PRIESTLEY 



15 



had failed to confirm the beneficient effect of plants on air; and now 

 Priestley too found himself unable to obtain positive results regularly. 

 Instead of his earlier categorical statements, he now wrote: "Upon the 

 whole, I think it probable that the vegetation of healthy plants, growing 

 in situations natural to them, has a salutarv effect on the air." Before 



^ DoC/YJ/i PiFLOa/STOX. 



//'//'//,v// //;•/..;// 



Fig. 2. — A contemporary cartoon of 

 Joseph Prie.stley. 



he succeeded in finding an explanation for the irregular results (which, 

 we think now, might have been caused by poor illumination), Priestlej^'s 

 attention was diverted by an observation which he called "the most 

 extraordinar}^ of all ixvy unexpected discoveries." He found, namely, 

 that a "green matter" deposited on the walls of many of his water con- 

 tainers, formed bubbles of pure "dephlogisticated air" (that is, oxygen), 

 whenever it was illuminated by the sun. At first, Priestle}^ thought this 



