xvi WATER AND THE ELEMENTS COMPOSING IT 235 



after the sodium has all disappeared, and test it with litmus. 

 Evaporate away the water. Note the residue. 



In this case we could only see that the action was energetic 

 and a new product was formed, while the soapy feel of the water, 

 and the action on litmus, appeared to indicate that this product 

 was that previously obtained when sodium burned in oxygen and 

 the fumes were dissolved in water. We could not, however, see 

 whether any gas came off, as the experiment was done in the 

 open air and the sodium floated on the surface of the water. We 

 must therefore try to alter our experiment so as to collect any 

 gas which may be evolved. 



EXPT. 236. Place the sodium in a small piece of lead tubing, 

 the ends of which are nearly closed, and drop into water (or 

 place down over the sodium a thimble with holes in the top). 

 A gas is seen to come off. 

 Collect this in an inverted tube 

 full of water, and by this means 

 obtain three test-tubes of the 

 gas, and observe that it is 

 colourless and odourless. 



EXPT. 237. Take out two 

 tubes and hold them for the 

 same time, say, 30 seconds, 

 (1) with mouth up, (2) with 

 mouth down. Then apply a 

 light to the mouth of both. 



The slight explosion of (2) 

 shows it to contain an explosive 

 gas, while the absence of any 

 effect with (1) shows that the gas 

 has disappeared. Hence it is seen 

 that the gas escapes from a tube held mouth upwards, but not 

 from one held mouth downwards. It is therefore lighter than 

 air, being in fact the lightest gas known. 



EXPT. 238. Now try the 3rd tube, holding it mouth down, 

 and place a lighted match up into the tube. Note that the 

 gas does not explode, but burns quietly, while the match is 

 extinguished (Fig. 121). 



FIG. 119. Action of Sodium on 

 Water. 



