Chap, xxni.] DENSITY OF GASES. 273 



tubes of the same length and capacity, closed at one 

 end. Each tube contained the same volume of gas, 

 and they were all plunged into a vessel filled with 

 mercury. This was then placed in a stout glass 

 cylinder filled with water and fitted with a screw piston. 

 The pressure exerted by the piston was communicated 

 through the water to the mercury, which was thus 

 forced up the tubes, and compressed the gas. The 

 different heights of the columns of mercury in the 

 different tubes showed the different compressibility of 

 the various gases. Though the difference between 

 any two gases is very slight, yet each has its own 

 degree of compressibility. 



Weight of gases. That gases have weight is 

 easily proved by suspending a globe, exhausted of air, 

 from the scale of a balance and counterpoising it. On 

 permitting air or other gas to enter, the beam will go 

 down to the globe side, indicating increased weight. 

 A litre of dry air at C. is 1*293 grammes. A litre 

 is 1,000 cubic centimetres, and 1,000 cubic centi- 

 metres of water weigh 1,000 grammes. So that the 

 weight of air is to the weight of water as 1-293 is to 



1,000 ; that is, the ratio is = ~. Water is thus 



773 times heavier than air. Hydrogen weighs only 

 0-089 gramme per litre, oxygen, 143, carbonic 

 acid, 1-97. 



The density of a gas can be measured in a 

 similar way to that of liquids. Its ratio to that of 

 water has been shown to be -00 1 2 9 3. A given volume 

 of gas may then be considered as a volume of a 

 liquid of very much less density than water. It is, 

 then, understood how laws, applicable to liquids, are 

 similarly applicable to gases. Suppose we have a 

 mass of gas, and a body somewhere within the mass. 

 Just as in the case of liquids (page 180), the body will 

 be pressed upon on all sides. If we consider the mass 

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