A History of Science, vol 4 by Henry Smith Williams (the two towers ebook .TXT) 📕

statement that a metal, for

example, was composed of phlogiston and an element was much less

enigmatic, even if wrong, than the statement of the alchemist

that “metals are produced by the spiritual action of the three

principles, salt, mercury, sulphur”—particularly when it is

explained that salt, mercury, and sulphur were really not what

their names implied, and that there was no universally accepted

belief as to what they really were.

 

The metals, which are now regarded as elementary bodies, were

considered compounds by the phlogistians, and they believed that

the calcining of a metal was a process of simplification. They

noted, however, that the remains of calcination weighed more than

the original product, and the natural inference from this would

be that the metal must have taken in some substance rather than

have given off anything. But the phlogistians had not learned

the all-important significance of weights, and their explanation

of variation in weight was either that such gain or loss was an

unimportant “accident” at best, or that phlogiston, being light,

tended to lighten any substance containing it, so that driving it

out of the metal by calcination naturally left the residue

heavier.

 

At first the phlogiston theory seemed to explain in an

indisputable way all the known chemical phenomena. Gradually,

however, as experiments multiplied, it became evident that the

plain theory as stated by Stahl and his followers failed to

explain satisfactorily certain laboratory reactions. To meet

these new conditions, certain modifications were introduced from

time to time, giving the theory a flexibility that would allow it

to cover all cases. But as the number of inexplicable experiments

continued to increase, and new modifications to the theory became

necessary, it was found that some of these modifications were

directly contradictory to others, and thus the simple theory

became too cumbersome from the number of its modifications. Its

supporters disagreed among themselves, first as to the

explanation of certain phenomena that did not seem to accord with

the phlogistic theory, and a little later as to the theory

itself. But as yet there was no satisfactory substitute for this

theory, which, even if unsatisfactory, seemed better than

anything that had gone before or could be suggested.

 

But the good effects of the era of experimental research, to

which the theory of Stahl had given such an impetus, were showing

in the attitude of the experimenters. The works of some of the

older writers, such as Boyle and Hooke, were again sought out in

their dusty corners and consulted, and their surmises as to the

possible mixture of various gases in the air were more carefully

considered. Still the phlogiston theory was firmly grounded in

the minds of the philosophers, who can hardly be censured for

adhering to it, at least until some satisfactory substitute was

offered. The foundation for such a theory was finally laid, as

we shall see presently, by the work of Black, Priestley,

Cavendish, and Lavoisier, in the eighteenth century, but the

phlogiston theory cannot be said to have finally succumbed until

the opening years of the nineteenth century.

 

II. THE BEGINNINGS OF MODERN CHEMISTRY

 

THE “PNEUMATIC” CHEMISTS

 

Modern chemistry may be said to have its beginning with the work

of Stephen Hales (1677-1761), who early in the eighteenth century

began his important study of the elasticity of air. Departing

from the point of view of most of the scientists of the time, be

considered air to be “a fine elastic fluid, with particles of

very different nature floating in it” ; and he showed that these

“particles” could be separated. He pointed out, also, that

various gases, or “airs,” as he called them, were contained in

many solid substances. The importance of his work, however, lies

in the fact that his general studies were along lines leading

away from the accepted doctrines of the time, and that they gave

the impetus to the investigation of the properties of gases by

such chemists as Black, Priestley, Cavendish, and Lavoisier,

whose specific discoveries are the foundation-stones of modern

chemistry.

JOSEPH BLACK

The careful studies of Hales were continued by his younger

confrere, Dr. Joseph Black (1728-1799), whose experiments in the

weights of gases and other chemicals were first steps in

quantitative chemistry. But even more important than his

discoveries of chemical properties in general was his discovery

of the properties of carbonic-acid gas.

 

Black had been educated for the medical profession in the

University of Glasgow, being a friend and pupil of the famous Dr.

William Cullen. But his liking was for the chemical laboratory

rather than for the practice of medicine. Within three years

after completing his medical course, and when only twenty-three

years of age, he made the discovery of the properties of carbonic

acid, which he called by the name of “fixed air.” After

discovering this gas, Black made a long series of experiments, by

which he was able to show how widely it was distributed

throughout nature. Thus, in 1757, be discovered that the bubbles

given off in the process of brewing, where there was vegetable

fermentation, were composed of it. To prove this, he collected

the contents of these bubbles in a bottle containing lime-water.

When this bottle was shaken violently, so that the lime-water and

the carbonic acid became thoroughly mixed, an insoluble white

powder was precipitated from the solution, the carbonic acid

having combined chemically with the lime to form the insoluble

calcium carbonate, or chalk. This experiment suggested another.

Fixing a piece of burning charcoal in the end of a bellows, he

arranged a tube so that the gas coming from the charcoal would

pass through the lime-water, and, as in the case of the bubbles

from the brewer’s vat, he found that the white precipitate was

thrown down; in short, that carbonic acid was given off in

combustion. Shortly after, Black discovered that by blowing

through a glass tube inserted into lime-water, chalk was

precipitated, thus proving that carbonic acid was being

constantly thrown off in respiration.

