S50 Dr. M^Keever on the Influence of Solar Light [Nov. 



in common air, though exposed only to a weak light, or protected 

 from the action of the sun. The nitrogenous atmosphere at the 

 end of two months was increased in bulk, and contained -t^ths 

 of oxygen gas ; whereas when similar plants were confined in 

 pure nitrogen gas, and kept in perfect darkness, though they 

 were renewed every twelve hours, lest their vegetation might 

 languish ; yet they produced no oxygen gas, but augmefiited 

 their atmosphere by a quantity of carbonic acid.* ' {Itnofiiqqi, 



The production of the green colour in plants (a prCftJdisiIfifl&y 

 observe intimately connected with the discharge of okygen)f 

 has been clearly demonstrated to be influenced by the pdrtion of 

 the spectrum to which the plant may have been subjected ; and 

 these changes, it has been further proved, were occasioned not 

 by the heating or illuminating rays, but by the peculiar properties 

 of the chemical rays associated with them. Senebier having 

 sowed different quantities of lettuce seeds in several small cups, 

 subjected them to the influence of hght transmitted through 

 fluids tinged of various colours. The leaves exposed to yellow 

 light were at first of a faint green, but afterwards became yellbw : 

 those exposed to violet light were of a bright green, and their 

 colour augmented with their age ; while those raised in obscu- 

 rity possessed no verdure whatever, ij; - t ■ j > 



As animal substances (according to the experiments of Abfet- 

 nethy, Cruickshank, and Jurin) deteriorate the air after the 

 manner of vegetables ; namely, by the absorption of oxygen and 

 the emission of an equivalent quantity of carbonic acid gas, the 

 supposition seems not improbable, from their similarity in struc- 

 ture and other circumstances, that they also when exposed to 

 sunshine would give out oxygen gas. But for this, we have as 

 yet no data. I believe the experiment has not been ti'ied, and 

 I merely mention it as affording a field for curious and intere'sting 

 inquiry both to the chemist and physiologist. With regard to 

 inorganic matter, we find that the most compact and solid mate- 

 rials with which we are acquainted are incessantly subjected to 

 those slow and silent changes to which we commonly a])ply the 

 term decay. By this in truth we understand, that the elements 

 of which they are composed are constantly entering into new and 

 varied states of combination. Influenced by the physical ^^nts 

 that surround them, the results of this mutual interchange of 



■t; 



♦ We can readily conceive that during the ordinary light of day, the chemical ri^s, 

 from their dilute and feeble state, may not possess sufficient power to interfere (unless 

 in a few instances such as those to which I have alluded) with the usual functions of 

 plants ; but that during sunshine, their intensity may become so mudi increased w to 

 occasion a directly opposite round of changes, the air that had hitherto been contaminated 

 by vegetation being now restored to its original state of purity. 



•f ** Tlie emission of oxygen and the production of the green colour in plants aj^ar 

 both to depend on the same cause — the decomposition of carbonic add; sothi^^iee 

 •cannot so properly affirm th&t the green parts afford oxygen as that thdy becotai grflen 

 Whiea feat gaa ia ex pelled."— (See Ellis^s Further Enquiries.) > -. • 



1^ Further Enqnirie?, A:c. p. 78; Saussute's RdcheroIi«s,M]^.'Mrrr" 



