VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 36 1 



stopper now and then, so as to let in air to supply the place of the diminished 

 air. In order to know when the air is as much diminished as it can be, the best 

 way is, when the air is supposed to be nearly phlogisticated, to place the bottle 

 with its mouth under water, still keeping it stopped, and to loosen the stopper 

 now and then, while under water, so as to let in water to supply the place of the 

 diminished air, by which means the alteration of weight of the bottle shows 

 whether the air is diminished or not. If the solution of liver of sulphur be 

 made by boiling together fixed alkali, lime, and flowers of sulphur, which is 

 the most convenient way of procuring it, the air phlogisticated by it will be per- 

 fectly free from fixed air: whether it will be so if the liver of sulphur be made 

 without lime, I am not sure. A still more convenient way however, of procuring 

 phlogisticated air, is by a mixture of iron filings and sulphur; and, as far as I 

 can perceive, the air procured this way is as completely phlogisticated as that 

 prepared by liver of sulphur. 



Where the impurities mixed with the air have any considerable smell, our 

 sense of smelling may be able to discover them, though the quantity is vastly 

 too small to phlogisticate the air in such a degree as to be perceived by the ni- 

 trous test, even though those impurities impart their phlogiston to the air very 

 freely. For instance, the great and instantaneous power of nitrous air in phlo- 

 gisticating common air is well known; and yet 10 ounce measures of nitrous air, 

 mixed with the air of a room upwards of 12 feet each way, is sufficient to com- 

 municate a strong smell to it, though its effects in phlogisticating the air must 

 be utterly insensible to the nicest eudiometer; for that quantity of nitrous air is 

 not more than the 140000th part of the air of the room, and therefore can 

 hardly alter its test by more than — -<nnnr or tttto P art - Liver of sulphur also 

 phlogisticates the air very freely, and yet the air of a room will acquire a very 

 strong smell from a quantity of it vastly too small to phlogisticate it in any sen- 

 sible degree. In like manner it is certain, that putrifying animal and vegetable 

 substances, paint mixed with oil, and flowers, have a great tendency to phlogis- 

 ticate the air; and yet it has been found, that the air of a privy, of a fresh 

 painted room, and of a room in which such a number of flowers were kept as to 

 be very disagreeable to many persons, was not sensibly more phlogisticated than 

 common air. There is no reason to suppose from these instances, either that 

 these substances have not more tendency to phlogisticate the air, or that nitrous 

 air is not a true test of its phlogistication, as both these points have been suffi- 

 ciently proved by experiment; it only shows, that our sense of smelling can, in 

 many cases, perceive infinitely smaller alterations in the purity of the air than 

 can be perceived by the nitrous test, and that in most rooms the air is so fre- 

 quently changed, that a considerable quantity of phlogisticating materials may be 

 kept in them without sensibly impairing the air. But it must be observed, that 



vol xv. 3 A 



