

LOSS OF WEIGHT IN AIR. 221 



scale-pan with the flask descends; l gr or more will have 

 to bo placed in the other scale-pan to produce equili- 

 brium: that is, the air which enters the flask weighs one 



gramme or more. 



The loss of weight which bodies undergo in air is gene- 

 rally very small in comparison with the weight of the 

 bodies; hence the loss is practically disregarded in the 

 transactions of common life. But if bodies really lose 

 part of their weight when surrounded by air, it follows 

 ; that bodies will float in it if their weight is less than 

 that of an equal volume of air. Some gases are indeed 

 lighter than air, for example common coal-gas, which 

 ; is about half as heavy as air; and there is a gas, 

 hydrogen, which is the lightest of all known bodies, 

 and weighs less than one-fourteenth of the weight of 

 air. Both these gases consequently float on air, that 

 is, they rise in it. This may be shown by holding a 

 stout test-tube, or a lamp-cylinder which is closed at 

 one end by a tight cork fixed with sealing-wax, over 

 a common gas-burner, so that the nozzle of the burner 

 is well within the tube. The cock being opened, allow 

 the gas to issue from the burner while you are counting 

 twenty, that is, for about twenty seconds. Close the 

 | cock, hold the tube or cylinder in the same position for 

 another twenty seconds, and now hold a lighted lucifer- 

 1 match to the open end. The coal-gas having ascended 

 in the tube in virtue of its being lighter than air, 

 which* it has pushed downwards and out of the 

 tube, fills the latter completely, remains in it for some 

 time afterwards, and manifests its presence by burning 

 when the lighted match is applied to it. If the experi- 

 ment is repeated with this alteration, that immediately 







