According to Sachs, Ray made some interesting observations on the transmission of water, but on the whole what he says on this subject is not important. There is no evidence that Ray influenced Hales.
Mariotte, the physicist, came to one physiological conclusion of great weight; {119} namely, that the different qualities of plants, _e.g._ taste, odour, etc., do not depend on the absorption from the soil of differently scented or flavoured principles, as the Aristotelians imagined, but on _specific differences_ in the way in which different plants deal with identical food material-an idea which is at the root of a sane physiological outlook. These views were published in 1679, {120} and may have been known to Hales. He certainly was interested in such ideas, as is indicated by his attempts to give flavour to fruit by supplying them with medicated fluids. He probably did not expect success, for he remarks (p. 360): "The specifick differences of vegetables, which are all sustained and grow from the same nourishment, is [_sic_] doubtless owing to the very different formation of their minute vessels, whereby an almost infinite variety of combinations of the common principles of vegetables is made." He continues in the following delightful pa.s.sage: "And could our eyes attain to a sight of the admirable texture of the parts on which the specific differences in plants depends, [_sic_] what an amazing and beautiful scene of inimitable embroidery should we behold? what a variety of masterly strokes of machinery? what evident marks of consummate wisdom should we be entertained with?" To conclude what has been said on Hales"
chronological position-Ingenhousz, the chief founder of the modern point of view on plant nutrition, was born 1730 and published his book, _On Vegetables_, etc., in 1779. So that what was said of Hales" chemical position is again true of him considered in relation to nutrition; he did not live to see the great discoveries made at the close of the 18th century.
There is in his writing a limpid truthfulness and simplicity, unconsciously decorated with pretty 18th century words and half-rusticities which give it a perennial charm. And inasmuch as I desire to represent Hales, not only as a man to be respected but also to be loved, it will be as well to give what is known of the personal side of his character before going on to a detailed account of his work.
He was, as we have seen, entered at Corpus Christi College, Cambridge, in June 1696. In February 17023 he was admitted a fellow of the College.
It was during his life as a fellow that he began to work at chemistry in what he calls "the elaboratory in Trinity College." The room is now occupied by the Senior Bursar, and forms part of the beautiful range of buildings in the bowling green, which, freed from stucco and other desecration, are made visible in their ancient guise by the piety of a son of Trinity and the wisdom of the College authorities. It was here, according to Dr. Bentley, that "the thieving Bursars of the old set embezzled the College timber," {121} and it was this room that was fitted up as "an elegant laboratory" in 1706 for John Francis Vigani, an Italian chemist, who had taught unofficially in the University for some years, and became, in 1703, the first Professor of Chemistry at Cambridge.
Judging from his book, _Medulla Chymiae_, 1682, Vigani was an eminently practical person, who cared greatly about the proper make of a furnace and the form of a retort but was not c.u.mbered with theories.
Hales vacated his fellowship and became minister or perpetual curate of Teddington {122} in 17089, and there he lived until his death, fifty-two years afterwards. He was married (? 1719) and his wife died without issue in 1721.
He attracted the attention of Royalty, and received plants from the King"s garden at Hampton Court. Frederick Prince of Wales, the father of George III., is said to have been fond of surprising him in his laboratory at Teddington. This must surely be a unique habit in a prince, but we may remember that, in the words of the Prince"s mock epitaph, "Since it is only Fred there"s no more to be said." He became Clerk of the Closet to the Dowager Princess, and this "mother of the best of Kings," as she calls herself, put up his monument in Westminster Abbey. Hales had the honour of receiving the Copley Medal from the Royal Society in 1739, and Oxford made him a D.D. in 1733.
Some years ago I made a pilgrimage to Teddington, and found in the parish registers many interesting entries by his hand; the last, in a tremulous writing, is on November 4th, 1760, two months before he died. He was clearly an active parish priest. He made his female parishioners do public penance when he thought they deserved it. He did much for the fabric of the church. "In 1754 {123a} he helped the parish to a decent water supply and characteristically records in the parish register that the outflow was such as to fill a two-quart vessel in "three swings of a pendulum beating seconds, which pendulum was 39+2/10 inches long from the suspending nail to the middle of the plumbet or bob."" Under the tower he helped to build (which now serves as a porch) Stephen Hales is buried, and the stone which covers his body is being worn away by the feet of the faithful. By the piety of a few botanists a mural tablet, on which the epitaph is restored, has been placed near the grave.
