VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 35y 



than another ; hut found no difference that I could be sure of, though the wind 

 and weather on those days were very various ; some of them being very clear 

 and fair, others very wet, and others very foggy. My way was, to fill bottles with 

 glass stoppers every now and then with air from without doors, and preserve them 

 stopped and inverted into water, till I had got 7 or 8, and then take their test ; 

 and whenever I observed their test, I filled 1 bottles, one of which was tried 

 that day, and the other was kept till the next time of trying, in order to see how 

 nearly the test of the same air, tried on different days, would agree. The expe- 

 riment was always made with distilled water, and care was always taken to ob- 

 serve the diminution which nitrous air suffered by being shaken in the water- 

 The heat of the water in the tub also was commonly set down. Most of the 

 bottles were tried only in the first method ; but some of them were also tried bv 

 the second, and by the method of Fontana. 



I would by all means recommend it to those who desire to compare the air of 

 different places and seasons, to fill bottles with the air of those places, and to try 

 them at the same time and place, rather than to try them at the time they were 

 filled, as all the errors to which this experiment is liable, as well those which pro- 

 ceed from small differences in the manner of trying the experiment, as those 

 which proceed from a difference in the nature of the water and nitrous air, will 

 commonly be much less when the different parcels of air are tried at the same 

 time and place, than at different ones ; provided only, that air can be kept in 

 this manner a sufficient time without being injured, which I believe it may, if 

 the bottles are pretty large, and care be taken that they, as well as the water used 

 in filling them with air, are perfectly clean. I have tried air kept in the above- 

 mentioned manner for upwards of three quarters of a year in bottles holding 

 about a pint, which I have no reason to think was at all injured ; but then I 

 have tried some kept not more than one third part of that time which seemed to 

 have been a little impaired, though I do not know what it could be owing to, 

 unless it was that the bottles were smaller, namely, holding less than J- of a pint, 

 and that in all of them, except 2, which were smaller than the rest, the stopper 

 which, however, fitted in very tight, was tied down by a piece of bladder. 



I made some experiments also to try whether the air was sensibly more dephlo- 

 gisticated at one time of the day than another, but could not find any difference. 

 I also made several trials with a view to examine whether there was any difference 

 between the air of London and the country, by filling bottles with air on the 

 same day, and nearly at the same hour, at Marlborough-street and at Kensing- 

 ton. The result was, that sometimes the air of London appeared rather the 

 purer, and sometimes that of Kensington; but the difference was never more 

 than might proceed from the error of the experiment; and by taking a mean of 

 all, there did not appear to be any difference between them. The number of 



