306 Dr, Thomson on the hifluence of Humidity [April, 



Then, by a well known principle in pneumatics, 



^ A + B 58 



The presence of the vapour, it is obvious, diminishes the 

 specific gravity of the air a little. The true specific gravity of 

 air should have been 1*0000 ; but we have obtained only 

 0-9829, which is less by 0-0171, or somewhat more than l-58th 

 part. 



If the temperature, instead of 60°, had been only 32^, the 

 j^TTOT would have been less. At that temperature, the elasticity 

 of aqueous vapour is 0*2 inch, and its specific gravity 0-00314; 

 so that the volume of air is 149, and that of vapour 1. 

 Here we have A = 149 



a = 1 

 B = 1 

 b =z 0-00314 

 Consequently 



or little more than l-150th part below the truth. 



We see from these examples, that the specific gravity of air is 

 diminished very nearly by the volume of vapour mixed with it. 

 And the lower the temperature, the more nearly does this 

 approach to accuracy ; because the specific gravity becomes 

 always less and less considerable. 



When the gas under examination is heavier than common air, 

 the error becomes more considerable. The heaviest gas, whose 

 specific gravity can be taken over water, is chlorine. Its specific 

 gravity is 2-5. Let us determine the error, when we weigh it, 

 jjjtanding over water at the temperature of 32®. Here we have 



A = 149 

 a = 2-5 

 B = 1 

 b = 0-00314 and 



Aa + Bb 149 X 2-5 + 000314 362-50314 ^ A^ot^on 



jc = --7 — — - = ;— = — ^^ — = 2-416687 



A + B loO 150 



Here the error is 0-083313, or l-30th part. 



When the gas is lighter than common air, the error dimi- 

 nishes ; but still continues too great to be neglected. The 

 lightest gas with which we are acquainted is hydrogen gas. Its 

 specific gravity is 0-0694. Let us determine what the specific 

 gravity would be when weighed over water at the temperature 

 of32^ We have 



A = 149 



a = 0-0694 



B = 1 

 b = 0-00314 and 



Aa+ Bb J 0*347 2* + 000314 10-35036 n r\cr\rk 



J ss C3 ■ .. . .,, . s= » ss U*0oy0» 



A + B 150 J50 