Horace Walpole called Hales "a poor, good, primitive creature" and Pope {123b} (who was his neighbour) said, "I shall be very glad to see Dr.
Hales, and always love to see him, he is so worthy and good a man."
Peter Collinson writes of "his constant serenity and cheerfulness of mind"; it is also recorded that "he could look even upon wicked men, and those who did him unkind offices, without any emotion of particular indignation; not from want of discernment or sensibility, but he used to consider them only like those experiments which, upon trial, he found could never be applied to any useful purpose, and which he therefore calmly and dispa.s.sionately laid aside."
Hales" work may be divided into three heads:
I Physiological, animal and vegetable; II Chemical; III Inventions and miscellaneous essays.
Under No. I I shall deal only with his work on plants. The last heading (No. III) I shall only refer to slightly, but the variety and ingenuity of his miscellaneous publications is perhaps worth mention here as an indication of the quality of his mind. It seems to me to have had something in common with the versatile ingenuity of Erasmus Darwin and of his grandson Francis Galton. The miscellaneous work also exhibits Hales as a philanthropist, who cared pa.s.sionately for bettering the health and comfort of his fellow creatures by improving their conditions of life.
His chief book from the physiological and chemical point of view is his _Vegetable Staticks_. It will be convenient to begin with the physiological part of this book, and refer to the chemistry later.
_Vegetable Staticks_ is a small 8vo of 376 pages, dated on the t.i.tle-page 1727. The "_Imprimatur_ Isaac Newton Pr. Reg. Soc." is dated February 16, 1720, and this date is of some slight interest, for Newton died on March 20, and _Vegetable Staticks_ must have been one of the last books he signed.
The dedication is to George Prince of Wales, afterwards George III. The author cannot quite avoid the style of his day, for instance: "And as _Solomon_ the greatest and wisest of men, disdeigned {124} not to inquire into the nature of Plants, _from the __Cedar of Lebanon_, _to the Hyssop that springeth out of the wall_: So it will not, I presume, be an unacceptable entertainment to your Royal Highness," etc.
But the real interest of the dedication is its clear statement of his views on the nutrition of plants. He a.s.serts that plants obtain nourishment, not only from the earth, "but also more sublimed and exalted food from the air, that wonderful fluid, which is of such importance to the life of Vegetables and Animals," etc. We shall see that his later statement is not so definite, and it is well to rescue this downright a.s.sertion from oblivion.
His book begins with the research for which he is best known, namely that on transpiration. He took a sunflower growing in a flowerpot, covering the surface of the earth with a plate of thin milled lead, and cemented it so that no vapour could pa.s.s, leaving a corked hole to allow of the plant being watered. He did not take steps to prevent loss through the pot, but at the end of the experiment cut off the plant, cemented the stump, and found that the "unglazed porous pot" perspired 2 ozs. in 12 hours, and for this he made due allowance.
The plant so prepared he proceeded to weigh at stated intervals. He obtained the area of the leaves by dividing them into parcels according to their several sizes, and measuring one leaf {125} of each parcel. The loss of water in 12 hours converted to the metric system is 1.3 c.c. per 100 sq. cm. of leaf-surface; and this is of the same order of magnitude as Sachs" result, {126a} namely, 2.2 c.c. per 100 sq. cm.
He goes on to measure the surface of the roots {126b} and to estimate the rate of absorption per area. The calculation is of no value, since he did not know how small a part of the roots is absorbent, nor how enormously the surface of that part is increased by the presence of root-hairs. He goes on to estimate the rate of the flow of water up the stem; this would be 34 cubic inches in 12 hours if the stem (which was one square inch in section) were a hollow tube. He then allowed a sunflower stem to wither and to become completely dry, and found that it had lost of its weight, and a.s.suming that the of the "solid parts"
left was useless for the transmission of water he increases his 34 by ?
and gives 45? cubic inches in 12 hours as the rate. But the solid matter which he neglected contained the vessels, and he would have been nearer to the truth had he corrected his figures on this basis. The simplest plan is to compare his results with those obtained by Sachs {126c} in allowing plants to absorb solutions of lithium-salts. If the flow takes place through conduits equivalent to a quarter of a square inch in area, the fluid will rise in 12 hours to a height of 4+34 or 136 inches, or in one hour to 28.3 cm. {126d} This is a result comparable to, though very much smaller than, Sachs" result with the sunflower, viz. 63 cm. per hour.
