1820.] • the Rev. Dr. Hales. 165 



Dr. Priestley. This produced the taste for pneumatic chemistry, 

 and of course was the occasion of all the subsequent discoveries 

 in this most fertile region of the science. 



Many interesting experiments had been made on air for a 

 whole century before Hales began his career. But hitherto it 

 had been examined only as a heavy, transparent, and elastic 

 body. No one had thought that air was capable of existing 

 in a great variety of substances, deprived of its elastic state, but 

 capable, when favourable circumstances occurred, of assuming 

 all the elasticity of air. Hales demonstrated that almost aS 

 vegetable, animal, and mineral substances, contain air, and that 

 the quantity in many cases is astonishingly great. Thus from a 

 piece of oak wood he extracted a quantity of air equivalent to 

 200 times its bulk, and constituting about a third part of the 

 weight. It is true that Dr. Hales did not suspect that the 

 elastic fluids thus obtained from vegetable, animal, and even 

 mineral bodies, differed entirely in their nature from common 

 air, and consisted of elastic bodies of a very different nature. 

 But it was a great step to call the attention of chemists to the 

 extrication of airs from different bodies. It was natural, after 

 being aware of the fact, that elastic fluids are extricated from a 

 great variety of bodies for chemists to turn their attention to the 

 properties of the elastic fluids themselves. This accordingly 

 was done. by Cavendish, Priestley, and Scheele. These bodies 

 were characterized by names according to their characters. The 

 next step was to determine the constituents of these elastic 

 fluids. This was the occupation of Lavoisier and his followers. 

 And of late years, chiefly in consequence of the atomic theory 

 and the theory of volumes, for which we are indebted respect- 

 ively to Dalton and Gay-Lussac, this important department of 

 chemistr}' is in a great measure completed. Thus a new and 

 most important chemical rout which was first opened by Dr 

 Hales has only been completely explored in our own day and 

 within these few years. 



Hales's experiments on respiration, on the quantity of air 

 deprived of its elasticity by combustion, and by various kinds of 

 vapours, deserve a careful perusal. If he did not see or even 

 suspect the whole truth, he at least greatly facilitated the future 

 investigations of chemists after the composition of atmospherical 

 air, the true nature of combustion, and the important part which 

 oxygen performs in it, were known. This work of Dr. Hales 

 was translated into French by Buffbn ; and the translation 

 speedily raised the reputation of the author as high on the con- 

 tment as the original work had already done in Great Britain. 



The success of his experiments on the motion of the sap in 

 plants naturally led him to examine the motion of the blood in 

 animals, previously much better knovvn than the former. Accord- 

 ingly in 1733 he pubhshed, by order of the Royal Society, a 



