His first experiment (Vegetable Staticks, p. 100), was with a vine, to which he attached a vertical pipe made of three lengths of glass-tubing jointed together. His method is worth notice. He attached the stump to the manometer with a “stiff cement made of melted Beeswax and Turpentine, and bound it over with several folds of wet 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 32½ inches or 36 feet 5½ 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 gas from another, and almost the only quality noted is a want of permanence, e.g. when the CO2 produced
was dissolved by the water over which he collected it. Sir E. Thorpe [134a] points out that Hales must have prepared hydrogen, carbonic acid, carbonic oxide, sulphur dioxide, and marsh gas. It may, I think, be said that Hales deserved the title usually given to Priestley, viz. “the father of pneumatic [134b] chemistry.”
Perhaps the most interesting experiment made by Hales is the heating of minium (red-lead) with the production of oxygen. It proves that he knew, as Boyle, Hooke and Mayow did before him, that a body gains weight in oxidation. Thus Hales remarks: “That the sulphurous and aereal particles of the fire are lodged in many of those bodies which it acts upon, and thereby considerably augments their weight, is very evident in Minium or Red Lead, which is observed to increase in weight in undergoing the action of the fire. The acquired redness of the Minium indicating the addition of plenty of sulphur in the operation.” He also speaks of the gas distilled from minium, and remarks: “It was doubtless this quantity of air in the Minium which burst the hermetically sealed glasses of the excellent Mr. Boyle, when he heated the Minium contained in them by a burning glass” (p. 287).
This was the method also used by Priestley in his celebrated experiment of heating red-lead in hydrogen, whereby the metallic lead reappears