338 Qn the prefctit State of Aerojlilthn. 



incapable of exerting a chemical aAioa thereon : but if water 

 palles the pores of paper, it is moft probably occafioned bv 

 its fird eflecling a foiution of the mucilage, by which means 

 the reft of the fluid imds an eafy paffage ; for, the hotter the 

 water, the quicker does it percolate; and this is not attributable 

 to any fuppofed attenuation of the individual particles of the 

 fluid, but to an actual foiution of the paper itlelf; olherwife 

 the Itrength of each fingle fibre would remain unimpaired 

 by the ditference of temperature in the water; the contrary 

 to which the manufa^Slure of paper itfelf will evince. 



The difficulty, therefore, of obtaining the gafes in a flate 

 of drynefs in fo' large a quantity as is required for aeroftatic 

 experiments, will ever remain an impediment to the ufe of 

 paper for any other than balloons of fmall diameter, and on 

 the principle of Montgolfier. Two brothers of this name, 

 uatives of France, confidering the difference in the fpecific 

 gravity of heated and cool air, juflly imagined, that if a hag, 

 lufficiently large, were filled with the former, the weight of 

 the inclofed heated air and bag together would be lefs than 

 an equal bulk of atmofpheric air of ordinary temperature, and 

 that fuch an apparatus would afcend till it fiiould attain that 

 elevation where the refpeilive gravities would correfpond. 



Experiment fullv verified the opinion they had formed, 

 and M. Pilatre de Rozier was the firft to make trial of its 

 fecurity. 



The inconveniences peculiar to machines rendered buoy- 

 ant by heated air, arofe from the impofl'ibility of keeping up 

 the elevated temperature of the inclofed air without the con- 

 tinued renewal of fuel, and that in large quantity; whereby 

 tjie travellers were expofed to great danger from the occafion- 

 ^lly fudden and unavoidable expanfion of the flames, and their 

 inability to command that uniformity of rarefad ion fo necef- 

 fary to the fafety of the vovasre. 



As aerial chemiftry had been before this time making rapid 

 advances, fo the phiiofophical world, through the indefati- 

 gable labours of the honourable Mr. Cavendifli, had beea 

 made ac(juainted with the properties of inflammable air, 

 whofe fpecific gravity, in a tolerably pure flate, is at leall 

 twelve times lighter than atmofpheric air. DoAor Black 

 firft applied this newly difcovered gas to balloons, by fuggcfi- 

 ing its inclofure in an air-tight bag, as capable of raifing it- 

 felf into the atmofphere, agreeably to the common hydrofta- 

 tical axiom, that bodies immerfed in a fluid heavier than 

 themfelves muft inevitably float in that fluid ; and that, as 

 the denfity of fluids is proportionate to their heights, fo the 

 lighter body will continue to rife till its gravity Ihall corre- 

 fpond 



