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

same air to supply the

place of that destroyed by the explosion, which was again fired,

and the operation continued till almost the whole of the mixture

was let into the globe and exploded. By this means, though the

globe held not more than a sixth part of the mixture, almost the

whole of it was exploded therein without any fresh exhaustion of

the globe.”

 

At first this condensed matter was “acid to the taste and

contained two grains of nitre,” but Cavendish, suspecting that

this was due to impurities, tried another experiment that proved

conclusively that his opinions were correct. “I therefore made

another experiment,” he says, “with some more of the same air

from plants in which the proportion of inflammable air was

greater, so that the burnt air was almost completely

phlogisticated, its standard being one-tenth. The condensed

liquor was then not at all acid, but seemed pure water.”

 

From these experiments he concludes “that when a mixture of

inflammable and dephlogisticated air is exploded, in such

proportions that the burnt air is not much phlogisticated, the

condensed liquor contains a little acid which is always of the

nitrous kind, whatever substance the dephlogisticated air is

procured from; but if the proportion be such that the burnt air

is almost entirely phlogisticated, the condensed liquor is not at

all acid, but seems pure water, without any addition

whatever.”[2]

 

These same experiments, which were undertaken to discover the

composition of water, led him to discover also the composition of

nitric acid. He had observed that, in the combustion of hydrogen

gas with common air, the water was slightly tinged with acid, but

that this was not the case when pure oxygen gas was used. Acting

upon this observation, he devised an experiment to determine the

nature of this acid. He constructed an apparatus whereby an

electric spark was passed through a vessel containing common air.

After this process had been carried on for several weeks a small

amount of liquid was formed. This liquid combined with a solution

of potash to form common nitre, which “detonated with charcoal,

sparkled when paper impregnated with it was burned, and gave out

nitrous fumes when sulphuric acid was poured on it.” In other

words, the liquid was shown to be nitric acid. Now, since nothing

but pure air had been used in the initial experiment, and since

air is composed of nitrogen and oxygen, there seemed no room to

doubt that nitric acid is a combination of nitrogen and oxygen.

 

This discovery of the nature of nitric acid seems to have been

about the last work of importance that Cavendish did in the field

of chemistry, although almost to the hour of his death he was

constantly occupied with scientific observations. Even in the

last moments of his life this habit asserted itself, according to

Lord Brougham. “He died on March 10, 1810, after a short

illness, probably the first, as well as the last, which he ever

suffered. His habit of curious observation continued to the end.

He was desirous of marking the progress of the disease and the

gradual extinction of the vital powers. With these ends in view,

that he might not be disturbed, he desired to be left alone. His

servant, returning sooner than he had wished, was ordered again

to leave the chamber of death, and when be came back a second

time he found his master had expired.[3]

JOSEPH PRIESTLEY

While the opulent but diffident Cavendish was making his

important discoveries, another Englishman, a poor country

preacher named Joseph Priestley (1733-1804) was not only

rivalling him, but, if anything, outstripping him in the pursuit

of chemical discoveries. In 1761 this young minister was given a

position as tutor in a nonconformist academy at Warrington, and

here, for six years, he was able to pursue his studies in

chemistry and electricity. In 1766, while on a visit to London,

he met Benjamin Franklin, at whose suggestion he published his

History of Electricity. From this time on he made steady

progress in scientific investigations, keeping up his

ecclesiastical duties at the same time. In 1780 he removed to

Birmingham, having there for associates such scientists as James

Watt, Boulton, and Erasmus Darwin.

 

Eleven years later, on the anniversary of the fall of the Bastile

in Paris, a fanatical mob, knowing Priestley’s sympathies with

the French revolutionists, attacked his house and chapel, burning

both and destroying a great number of valuable papers and

scientific instruments. Priestley and his family escaped violence

by flight, but his most cherished possessions were destroyed; and

three years later he quitted England forever, removing to the

United States, whose struggle for liberty he had championed. The

last ten years of his life were spent at Northumberland,

Pennsylvania, where he continued his scientific researches.

 

Early in his scientific career Priestley began investigations

upon the “fixed air” of Dr. Black, and, oddly enough, he was

stimulated to this by the same thing that had influenced

Black—that is, his residence in the immediate neighborhood of a

brewery. It was during the course of a series of experiments on

this and other gases that he made his greatest discovery, that of

oxygen, or “dephlogisticated air,” as he called it. The story of

this important discovery is probably best told in Priestley’s own

words:

 

“There are, I believe, very few maxims in philosophy that have

laid firmer hold upon the mind than that air, meaning atmospheric

air, is a simple elementary substance, indestructible and

unalterable, at least as much so as water is supposed to be. In

the course of my inquiries I was, however, soon satisfied that

atmospheric air is not an unalterable thing; for that, according

to my first hypothesis, the phlogiston with which it becomes

loaded from bodies burning in it, and the animals breathing it,

and various other chemical processes, so far alters and depraves

it as to render it altogether unfit for inflammation,

respiration, and other purposes to which it is subservient; and I

had discovered that agitation in the water, the process of

vegetation, and probably other natural processes, restore it to

its original purity….