 

The effect of Black’s discoveries was revolutionary, and the

attitude of mind of the chemists towards gases, or “airs,” was

changed from that time forward. Most of the chemists, however,

attempted to harmonize the new facts with the older theories—to

explain all the phenomena on the basis of the phlogiston theory,

which was still dominant. But while many of Black’s discoveries

could not be made to harmonize with that theory, they did not

directly overthrow it. It required the additional discoveries of

some of Black’s fellow-scientists to complete its downfall, as we

shall see.

HENRY CAVENDISH

This work of Black’s was followed by the equally important work

of his former pupil, Henry Cavendish (1731-1810), whose discovery

of the composition of many substances, notably of nitric acid and

of water, was of great importance, adding another link to the

important chain of evidence against the phlogiston theory.

Cavendish is one of the most eccentric figures in the history of

science, being widely known in his own time for his immense

wealth and brilliant intellect, and also for his peculiarities

and his morbid sensibility, which made him dread society, and

probably did much in determining his career. Fortunately for him,

and incidentally for the cause of science, he was able to pursue

laboratory investigations without being obliged to mingle with

his dreaded fellow-mortals, his every want being provided for by

the immense fortune inherited from his father and an uncle.

 

When a young man, as a pupil of Dr. Black, he had become imbued

with the enthusiasm of his teacher, continuing Black’s

investigations as to the properties of carbonic-acid gas when

free and in combination. One of his first investigations was

reported in 1766, when he communicated to the Royal Society his

experiments for ascertaining the properties of carbonic-acid and

hydrogen gas, in which he first showed the possibility of

weighing permanently elastic fluids, although Torricelli had

before this shown the relative weights of a column of air and a

column of mercury. Other important experiments were continued by

Cavendish, and in 1784 he announced his discovery of the

composition of water, thus robbing it of its time-honored

position as an “element.” But his claim to priority in this

discovery was at once disputed by his fellow-countryman James

Watt and by the Frenchman Lavoisier. Lavoisier’s claim was soon

disallowed even by his own countrymen, but for many years a

bitter controversy was carried on by the partisans of Watt and

Cavendish. The two principals, however, seem. never to have

entered into this controversy with anything like the same ardor

as some of their successors, as they remained on the best of

terms.[1] It is certain, at any rate, that Cavendish announced

his discovery officially before Watt claimed that the

announcement had been previously made by him, “and, whether right

or wrong, the honor of scientific discoveries seems to be

accorded naturally to the man who first publishes a demonstration

of his discovery.” Englishmen very generally admit the justness

of Cavendish’s claim, although the French scientist Arago, after

reviewing the evidence carefully in 1833, decided in favor of

Watt.

 

It appears that something like a year before Cavendish made known

his complete demonstration of the composition of water, Watt

communicated to the Royal Society a suggestion that water was

composed of “dephlogisticated air (oxygen) and phlogiston

(hydrogen) deprived of part of its latent heat.” Cavendish knew

of the suggestion, but in his experiments refuted the idea that

the hydrogen lost any of its latent heat. Furthermore, Watt

merely suggested the possible composition without proving it,

although his idea was practically correct, if we can rightly

interpret the vagaries of the nomenclature then in use. But had

Watt taken the steps to demonstrate his theory, the great “Water

Controversy” would have been avoided. Cavendish’s report of his

discovery to the Royal Society covers something like forty pages

of printed matter. In this he shows how, by passing an electric

spark through a closed jar containing a mixture of hydrogen gas

and oxygen, water is invariably formed, apparently by the union

of the two gases. The experiment was first tried with hydrogen

and common air, the oxygen of the air uniting with the hydrogen

to form water, leaving the nitrogen of the air still to be

accounted for. With pure oxygen and hydrogen, however, Cavendish

found that pure water was formed, leaving slight traces of any

other, substance which might not be interpreted as being Chemical

impurities. There was only one possible explanation of this

phenomenon—that hydrogen and oxygen, when combined, form water.

 

“By experiments with the globe it appeared,” wrote Cavendish,

“that when inflammable and common air are exploded in a proper

proportion, almost all the inflammable air, and near one-fifth

the common air, lose their elasticity and are condensed into dew.

And by this experiment it appears that this dew is plain water,

and consequently that almost all the inflammable air is turned

into pure water.

 

“In order to examine the nature of the matter condensed on firing

a mixture of dephlogisticated and inflammable air, I took a glass

globe, holding 8800 grain measures, furnished with a brass cock

and an apparatus for firing by electricity. This globe was well

exhausted by an air-pump, and then filled with a mixture of

inflammable and dephlogisticated air by shutting the cock,

fastening the bent glass tube into its mouth, and letting up the

end of it into a glass jar inverted into water and containing a

mixture of 19,500 grain measures of dephlogisticated air, and

37,000 of inflammable air; so that, upon opening the cock, some

of this mixed air rushed through the bent tube and filled the

globe. The cock was then shut and the included air fired by

electricity, by means of which almost all of it lost its

elasticity (was condensed into water vapors). The cock was then

again opened so as to let in more of the