482 PHILOSOPHICAL TRANSACTIONS. [ANNO J/84. 



owing to this cause ; my first experiments therefore were made in order to ascer- 

 tain whether any fixed air is really so produced. Now, as all animal and vegetable 

 substances contain fixed air, and yield it by burning, distillation, or putrefaction, 

 nothing can be concluded from experiments in which the air is phlogisticated by 

 them. The only methods I know, which are not liable to objection, are by the 

 calcination of metals, the burning of sulphur or phosphorus, the mixture of 

 nitrous air, and the explosion of inflammable air. Perhaps it may be supposed 

 that I ought to add to these the electric spark ; but I think it much more likely 

 that the phlogistication of the air, and production of fixed air, in this process, is 

 owing to the burning of some inflammable matter in the apparatus. When the 

 spark is taken from a solution of tournsol, the burning of the tournsol may pro- 

 duce this effect ; when it is taken from lime-water, the burning of some foulness 

 adhering to the tube, or perhaps of some inflammable matter contained in the 

 lime, may have the same effect ; and when quicksilver or metallic knobs are used, 

 the calcination of them may contribute to the phlogistication of the air, though 

 not to the production of fixed air. 



There is no reason to think that any fixed air is produced by the first method 

 of phlogistication. Dr. Priestley never found lime-water to become turbid by the 

 calcination of metals over it:* Mr. Lavoisier also found only a very slight and 

 scarce perceptible turbid appearance, without any precipitation, to take place 

 when lime-water was shaken in a glass vessel full of the air in which lead had 

 been calcined ; and even this small diminution of transparency in the lime-water 

 might very likely arise, not from fixed air, but only from its being fouled by par- 

 ticles of the calcined metal, which we are told adhered in some places to the glass. 

 This want of turbidity has been attributed to the fixed air uniting to the metallic 

 calx, in preference to the lime ; but there is no reason for supposing that the calx 

 contained any fixed air ; for I do not know that any one has extracted it from 

 calces prepared in this manner ; and though most metallic calces prepared over 

 the fire, or by long exposure to the atmosphere, where they are in contact with 

 fixed air, contain that substance, it by no means follows that they must do so 

 when prepared by methods in which they are not in contact with it. 



Dr. Priestley also observed, that quicksilver, fouled by the addition of lead or 

 tin, deposits a powder by agitation and exposure to the air, which consists in 

 great measure of the calx of the imperfect metal. He found too some powder of 

 this kind to contain fixed air ;-\- but it is by no means clear that this air was pro- 

 duced by the phlogistication of the air in which the quicksilver was shaken ; as 

 the powder was not prepared on purpose, but was procured from quicksilver fouled 

 by having been used in various experiments, and may therefore have contained 

 other impurities besides the metallic calces. 



* Experiments on Air, vol. i. p. 137. + Exper. in Nat. Phil. vol. i. p. 144. 



