222 BODIES FLOATING IN AIR. 





after the cock is closed the tube is inverted and kept 

 for twenty seconds mouth upwards, the lighted match 

 will not inflame the gas in the tube, for the coal-gas has 

 now escaped upwards, out of the tube, and the latter 

 is filled by the heavier air. 



If the experiment be performed precisely in the manner described, 

 it will always succeed ; but a bottle should never be used for it. 

 The air or the gas cannot within twenty seconds escape so rapidly 

 through the narrow neck of a bottle as not to leave some of it in 

 the bottle ; a mixture of both is therefore obtained which does not 

 burn quietly, but explodes with greater or less vehemence according 

 to the proportion in which the two gases are mixed. If the mixture 

 should accidentally contain air and gas in a proportion very favour- 

 able to an explosion, the latter will be very violent and loud, and 

 the bottle may be shattered into fragments, which will be thrown 

 in all directions and may cause serious accidents. A vessel which 

 has an equal width throughout, or becomes only somewhat wider at 

 the mouth, such as a test-tube or a lamp cylinder, will not be broken 

 even if, in consequence of an insufficient supply or discharge of gas, 

 an explosive mixture should have remained behind. 



Soap-bubbles filled with coal-gas or hydrogen rise 

 .rapidly in the air, because the thin liquid envelope 

 together with the gas is still lighter than the air 

 which they displace. A balloon is nothing else but a 

 capacious envelope filled with gas, forming together a 

 body lighter than an equal volume of air. A spherical 

 balloon of 12 m or 120 dcm diameter has by the rulegivenin 



i t f 120 x 120 x 120 x 3-1416 



art. 1 (page 6) a volume oi - - 





= 904,780-8 litres. A globe of water of this volume 

 would weigh an equal number of kilogrammes, for*l litre 

 of water weighs l kgr ; but the weight of air is -^-J-Q oi 

 that of water, hence a volume of air equal to that oi 

 the balloon weighs only L x 904,780 kg ^8, that is, 

 nearly l,131 kgr . When the balloon is filled with coal- 



