that he was, at all times* fully aware of the i 
tance of the agency of light. In many instances, how- 
ever, he has duly remarked it. Thus, for exam* 
pie, in his experiments on the green matter that forms 
on the sides of vessels filled with stagnant water, he 
asserts that the purr air is never produced in the 
shade, but only und er a direct exposure to light f 
and no degree of warmth, he adds, will supply the 
place of the sun's ra ys. He farther observed, that 
the water, which contained most carbonic acid, af-* 
forded oxygen gas most abundantly ; and that this 
gas was entirely dissipated by the action of the sun, 
leaving a residue of pure oxygen. When the glass 
vessels, containing this matter, were exposed to the 
sun, and were in the act of yielding pure air most 
abundantly, he found that the process ceased entire- 
ly, if he intercepted the solar rays by covering the 
vessels with black wax, or by removing them into a 
dark room *. These facts clearly demonstrate the 
power of light in the development of oxygen gas, 
and likewise furnish evidence of the decomposition of 
carbonic acid by the operation of the same agent. 
289. The experiments of Priestley were prosecu- 
ted by his illustrious contemporary Scheele, who, as 
we have already seen, (255. )could never succeed in his 
attempts to purify noxious air by vegetation, whether 
his experiments were conducted under exposure to 
the sun, or in the shade. The cause of uncertainty in 
the experiments of Priestley, and of entire failure in 
those of Scheele, is ascribed, by Dr Ingenhousz, to 
* Obs. on Air, vol. iv, p. 337. et se<j. 
