HOW TO SET THE THAMES ON FIRE. 213 



floor and pouring a little hot water over it. Even in this case, one 

 may easily be burnt by the flying about of burning bits of sodium. 



Expt. 247. To set Ice on Fire. The metal potassium is even 

 more energetic in its action on water than sodium, so that if 

 thrown on to cold water, the hydrogen evolved is almost always 

 lighted. The same result takes place with ice ; put a small pellet 

 of potassium on the top of a block of ice, and press the two 

 gently together; action will commence, and the heat produced 

 will generally be sufficient to inflame the hydrogen. 



This experiment illustrates a somewhat rare kind of action, viz., 

 where two solids directly act on one another. As a general rule 

 in order to bring about chemical change between substances in 

 the solid state, one or other of the substances present must either 

 be melted by heat, or reduced to a quasi-liquid condition by solu- 

 tion in some solvent (vide also Expt. 263). 



On account of the great tendency of sodium and potassium not 

 only to oxidize in the air, but also to decompose water and water- 

 vapour, these metals require to be preserved either in hermetically 

 sealed vessels, or in bottles filled up with paraffin oil or some 

 similar fluid incapable of acting on the metals. Sometimes the 

 metals are stored by casting them in brick-like ingots, which are 

 then dipped in melted paraffin wax which forms a protecting 

 coating over their surface. 



Expt. 248. To set a Pond on Fire. Into a dry gallipot pour 

 two or three teaspoonfuls of ether, and also put therein a few 

 fragments of potassium ; if the ether is pure the potassium will 

 not be acted upon. Now drop the whole into a pond, or into a 

 bucket of water ; the ether and the potassium will float up, and 

 the latter will act vigorously on the water evolving hydrogen and 

 setting fire thereto, and to the ether as well. 



Expt. 249. To decompose Steam by Red Hot Iron. Obtain a 

 piece of hard glass tubing (" combustion tubing," made of a parti- 

 cular kind of glass capable of standing a pretty high temperature 

 without melting) about a foot long, and fill it with fragments of 

 iron turnings ; at each end fix tightly a cork or india-rubber bung 

 with a piece of glass tube passing through. Connect one end with 

 a flask of boiling water, after heating the central part of the com- 

 bustion tube as hot as possible by means of one (or preferably 

 more) Bunsen burners, or better still, a series of burners playing 

 into a furnace made of tiles (fig. 93). If the current of steam be 

 not so rapid as to chill the hot iron borings too much, the iron 

 will decompose part of the steam, evolving hydrogen gas and pro- 

 ducing a film of oxide of iron on the surface of the borings, the 

 action being closely akin to that of sodium on water, excepting 



