35i On the pinport'ton of 



ffas it finds in Hiat volume of air. But 12-5 crains of the 

 iime water used require 70 grain measured ot carbonic acid 

 gas to saturate it; tiierc-rore, the l(>)2,-t'00 grain measures of 

 eommon air contain 70 of carbonic acid, or ttW^'^ '^^ ^'*® 

 whole. The weight of the carbonic acid atni(js>phere then- 

 is to that of the whole coniptvund as 1 r.l-lSOj but the 

 weight of carbonic acid gia^ \n a given- portion' of aii*at the 

 earth's surface, is nearly yj^'„odih of the whole ; because 

 the specific gravity of the gas is l-J- that of conmion air.. 

 1 have since found that the air in an assembly, iii which 

 t%vo hundred peo])le had breatlied for two hours^, witli the 

 windows and doors- shut, contained little more than 1 pci". 

 cent, of carbonic acid gas-. 



Having now determined tlie force with wkich cash at- 

 mosphere presses on. the earth's stn4ace, or in other words, 

 its weight, it remains next to inquire iuto their specilie 

 gravities. 



'l-hcsi; may be seen in the following table. 



Atmospheric air - - I'OOO 



Azotic gas- - - - '960^ 



0xvgenous gas - - I'l'ST 



Carbonic acid gas - 1'500 



Aqueous vapour - -'00 



IlydrogGuous gas- - - "077* 



Kirwau and Lavoisier are my authorities for these num- 

 bers, except oxygenous gas and aqueous vapour. For the 

 former I am indebted to Mr..])avy';j Chemical Res<.'arehes ;-, 

 his number is something gr-eatcr than theirs :- L prefer it,, 

 because, being determined wi-th at least equalat-tention to 

 accuracy witli the others, itiias thus- further claim for cre- 

 dit, that'll parts of gas of this specific gravity, mixed with- 

 79 parts of azotic gas, make a coiiipomid of exacllv' the 

 same spccUic gravity as the atmosphere, as they evidently 

 ought to do, setting aside the unhnuided notion of their 

 forming a chcniieai compound. The specific gravity of 

 aqueous vapour I have determined mvselfj.both by analytic 

 and synthetic methods-, after the manner, of De Saussure ; 

 that is, bv abstractint; aqueous vapour of a known force 

 from a orlven quantity of' air, and weighing the water ob- 

 tuined — and admiitiug » given weight of water to dry air,. 

 and comparing the loss- with- the increased elasticity. Dc 

 Saussure makes the specific gravity to-be ,71 of ,75 }- but> 



" TUe specific {jravity of hydrogen must be mtcd too low : if 100 oxygen 

 require 180 livdrogeu by measure, according to this 8!) oxygen would re- 

 i{uiifoiilv il iiydrogca to form -vviitt-r ; whereas H5 rec]uire ij. Hydrogen 

 eight to he found uijout l-.iOlh part of the ^veiglu of common air. 



be. 



