on the Production of Dephlogijlicated Air . 89 
«xprefling the volume of the two airs deftroyed, will become a 
whole number. 
But, inftead of writing 100^ + 400;;- 13^, &c. I fhall con- 
tinue to write la \n — 1,35, and (hall exprefs the laft number 
(3,65) as a whole number notwithftanding ; and I (hall fome- 
times (following the example of Dr. Ingen-housz) write this 
number only, in noting the goodnefs of any air in queflion. 
I would juft obferve, with refpeCt to the procefs of proving 
the goodnefs of any kind of air, by the teft of nitrous air, 
that I mix the two airs in a phial, about 1 inch in diameter and 
4 inches long, putting the air to be proved into the phial firft, 
and then introducing the nitrous air, 011c meafure after ano- 
ther, till the volume of the two airs after the diminution has 
taken place, amounts to more than one meafure, and is lefs 
than two meafures. 
Immediately after the introduction of each meafure of ni- 
trous air, I give the phial a couple of (hakes ; after which I 
fuffer it to (land at reft, while I prepare another meafure of 
nitrous air, which commonly takes up about 20 feconds. 
The meafure of the eudiometer being filled with air, I fuffer it 
to remain quiet under water 15 feconds, or while I can leifurely 
count 30, in order that the air may have time to acquire the 
temperature of the water in the trough, and that the water in 
the meafure may have time to run down from the fides of the 
glafs tube ; and in (hutting the flider I take care to bring it to 
be exa&ly even with the furface of the water in the trough. 
Similar precautions are likewife made ufe of in meafuring the 
volume of the two airs in the tube of the eudiometer, after 
they have been mixed and diminifhed in the phial. 
In order that I may know when I have added nitrous air 
enough to the air in the phial, fo that the volume of the two 
Vol. LXXVII. N airs 