 

“Having procured a lens of twelve inches diameter and twenty

inches local distance, I proceeded with the greatest alacrity, by

the help of it, to discover what kind of air a great variety of

substances would yield, putting them into the vessel, which I

filled with quicksilver, and kept inverted in a basin of the same

…. With this apparatus, after a variety of experiments …. on

the 1st of August, 1774, I endeavored to extract air from

mercurius calcinatus per se; and I presently found that, by means

of this lens, air was expelled from it very readily. Having got

about three or four times as much as the bulk of my materials, I

admitted water to it, and found that it was not imbibed by it.

But what surprised me more than I can express was that a candle

burned in this air with a remarkably vigorous flame, very much

like that enlarged flame with which a candle burns in nitrous

oxide, exposed to iron or liver of sulphur; but as I had got

nothing like this remarkable appearance from any kind of air

besides this particular modification of vitrous air, and I knew

no vitrous acid was used in the preparation of mercurius

calcinatus, I was utterly at a loss to account for it.”[4]

 

The “new air” was, of course, oxygen. Priestley at once

proceeded to examine it by a long series of careful experiments,

in which, as will be seen, he discovered most of the remarkable

qualities of this gas. Continuing his description of these

experiments, he says:

 

“The flame of the candle, besides being larger, burned with more

splendor and heat than in that species of nitrous air; and a

piece of red-hot wood sparkled in it, exactly like paper dipped

in a solution of nitre, and it consumed very fast; an experiment

that I had never thought of trying with dephlogisticated nitrous

air.

 

“… I had so little suspicion of the air from the mercurius

calcinatus, etc., being wholesome, that I had not even thought of

applying it to the test of nitrous air; but thinking (as my

reader must imagine I frequently must have done) on the candle

burning in it after long agitation in water, it occurred to me at

last to make the experiment; and, putting one measure of nitrous

air to two measures of this air, I found not only that it was

diminished, but that it was diminished quite as much as common

air, and that the redness of the mixture was likewise equal to a

similar mixture of nitrous and common air…. The next day I was

more surprised than ever I had been before with finding that,

after the above-mentioned mixture of nitrous air and the air from

mercurius calcinatus had stood all night, … a candle burned

in it, even better than in common air.”

 

A little later Priestley discovered that “dephlogisticated air .

. . is a principal element in the composition of acids, and may

be extracted by means of heat from many substances which contain

them…. It is likewise produced by the action of light upon

green vegetables; and this seems to be the chief means employed

to preserve the purity of the atmosphere.”

 

This recognition of the important part played by oxygen in the

atmosphere led Priestley to make some experiments upon mice and

insects, and finally upon himself, by inhalations of the pure

gas. “The feeling in my lungs,” he said, “was not sensibly

different from that of common air, but I fancied that my

breathing felt peculiarly light and easy for some time

afterwards. Who can tell but that in time this pure air may

become a fashionable article in luxury? … Perhaps we may from

these experiments see that though pure dephlogisticated air might

be useful as a medicine, it might not be so proper for us in the

usual healthy state of the body.”

 

This suggestion as to the possible usefulness of oxygen as a

medicine was prophetic. A century later the use of oxygen had

become a matter of routine practice with many physicians. Even in

Priestley’s own time such men as Dr. John Hunter expressed their

belief in its efficacy in certain conditions, as we shall see,

but its value in medicine was not fully appreciated until several

generations later.

 

Several years after discovering oxygen Priestley thus summarized

its properties: “It is this ingredient in the atmospheric air

that enables it to support combustion and animal life. By means

of it most intense heat may be produced, and in the purest of it

animals will live nearly five times as long as in an equal

quantity of atmospheric air. In respiration, part of this air,

passing the membranes of the lungs, unites with the blood and

imparts to it its florid color, while the remainder, uniting with

phlogiston exhaled from venous blood, forms mixed air. It is

dephlogisticated air combined with water that enables fishes to

live in it.”[5]

KARL WILHELM SCHEELE

The discovery of oxygen was the last but most important blow to

the tottering phlogiston theory, though Priestley himself would

not admit it. But before considering the final steps in the

overthrow of Stahl’s famous theory and the establishment of

modern chemistry, we must review the work of another great

chemist, Karl Wilhelm Scheele (1742-1786), of Sweden, who

discovered oxygen quite independently, although later than

Priestley. In the matter of brilliant discoveries in a brief

space of time Scheele probably eclipsed all his great

contemporaries. He had a veritable genius for interpreting

chemical reactions and discovering new substances, in this

respect rivalling Priestley himself. Unlike Priestley, however,

he planned all his experiments along the lines of definite

theories from the beginning, the results obtained