VOL. XLVI.] FHILOSOPHICAL TRANSACTIONS. Ill 



admitted. Now, I found the sum total of the fresh air thus destroyed to be 

 nearly equal to the first quantity of sulphureous air in the inverted glass. 



" Since we have in this experiment a full proof of the brisk agitation and effer- 

 vescence which arises from the mixture of fresh air with air that is impregnated 

 with sulphureous vapours, which arise from several mineral substances, espe- 

 ciallv from the pyrites, which abounds in many parts of the earth ; may we not 

 witli good reason conclude, that the irksome heat, which we feel in what is 

 called a close sultry temperature of the air, is occasioned by the intestine motion 

 between the air and the sulphureous vapours, which are exhaled from the earth ? 

 which effervescence ceases, as soon as the vapours are equably and uniformly 

 mixed in the air ; as happens also in the effervescences and ferments of other li- 

 quors. The common observation therefore, that lightning cools the air, seems 

 to be founded on good reason ; that being the utmost and last effort of this ef- 

 fervescence. 



" May we not hence also, with good probability, conclude, that the first 

 kindling of lightning is effected by the sudden mixture of the pure serene air 

 above the clouds, with the sulphureous vapours, which are sometimes raised in 

 plenty, immediately below the clouds ? the most dreadful thunders being usually 

 when the air is very black with clouds ; it rarely thundering without clouds : 

 clouds serving, in this case, like the above-mentioned inverted glasses, as a par- 

 tition between the pure and sulphureous airs : which must therefore, on their 

 sudden admixture through the interstices of the clouds, make (like the two airs 

 in the glass) a more violent effervescence, than if those airs had, without the 

 intervention of the clouds, more gradually intermixed, by the constant more 

 gradual ascent of the warmer sulphureous vapours from the earth, and descent 

 of the cold serene air from above. And though there was no luminous flash of 

 light in the glass, yet, when such sudden effervescence arises, among avast 

 quantity of such vapours in the open expanse of air, it may, not improbably, ac- 

 quire so rapid a velocity, as to kindle the sulphureous vapours, and thereby be- 

 come luminous. 



" And since, from the effects that lightning is observed to have on the lungs 

 of animals, which it often kills, by destroying the air's elasticity in them, as 

 also from its bursting windows outwards, by destroying the air's elasticity on the 

 outside of those windows : since, I say, it is hence probable, that the sulphu- 

 reous fumes do destroy a great quantity of elastic air ; it should therefore cause 

 great commotions and concussions in the air, when the air rushes into those 

 evacuated places ; which it must necessarily do with great velocity. 



" Dr. Papin has calculated the velocity with which air rushes into an exhausted 

 receiver, when driven by the whole pressure of the atmosphere, to be at the 

 rate of 1305 feet in a second of time ; which is at the rate of 889 miles in an 



