GO 
light to be decompounded in vegetables, and to as- 
sist in the formation of their resinous and inflamma- 
ble matter *. In the year 1774, Dr Robison ob- 
served, that the leaves of tansey, which had grown 
in a coal mine, and were white, afforded no aromatic 
smell, when rubbed between the fingers ; but after 
they were planted in a place exposed to light, their 
white leaves died down, and fresh green sprouts shot 
up, which possessed the smell and all the other pro- 
perties of common tansey. He repeated the experi- 
ment with great care on lovage, on mint, and other 
plants. They all throve well in darkness, but with 
a blanched foliage, that had no resemblance to the 
ordinary foliage of the respective plants. When 
brought into day light, they all died down, and the 
stocks then produced the proper plants in their usual 
dress, and having all their distinguishing smells f 
288. After these general remarks on the agency 
of light in producing the colours, and otherwise 
changing the sensible properties of plants, we proceed 
to consider its power in modifying the qualities of the 
atmosphere, in which vegetables are made to grow, 
From the history of facts already given (243.), it ap- 
pears that Dr Priestley first advanced the opinion, 
that plants, in certain circumstances, purified the air. 
This effect he considered to depend on the vegetating 
state of the plant ; but as, on most occasions, he has 
omitted to state whether his experiments were con- 
4ucted in sunshine, or in the shade, it cannot be said 
* On Air and Fire, p. 158. 
f Black's Chem. Lectures, vol. i. p. 532. 
