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by the legal profession, he turned his attention to
medicine, and became a medical practitioner at
Bath, where he lived during the fashionable season.
When not more than twenty - five years of age,
he wrote two essays on Eespiration, ascribing the
inflation of the lungs to the action of the inter-
costal muscles. These " Tractatus duo " were
published in 1668. Some years later he produced
the treatise on which his fame rests; it is en-
titled " Tractatus quinque medico-physici, quorum
primus agit de sal-nitro et spiritu nitro-aereo;
secundus, de respiratione ; tertius, de respiratione
foetus in utero et ovo ; quartus, de motu muscu-
lari, et spiritibus animalibus ; ultimus, de rhachi-
tide ; studio Joh. Mayow, LL.D. & Medici, nee



i BOYLE, MAYOW, AND HALES 17

non Coll. Omn. Anim. in Univ. Oxon. Socii. Oxonii
e Theatre Sheldoniano, An. Dom. MDCLXXIV." The
work was dedicated to Mr. Henry Coventry. It was
inserted in an abridged form in the Philosophical
Transactions of the Koyal Society some time after
its publication, but received only scant recognition,
for the fame of Newton and Boyle overshadowed
the labours of less well-known investigators. And
Mayow did not live to press his discoveries on
the attention of his contemporaries, for he died in
October 1679, five years after the publication
of his tracts, in his thirty-seventh year. Little is
known of Mayow's domestic life, save that he
married shortly before his death. His scientific
work proves that if he had been granted the usual
span of life, his extraordinary genius would have
furthered the knowledge of the true explanation
of the nature of air, and its function in sup-
porting combustion and respiration, and that his
views would have been accepted more than a
century before Lavoisier with fuller knowledge,
and with the scientific position which at once
gained a hearing forced precisely similar doc-
trines upon the attention of the scientific world.
Mayow was a contemporary of Boyle, and fre-



i8 THE GASES OF THE ATMOSPHERE CHAP.

quently made use of Boyle's experiments in support of
the theories which he advanced. Curiously enough,
while Boyle seems to have read Mayow's work, he
does not appear to have been favourably impressed
by his conclusions. Boyle, at the age of fifty-
two, had doubtless formed his own opinions, and
was unwilling that they should be disturbed by the
speculations, well founded though they were, of so
young a man. And shortly after Mayow's death,
the views of Becher, one of his contemporaries,
expounded and made definite by Stahl, regarding
the nature of combustion, were universally received.
After Lavoisier's theories had overthrown these
false views, attention was again directed to Mayow's
tracts, first by Blumenbach, in his Institutiones
Physiological; later by T. Beddoes, in 1790, who
wrote a digest of Mayow's work under the title
"Chemical Experiments and Opinions extracted
from a Work published in the Last Century " ; and
later by Johann Andreas Scherer, in a work pub-
lished at Vienna in 1793, and also by Dr. Yeats in
1798. Scherer gives a careful analysis of Mayow's
work, somewhat altering the order of his paragraphs,
with a paraphrase in German of the Latin text, which
he quotes in full. Yeats' treatise is more especially



I BOYLE, MAYOW, AND HALES 19

concerned with the medical aspect of Mayow's work,
although it also deals with the purely chemical por-
tion at considerable length. 1 In the following account
of Mayow's researches free use has been made of
both of these works, as well as of his own " Tracts."
Mayow's contributions to the chemistry of the
atmosphere may be classified thus :

1. The atmosphere consists of particles of two
kinds of gas at least : one of these, termed
" nitro- aerial particles," is necessary for the sup-
port of life and for the combustion of inflammable
bodies ; while the other, left after this constituent
has been removed, is incapable of supporting either
life or combustion. The portion which is necessary
for life enters, during respiration, into the blood. It
is the chief cause of motion in animals and in
plants.

2. These " nitro - aerial particles " are also
present in saltpetre or nitre, as can be shown by
mixing inflammable substances, such as sulphur and
charcoal, with nitre to form gunpowder, filling a
tube with the powder, and, after setting it on fire,
immediately plunging the open end of the tube

1 An English translation of Mayow's chemical researches has been
published by Prof. Crum Brown in the Alembic Club Reprints (James
Thin, Edinburgh, 1907).



20 THE GASES OF THE ATMOSPHERE CHAP.

under water. The sulphur and charcoal will be
as completely consumed as if burned in the open
air. Such combustion might, however, be ascribed
to a " sulphureous " constituent in saltpetre ; by
"sulphureous" is to be understood combustible,
for those substances capable of burning were
imagined to contain a " sulphur " which gave
them that property. That nitre does not con-
tain such " sulphur" can be shown by expos-
ing it alone to heat, when no change takes place,
except fusion. Besides, nitre is compounded of
" spirit of nitre " or nitric acid and pure alkali,
neither of which contains a combustible sulphur;
hence the particles of fire-air must be present in
nitre in no small amount. But it is probable that
it is the spirit of nitre which contains such fire-air
particles, because, as will be shown later, they are
not present in the alkali.

One difficulty occurs to Mayow. How is it
that so large a quantity of gas as is necessary
to support combustion can be contained in a
relatively small bulk of saltpetre ? He tries whether
a solution of saltpetre evolves air -bubbles when
placed in a vacuum, and finds that it effervesces
less than pure water does. He also prepares



I BOYLE, MAYOW, AND HALES 21

saltpetre by mixing nitric acid and alkali in a
vacuum ; a brisk effervescence occurs, and the
dried- up salt is ordinary saltpetre. Hence salt-
petre cannot contain elastic air. Mayow conse-
quently draws a distinction between "air" and
" air-particles."