The data are however hardly worth treating in this manner. But it is of historic interest to note that when Sachs was at work on his _Pflanzenphysiologie_, published in 1865, he was compelled to go back nearly 140 years to find any results with which he could compare his own.
We need not follow Hales into his comparison between the "perspiration"
of the sunflower and that of a man, nor into his other transpiration experiments on the cabbage, vine, apple, etc. But one or two points must be noted. He found {127a} the "middle rate of perspiration" of a sunflower in 12 hours of daylight to be 20 ounces, and that of a "dry warm night" about 3 ounces; thus the day transpiration was roughly seven times the nocturnal rate. This difference may be accounted for by the closure of the stomata at night, a phenomenon unknown to Hales.
Hales {127b} notes another point which a knowledge of stomatal behaviour might have explained, viz., that with "scanty watering the perspiration much abated"; he does not attempt an explanation, but merely refers to it as a "healthy lat.i.tude of perspiration in this sunflower."
In the course of his work on sunflowers he notices that the flower follows the sun. He says, however that it is "not by turning round with the sun," _i.e._ that it is not a twisting of the stalk, and goes on to call it _nutation_, which must be the _locus cla.s.sicus_ for the term used in this sense.
An experiment {128a} that I do not remember to have seen quoted elsewhere is worth describing. It is incidentally of interest as showing the generous scale on which his work was planned. An apple bough five feet long was fixed to a vertical gla.s.s tube nine feet long. The tube being above and the branch hanging below, the pressure of the column of water would act in concert with the suck of the transpiring leaves, instead of in opposition to this force. He then cut the bare stem of his branch in two, placing the apical half of the specimen (bearing side branches and leaves) with its cut end in a gla.s.s vessel of water; the basal and leafless half of the branch remained attached to the vertical tube of water. In the next 30 hours only 6 ounces dripped through the leafless branch, whereas the leafy branch absorbed 18 ounces. This, as he says, shows the great power of perspiration. And though he does not pursue the experiment, it is worthy of note as an attempt, like those of Janse {128b} and others, to correlate the flow of water under pressure with the flow due to transpiration.
It is interesting to find that Hales used the three methods of estimating transpiration which have been employed in modern times-namely, (i) weighing, (ii) a rough sort of potometer, (iii) enclosing a branch in a gla.s.s balloon and collecting the precipitated moisture, the well-known plan followed by various French observers.
He (_Vegetable Staticks_, p. 51) concluded his balance of loss and gain in transpiring plants by estimating the amount of available water in the soil to a depth of three feet, and calculating how long his sunflower would exist without watering. He further concludes (p. 57) that an annual rainfall of 22 inches is "sufficient for all the purposes of nature, in such flat countries as this about Teddington."
He constantly notes small points of interest, _e.g._ (p. 82) that with cut branches the water absorbed diminishes each day, and that the former vigour of absorption may be partly renewed by cutting a fresh surface.
{129a}
He also showed (p. 89) that the transpiration current can flow perfectly well from apex to base when the apical end is immersed in water.
These are familiar facts to us, but we should realise that it is to the industry and ingenuity of Hales that we owe them. In a repet.i.tion (p.
90) of the last experiment we have the first mention of a fact fundamentally important. He took two branches (which with a clerical touch he calls M and N), and having removed the bark from a part of the branch, dipped the ends in water, N with the great end downwards but M upside down. In this way he showed that the bark was not necessary for the absorption or transmission of water. {129b} I suspect that one branch was inverted out of respect for the hypothesis of sap-circulation.
He perhaps thought that water could travel apically by the wood, but only by the bark in the opposite direction.
Next in order (p. 95) comes his well-known experiment on the pressure exerted by peas increasing in size as they imbibe water. There are, however, pitfalls in this result of which Hales was unaware, and perhaps the chief interest to us now is that he considered the imbibition of the peas {130a} to be the same order of phenomenon as the absorption of water by a cut branch-notwithstanding the fact that he knew the absorption to depend largely on the leaves. {130b} It may be noticed that Sachs, in his imbibitional view of water-transport, may be counted a follower of Hales.
