from Water exposed to Light. 215 



proportion of 2 to i, and of 1734 to 1000;. that is to 

 say, as 3468 to 1000. 



Now, as the surface of 30 grains of raw silk amounts 

 to 476 square inches, the surface of 30 grains of poplar 

 cotton must amount to 1651 square inches, which gives 

 55 square inches of surface for each grain in weight ; 

 consequently, the. surface of the cotton made use of in 

 the foregoing experiment (No.. 16) did not amount to 

 less than 6600 square inches (for 120 grains, the weight 

 of the cotton, multiplied by 55, gives 6600), an enor- 

 mous surface indeed for a body whose solid contents did 

 not amount to quite half a cubic inch. 



From hence it appears evidently, that the quantities of 

 air furnished by water, in the experiments with raw silk 

 and with poplar cotton, were neither in proportion to the 

 quantities of these substances made use of, nor to the quan- 

 tities of their surfaces. It appears, likewise, from the two 

 last experiments, that the air which is furnished in the 

 beginning of the experiment, or when the water is first ex- 

 posed to the action of the sun's rays, is neither so good, 

 nor in so great abundance, as afterwards, at a more ad- 

 vanced period ; and that it totally ceases to be produced 

 after a certain time. 



To ascertain, with greater precision, the qualities of 

 the air furnished at different periods of the experiment, 

 or rather the period when the water begins to give good 

 air ; and also to determine the relative quantities and 

 qualities of the airs produced in the experiments with 

 raw silk and in those with poplar cotton, I made the 

 following experiments. 



Experiment No. 17. 

 A globe, about 4^ inches in diameter, containing just 



