198 
R. J. Harvey Gibson . 
sun alone has no power to purify air—it may, indeed, even tend 
to render it less pure—unless when acting in conjunction with 
green plants, and finally, that the degree of purity of the air given 
oft by green plants depends on many factors such as the intensity 
of the light falling on them, the extent of exposure of the leaves to 
sunlight and so on.” 
Not a bad record of results, one must admit, when it is 
remembered that they were all deduced in the year 1778, from 
something like 500 experiments performed with the crudest of 
apparatus in three months (June to September) in a villa garden 
near London in such intervals of leisure as were afforded to a 
court physician in the reign of George III. 
Sachs asserts that Ingenhousz “ not only discovered the 
assimilation of carbon and the true respiration of plants but also 
kept the conditions and the meaning of the two phenomena distinct 
from one another.” I can find no evidence in the “ Experiments 
on Vegetables ” that Ingenhousz had any idea that the “ foul air” 
or the “ common air ” absorbed by plants contributed anything to 
their sustenance beyond his quotation from Priestley, viz., “ that 
the air, spoiled and rendered noxious to animals by their breathing 
in it, serves to plants as a kind of nourishment”—Priestley’s own 
words are “ The pabulum which plants derive even from common 
air.” Not till long afterwards, in 1796, when he published his 
“ Essay on the Nutrition of Plants and the Fruitfulness of the 
earth,” was Ingenhousz able to interpret the gaseous interchanges 
correctly, as he himself admits. He confesses that since the new 
system of chemistry was not in existence when he wrote his 
“ Experiments on Vegetables,” he was not in a position to formulate 
a suitable theory to explain the facts. Now that he knew that 
“ fixed air ” or carbon dioxide was a compound of carbon and 
oxygen, he found no difficulty in putting forward a comprehensive 
theory of plant nutrition, so far at least as the plant’s relation to 
air was concerned. He held that green shoots gave off oxygen in 
sunlight and carbon dioxide in darkness, and that non-green parts 
gave off carbon dioxide both in the light and in the dark, but he 
thought that the carbon dioxide was the source both of the carbon 
retained by day and the oxygen retained at night. From the 
carbon and oxygen so acquired the plants manufactured their acids, 
oils, mucilage, etc., and these they combine with atmospheric 
nitrogen in the organs of their bodies. 
