Additional Notes. 481' 



Veries of several philosophers. The first step was made by 

 Dr. Priestley, in 1774, by the discovery of oxygen gas. 

 This gas, according to the prevailing theory of the time, 

 he considered as air totally deprived of phlogiston. Azotic 

 gas, on the other hand, was air satorated with phlogiston. 

 Hence he considered common air as oxygen gas combined 

 with an indefinite portion of phlogiston, and varying in pu- 

 rity according to that portion, being always pure in an in- 

 verse proportion to the quantity of phlogiston it contained. 



While Dr. Priestley was making experiments on oxy- 

 gen gas, ScHEELE, of Sweden, proceeded to the analysis of 

 air in a different manner. From his experiments he concluded 

 that common air is compounded of two different elastic fluids, 

 viz. foul air, which constitutes more than two thirds of the 

 whole, and another air, which is alone capable of supporting 

 flame and animal life, and to which he gave the name of ein- 

 pyreal air. The foul air of Scheele was the same with 

 &Qplilogisticated airoi Priestley ; and the eyiipyreal air o£ 

 the former was the same with the dephlogisticatcd air of the 

 latter, or with what is at present called oxygen gas. 



While Scheele was occupied with these experiments, 

 Lavoisier was assiduously employed on the same subject, 

 and was led, by a different road, to precisely the same con- 

 clusions. He found that air is composed of azotic and oxy- 

 gen gases ; and, from a variety of experiments, he determined 

 the proportions to be 73 parts of azotic gas, and 27 parts of 

 oxygen gas. These experiments were made in the year 1776. 



Secretion of Oxygen by Vegetables, p. S3. 



Dr. Priestley concluded from his experiments, and it has 

 been since generally believed by Ingenhouz, Sennebier, 

 and other vegetable physiologists, that vegetables, in the 

 course of their germination and growth, when exposed to 

 solar light, absorb azote and emit oxygen, and thus purify the 

 surrounding air. But, by a series of ingenious experiments 

 lately published, Professor Woodhouse, of Philadelphia, 

 has drawn into question the truth of these conclusions. From 

 the result of these experiments, he contends tliat the germina- 

 tion of seeds and the growth of plants do not purify atmos- 

 pherical air ; but that, whenever diey appear to atlord oxy- 

 gen gas, it is by devouring the coal of carbonic acid gas for 

 food, and leaving us oxygen in the form of pure air. 1 le has 