In order to ascertain "whether there was any lateral communication of the sap and sap vessels, as there is of blood in animals," Hales (p. 121) made the experiment which has been repeated in modern laboratories, {130c} _i.e._ cutting a "gap to the pith," and another opposite to it and a few inches above. This he did on an oak branch six feet long whose basal end was placed in water. The branch continued to "perspire" for two days, but gave off only about half the amount of water transpired by a normal branch. {130d} He does not trouble himself about this difference, being satisfied of "great quant.i.ties of liquor having pa.s.sed laterally by the gap."
He is interested in the fact of lateral transmission in connexion with the experiment of the suspended tree (Fig. 24, p. 126), which is dependent on the neighbours to which it is grafted for its water supply.
This seems to be one of the results that convinced him that there is a distribution of food material which cannot be described as circulation of sap in the sense that was then in vogue.
Hales (p. 143) was one of the first {131a} to make the well-known experiment-the removal of a ring of bark, with the result that the edge of bark nearest the base of the branch swells and thickens in a characteristic manner. He points out that if a number of rings are made one above the other, the swelling is seen at the lower edge of each isolated piece of bark, and therefore (p. 143) the swelling must be attributed "to some other cause than the stoppage of the sap in its return downwards," because the first gap in the bark should be sufficient to check the whole of the flowing sap. {131b} He must, in fact have seen that there is a redistribution of plastic material in each section of bark.
We now for the moment leave the subject of transpiration and pa.s.s on to that of root-pressure on which Hales is equally illuminating.
His first experiment (_Vegetable Staticks_, p. 100), was with a vine, to which he attached a vertical pipe made of three lengths of gla.s.s-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. 1067) used a mercury gauge, and registered a root-pressure of 32 inches or 36 feet 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.
a.s.similation.
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 t.i.tle-page as "An attempt to a.n.a.lyse 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 ill.u.s.trious 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-sh.e.l.ls, red-lead, gold, etc., and measuring the "air" given off from them. He also tried the effect of acid on iron filings, oyster-sh.e.l.ls, 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 t.i.tle 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 quant.i.ty of air in the Minium which burst the hermetically sealed gla.s.ses of the excellent _Mr. Boyle_, when he heated the Minium contained in them by a burning gla.s.s" (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 and the hydrogen disappears by combining with the oxygen set free. This was expressed in the language of the day as the reconstruction of metallic lead by the addition of phlogiston (the hydrogen) to the calx of lead (minium). Thorpe points out the magnitude of the discovery that Priestley missed, and it may be said that Hales too was on the track, and had he known as much as Priestley it would not have been phlogiston that kept him from becoming a Cavendish or Lavoisier. What chiefly concerns us, however, is the bearing of Hales" chemical work on his theories of nutrition. He concludes that "air makes a very considerable part of the substance of Vegetables," and goes on to say (p. 211) that "many of these particles of air" are "in a fixt state, strongly adhering to and wrought into the substance of" plants. {135a} He has some idea of the instability of complex substances, and of the importance of the fact, for he says {135b} that "if all the parts of matter were only endued with a strongly attracting power, [the] whole [of] nature would then become one unactive cohering lump." This may remind us of Herbert Spencer"s words: "Thus the essential characteristic of living organic matter, is that it unites this large quant.i.ty of contained motion with a degree of cohesion that permits temporary fixity of arrangement" (_First Principles_, -- 103). With regard to the way in which plants absorb and fix the "air"
which he finds in their tissues, Hales is not clear; he does not in any way distinguish between respiration and a.s.similation. But as I have already said, he definitely a.s.serts that plants draw "sublimed and exalted food" from the air.
As regards the action of light on plants, he suggests (p. 327) that "by freely entering the expanded surfaces of leaves and flowers" light may "contribute much to the enn.o.bling principles of vegetation." He goes on to quote Newton (_Opticks, query_ 30): "The change of bodies into light, and of light into bodies, is very conformable to the course of nature, which seems delighted with transformations." It is a problem for the antiquary to determine, whether or no Swift took from Newton the idea of bottling and recapturing sunshine as practised by the philosopher of Lagado. He could hardly have got it from Hales, since _Gulliver"s Travels_ was published in 1726, before _Vegetable Staticks_.