relative to Black, Watt, and Cavendish. . 497 



in an equal volume of common air*. But he advanced little 

 beyond his predecessor in demonstrating the air to be a com- 

 pound. It is not to be supposed he says that that aerial sup- 

 porter of combustion is the whole air, but only a part of it, 

 which is more active and subtle than the rest; since a light 

 enclosed under a glass expires, even whilst the vessel still con- 

 tains abundance of air: for we cannot believe that the par- 

 ticles of air which were in the said glass can be annihilated, 

 nor yet dissipated', since they cannot pass through the glass." 

 But this reasoning, though probable, is not conclusive ; since 

 it was certainly possible that the enclosed air might have been 

 diminished by condensation instead of abstraction, and have 

 become unfit to burn and to be breathed by a total vitiation, 

 instead of a partial loss. 



Yet, after all, Mayow's reasoning appears to advantage by 

 the side of Priestley's, or Scheele's, even when in the pro- 

 gress of experiment his nitro-aerial spirit, or fire-air, had 

 been actually divorced from " its consort," and when the latter 

 great chemist had approached a complete analysis of the at- 

 mosphere. For so difficult did Scheele find it to interpret 

 his own experiments, that when he had in his hands the " liver 

 of sulphur" which had produced a given diminution in a given 

 volume of air, — when he had found the specific gravity of the 

 diminished air to be less than that of common air, and the 

 " fire-air," which he had succeeded in separating from nume- 

 rous substances, to have a greater specific gravity, as well as 

 a greater power of supporting combustion, — when by re- 

 uniting them he had recomposed an air with all the proper- 

 ties of common air restored, — when he had arrived at the con- 

 clusion — " that the air consists of two different kinds of elastic 

 fluids," and that the "Jire-air" makes between a third and a 

 fourth of the whole bulk, — when coming finally to the ultimate 

 question of the analysis, he failed to find the « lost air " in the 

 liver of sulphur. Then he gave the reins to his imagination, 

 and embracing the idea, that heat is a compound of " fire-air " 

 with an imaginary substance invented by Stahl, concluded 

 that by the action of a double affinity the " fire-air " in his 

 experiment had combined with the phlogiston of the liver of 

 sulphur, and that the compound had passed through the pores 

 of the glass by which it had before been confined. Where 



* Tract, de parte a'erea igneaque Spiritus Nitri, cap. 7- p. 101. "Comperi 

 aerem per lucerna? deflagrationem in spatiutn ex parte circiter tricesima 

 minus quam antea reductum esse. Postquam fumi lucernae deflagrantis, 

 quibus cucurbita praedicta repleta est, prorsus evanuerunt, vitrunique intus 

 aeque ac prius pellucidum evasit, conatus sum secunda vice lucernam in 

 eadem accendere, radios solares in aliam camphorae portionem, in vitro eo 

 pariter suspensam, uti prius conjiciendo." 



