity than 

 before (Expt. 283, et. seq.) ; still the chemical activity is not en- 

 hanced so much as to enable the oxygen to combine directly with 

 the metal. It is different when other metals not belonging to 

 the noble class are similarly treated, i.e., obtained in a very fine 

 state of division by chemical means, and then brought in contact 

 with air or pure oxygen ; the oxygen attracted by the metal 

 will then often combine directly therewith, evolving heat and 

 light in so doing ; so that if the finely divided metal be prepared 

 in a closed vessel in such fashion that no air has access to it, on 

 opening the vessel and allowing the particles of metal to fall out 



* Although silver will not combine with oxygen and rust spontaneously, 

 yet it is speedily acted upon and blackened by sulphuretted hydrogen ; fre- 

 quently it will be found whilst experimenting with that gas that silver coins 

 in the pocket will become blackened, whilst the same result is speedily brought 

 about by putting on a shilling a little freshly prepared sulphuretted hydrogen 

 water or a drop of sulphide of ammonium (sulphuretted hydrogen dissolved 

 in ammonia water, Expt. 145). On the other hand, although gold is 

 unaffected by either oxygen or sulphuretted hydrogen, it is readily attacked 

 by chlorine (Expt. 229). 



