et bladder and pack-thread." We
cannot wonder that the making of water-tight connexions was a great
difficulty, and we can sympathise with his belief that he could have got
a column more than 21 feet high but for the leaking of the joints on
several occasions. He notes the familiar fact that the vine-stump
absorbed water before it began to extrude it.

He afterwards (pp. 106-7) used a mercury gauge, and registered a
root-pressure of 32.5 inches or 36 feet 5.5 inches of water, which he
proceeds to compare with his own determination of the blood-pressure of
the horse (8 feet) and of other animals. Perhaps the most interesting of
his root-pressure experiments was that (p. 110) in which several
manometers were attached to the branches of a bleeding vine, and showed a
result which convinced him that "the force is not from the root only, but
must proceed from some power in the stem and branches," a conclusion
which some modern workers have also arrived at.

Assimilation.

Hales' belief that plants draw part of their food from the air, and
again, that air is the breath of life, of vegetables as well as of
animals (p. 148), are based upon a series of chemical experiments
performed by himself. Not being satisfied with what he knew of the
relation between "air" (by which he meant gas) and the solid bodies in
which he supposed gases to be fixed, he delayed the publication of
_Vegetable Staticks_ for some two years, and carried out the series of
observations which are mentioned in his title-page as "An attempt to
analyse the air, by a great variety of chymio-statical experiments,"
occupying 162 pages of his book. {133}

The theme of his inquiry he takes (_Vegetable Staticks_, p. 165) from
"the illustrious Sir _Isaac Newton_," who believed that "dense bodies by
fermentation rarify into several sorts of Air; and this Air by
fermentation, and sometimes without it, returns into dense bodies."

Hales' method consisted in heating a variety of substances, _e.g._
wheat-grains, pease, wood, hog's blood, fallow-deer's horn,
oyster-shells, red-lead, gold, etc., and measuring the "air" given off
from them. He also tried the effect of acid on iron filings,
oyster-shells, etc. In the true spirit of experiment he began by
strongly heating his retorts (one of which was a musket barrel) to make
sure that no air arose from them. It is not evident to me why he
continued at this subject so long. He had no means of distinguishing one