100 ELEMENTS AND COMPOUNDS 



Before we bring a flame near a jet of hydrogen, we hold a test tube 

 over the outlet tube (Fig. 79, B) for a minute, and then carry the test 

 tube, with its mouth downward, to a flame at least 3 feet away. The 

 gas in the test tube burns rapidly, if it still contains air; but if it con- 

 sists of fairly pure hydrogen, it burns slowly. When we have lighted 

 the gas in the test tube, we carry the tube, with its mouth downward, 

 back to the jet of hydrogen. We do this until the test tube of burn- 

 ing hydrogen sets the jet of hydrogen on fire. 



Hydrogen burns with a colorless flame when pure. 

 The flame is very hot. When 1 gram of hydrogen is burned 

 in oxygen, the heat given off is sufficient to heat 342 grams 

 of water from C. to 100 C. 



Water is formed not only when hydrogen itself burns, 

 but also when compounds of hydrogen burn (cf. 52). 

 Thus it happens that when a kettle of cold water is placed 

 over a gas flame, water drops are deposited on the outside 

 of the kettle. When the kettle becomes hot 

 enough, the steam formed in the burning 

 escapes without being condensed. 



106. Diffusion of Gases and of Liquids. 



If we attach a porous cup (Fig. 82) tightly 

 to a tube ending under water, and place a 

 jar of hydrogen over the cup, an interesting 

 Hydrogen 2 pas- phenomenon takes place : bubbles of gas escape 

 porl n uscup from the bottom of the tube. The bottom of the 



a ?r pi Sn jar containing hydrogen is open; so the only 

 pass outward reagon ^y gag escapes through the water 



must be that hydrogen enters the porous cup. It can be 

 proved that air passes outward through the porous materi- 

 al ; but it does this so much more slowly than the hydrogen 

 goes inward that there is a sudden increase in the volume 



