which the plant grew. He allowed this
to go on growing for five years, and at the end of that period, thinking
his experiment had been conducted sufficiently long, he pulled up his
tree by the roots, shook all the earth off, dried the earth again,
weighed the earth and weighed the plant. He found that the plant now
weighed 169 lb. 3 ounces, whereas the weight of the soil remained very
nearly what it was--about 200 lb. It had only lost 2 ounces in
weight."[1]

The conclusion, therefore, come to by Van Helmont was that the source of
plant-food was _water_.[2]

_Digby's Theory._

Some fifty years later an extremely interesting book was published
bearing the following title: 'A Discourse concerning the Vegetation of
Plants, spoken by Sir Kenelm Digby, at Gresham College, on the 23d of
January 1660. (At a meeting of the Society for promoting Philosophical
Knowledge by Experiments. London: Printed for John Williams, in Little
Britain, over against St Botolph's Church, 1669.)' The author attributes
plant-growth to the influence of a _balsam_ which the air contains. This
book is especially interesting as containing the earliest recognition of
the value of saltpetre as a manure. The following is an extract from
this interesting old work:--

"The sickness, and at last the death of a plant, in its natural course,
proceeds from the want of that balsamick saline juice; which, I have
said, makes it swell, germinate, and augment itself. This want may
proceed either from a destitution of it in the place where the plant
grows, as when it is in a barren soil or bad air, or from a defect in
the plant itself, that hath not vigour sufficient to attract it, though
it be within the sphere of it; as when the root has become so hard,
obstructed and cold, as that it hath lost its vegetable functions. Now,
both these may be remedy'd, in a great measure, by one and the same
physick.... The watering of soils with cold hungray springs doth little
good; whereas muddy saline waters brought to overflow a piece of ground
enrich it much. But above all, well-digested dew makes all plants
luxuriate and prosper most. Now what may it be that endues these liquors
with such prolifick virtue? The meer water which is common to them all,
cannot be it; there must be something else enclosed within it, to which
the water serves but for a vehicle. Examine it by spagyric art, and you
will find that it is nothing else than a _nitrous salt_, which is
dilated in t