IG DENSITIES OK OXYtJEN ANlJ IIVDROIJEN 



COMPRESSION OF GLOBES DUE TO 76 CENTIMETRES' DIFFERENCE OF PRESSURE. 

 CUobe. Compression. 



1 1.30 CC. 



2 1.44 " 



3 6.28 " 



4 6.00 " 



5 8.07 " 



7 3'3 " 



8 3.89 •■ 



9 2-94 " 



10. — COUIiKOTION KOK ("OMI'IJESSIOX OK (iLOHKS WHKN EXHAUSTED. 



Tlie ett'ect of the change of volume of a glol)e wliicli is due to a change of 

 internal pressure may be eliminated by suitable manipulation, or the amount of a 

 correction to the weight of the glol)e may be computed from the amount of tlie 

 change of volume, together with the density of the air at the time of weighing. 

 Tiiere will doubtless be a ditference of opinion as to which is the more convenient. 



In the cjvse of one of my globe.s, the change of volume was about eight cubic 

 centimetres. Tii order, therefore, not to produce an error of one tenth of a milli- 

 gramme in the value of the correction, it would be necessary to observe the pjessui-e 

 and temperature of the air at each weighing. But, with the balance which I pro- 

 cured for the present series of expeiiments, numerou.s weighings were required in 

 order to determine a weight accui-ately enough, so that much lalior would be 

 involved in the observations and com[)utations for the corrections. It was accord- 

 ingly thought more convenient to make the needed correction a matter of manipula- 

 tion consuming no time after the [iroptn- apparatus had been constructed. Each 

 globe was therefore provideil witii a counterpoise e(pial in volume tt> the globe 

 viheii exhausted. Then, for each globe, was made a jiair of minute Hasks whose 

 volumes differed by the amount of the eom])re.ssion of the globe on exhaustion, 

 and who.se weights w-ere exactly the same when they were weighed in a vacuum. 

 For instance, the compression of globe No. 1, for the difference of pressure 

 usual in ray experiments, was 1.27 cubic centimetres. Two flasks were made whose 

 volumes were 2.080 and 0.810 cubic centimetres, and whose difference of weight 

 when weif^heil in air was equal to the weight of 1."_'7 <iil)ic centimetres of air at the 

 time, taking account of the true values of the weights emphiyed. 



Nt)W, when the globe No. 1 was exhausted, it was weighed against its 

 counterpoise, wliicli had the same volume. Wiien the globe was fidl of ga.s, the 

 0.81 cubic centimetres were placed with it on the balance, .and the 2.08 cubic centi- 

 metres were likewise added to the counterpoise. The true weights of the globe 

 and counteriioise suffered equal additions, for the true weights of the two additions 

 were e(pial williin a fiftieth of a milligramme. Therefore, the apparent difference 



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