measurements and calculations combine to form a living
picture of the whole subject. Malpighi endeavoured to discover the physiological functions of organs by the aid of
analogies and a reference to their structure; Mariotte discerned
the main features of the connection between plants and their
environment by combining together physical and chemical
facts ; Hales may be said to have made his plants themselves
speak ; by means of cleverly contrived and skilfully managed
experiments he compelled them to disclose the forces that
were at work in them by effects made apparent to the eye, and
thus to show that forces of a very peculiar kind are in constant
activity in the quiet and apparently passive organs of vegetation.
Penetrated with the spirit of Newton's age, which notwithstanding its strictly ideological and even theological conception
of nature did endeavour to explain all the phenomena of life
mechanically by the attraction and repulsion of material
particles, Hales was not content with giving a clear idea of the
phenomena of vegetation, but sought to trace them back to
mechanico-physical laws as then understood. He infused life
into the empirical materials which he collected by means of
ingenious reflections, which brought individual facts into
connection with more general considerations. Such a book
necessarily attracted great attention, and for us it is a source
of much valuable instruction on matters of detail, though we
now gather up the phenomena of vegetation into a somewhat
differently connected whole.
His investigations into transpiration and the movement of water in the wood were greeted with the warmest approbation. He measured the quantity of water sucked in by the roots and given off by the leaves, compared this with the supply of moisture contained in the earth, and endeavoured to calculate the rapidity with which the water rises in the stem, and to compare it with the rapidity of its entrance into the roots and its exit by the leaves. The experiments, by which he showed the force of suction in wood and roots, and that of the root-
