Pioneer Investigators of Photosynthesis. 201 
als Entdecker noch als Mitentdecker der Assimilation bezeichnet 
werden. Einiger Verdienst liegt in seinen Experimenten, nament- 
lich in denen ueber die Wirkung der Sonnenlichtes auf den 
Gaswechsel.er sammelt aber er baut nicht auf.” 
The publication of the “ Chemical Researches on Vegetation ” 
by De Saussure in 1804, marks an epoch in the history of plant 
nutrition. At last the phlogiston theory disappears from chemistry 
and we meet for the first time with a chemical terminology and 
chemical methods which may be regarded as, comparatively 
speaking, modern. 
A glance at the headings of the sections of De Saussure’s 
chapters, together with a study of the succinct resumes which he 
offers at the end of each, enables the reader to form a very clear 
conception of the general theory of plant nutrition held by the 
author, and hence it is all the more remarkable that Sachs should 
characterise the book as “ dry and unattractive ” and deficient in 
offering no “deductive exposition [of his doctrines] of a more 
didactic character.” Hansen’s estimate of De Saussure’s work is 
much more just: “Das waren die Leistungen eines echten 
Forschers, der nicht tastete, nicht versuchte, sondern der mit 
System der Wissenschaft im Kopfe an eine Frage heranging und 
mit sicherer Hand die Methode handhabte, dieselbe zu losen.” 
Investigators before De Saussure’s day had attempted little 
. . . . 1 
more than a demonstration of the conditions under which carbon 
dioxide was decomposed and oxygen given off; De Saussure tackled 
the problem by quantitative methods and so gave clearness and 
precision to what was previously vague and sketchy. 
That the presence of gaseous C0 2 was an essential condition 
of plant growth was De Saussure’s starting point. He recognised 
that plants could be grown under bell-jars in a CO 2 -free atmosphere, 
but he explained this by saying that they utilised the C0 2 they 
themselves formed in respiration, for if the C0 2 so formed was 
removed by KOH or other absorbent as fast as it was formed, then 
the plant succumbed. 
In the second chapter of his book De Saussure discusses the 
effect of carbonic acid gas on vegetation, and proposes to “ ascertain 
whether this gas is useful to plants when it is derived from the 
surrounding air.” A paper by T. Percival, of Manchester, published, 
so far as I can discover about 1785, apparently suggested to De 
Saussure an important line of research. Percival had observed 
that a plant of mint exposed to a current of air containing an 