The residue left after the " fire-air," or spiritus
igneo-aerius, has been' removed from ordinary air
by breathing or by combustion is proved to be
lighter than the fire-air itself; because a mouse
dies sooner if kept at the top of air in a con-
fined bell-jar than at the bottom ; and a candle
goes out sooner. Here the conclusion is right,
although the reason given is wrong ; for it is the
temperature of the respired air which makes it
rise, and not the fact that it is specifically lighter
than the oxygen.

Metallic antimony gains in weight when it is
set on fire by a lens, and burns ; if this gain in
weight, Mayow remarks, is not due to the absorp-
tion of nitro-aerial particles and to the fire, it is
difficult to say to what it is due.

The reason why substances burn so violently
in nitre compared with air, is because of the
proximity of the fire-air particles; and these are



22 THE GASES OF THE ATMOSPHERE CHAP.

evidently due to the nitric acid, because the
residue, the alkali, if mixed with sulphur and
inflamed, does not produce ignition.

2. All acids contain fire-air particles, for acids
have great similarity to each other. This is shown
as follows : Antimony made into a calx by the
sun's rays with a burning-glass gives the same
calx as when it is evaporated repeatedly with nitric
acid and converted into " Bezoar-mineral," i.e. oxide
of antimony. And iron-rust obtained from sulphide
of iron appears to be formed by the union of the
fire-air particles with the metallic " sulphur " of the
iron.

It has up till now been believed that sulphuric
acid is an ingredient of common sulphur. But
this is unlikely, for sulphur has a sweetish, and not
an acid taste. Moreover, quite a different sub-
stance from a vitriol (or sulphate) is obtained by
melting together alkali and sulphur ; and no
effervescence takes place during its preparation.
Sulphur, too, is precipitated out of the "liver of
sulphur" (potassium persulphide) by the addition
of sulphuric acid. Now, were sulphuric acid con-
tained in sulphur, it would hinder the union of the
sulphur with the alkali.



i BOYLE, MAYOW, AND HALES 23

It is to be noticed that the volatile sulphuric
acid, from the combustion of sulphur, is produced
in the following way : " The flame of the burning
sulphur consists, like every other flame, in the
violent motion of the sulphur particles with that
of the nitro-aerial particles ; hence the sulphur
particles, at first solid, become sharp and acid, and
probably form the ordinary * spirit of sulphur '
(sulphuric acid). If this be not so, I know not
in what manner this acid can be produced ; for, as
has been shown, it is very improbable that it
previously existed in the mass of the sulphur
before its deflagration. Such a change also, in
all probability, takes place in pyrites, when it is
converted to green vitriol ; because pyrites yields
sulphur on distillation; and the green vitriol on
distillation gives sulphuric acid, leaving red col-
cothar (iron oxide) behind."

Similarly, nitre appears to be a triple salt, formed
by the union of the fiery part of air with a salt-
like substance existing in the earthy material,
together forming nitric acid ; and this, added to
earthy salts (alkali), yields ordinary nitre. " I have
tried to show that all acids consist of certain saline
particles rendered fluid by the nitro-aerial particles.'



24 THE GASES OF THE ATMOSPHERE CHAP.

4. Boyle has shown that a flame is extin-
guished more rapidly in a vacuous space than in
a confined space containing air ; this is obviously
due to absence of nourishment in the air, rather
than to its choking by its own vapours ; for in the
vacuous vessel there is evidently more space for
such noxious vapours than in the air-filled vessel,
and yet the flame is more rapidly extinguished.
Moreover, no combustible matter can be kindled in
a vacuum by means of a burning-glass. But it
must not be concluded that this fire-air constitutes
the whole of ordinary air ; because a candle goes
out in air confined in a glass while a large quantity
of air is still contained in it.

While gunpowder burns owing to the fire-air
particles which it contains, and requires no susten-
ance from external air, the combustion of vege-
tables is supported partly by the igno- aerial
particles which they themselves contain, partly by
those of the external air.

Air which has supported combustion loses to
some extent its elasticity (i.e. diminishes in
volume), as shown by the burning of a candle
in air confined over water. This is to be ascribed
partly to actual loss of elasticity, partly to the



i BOYLE, MAYOW, AND HALES 25

absorption of the fire-air. The loss of volume
amounts to about three per cent of the whole
quantity of air taken.

All this is exceedingly clear, and in accordance
with our modern views, but Mayow's mind is some-
what confused with reference to flame and heat, since
he imagined that the diminution of the volume of air
in which combustible substances have been burned
is due to the escape of heat ; and inasmuch as a rise
of temperature was known to increase the volume
of air, so a loss of heat should, in his opinion, pro-
duce the opposite effect. The fire-air particles are
apparently regarded as a sort of compound of heat
with matter (as indeed in a certain sense they are) ;
and by combustion or by respiration both are
removed. The loss of volume is to be explained
by the removal of both from the air, and the gain
in weight by the union of the matter with the
combustible body, such as antimony.

Such is a brief account of Mayow's views on
the nature of atmospheric air. But the tale would
be incomplete without mention of the fact that he
prepared a gas by the action of nitric acid on iron,
viz. nitric oxide, which, when introduced into ordi-
nary air confined over water, decreased its volume;



26 THE GASES OF THE ATMOSPHERE CHAP

and he found that further admission of nitric oxide
produced no further diminution in the volume of
the air. A very little more, and he would have
recognised in this a means of analysing air, and
depriving it wholly of its oxygen. He goes so far
as to speculate that a compound is formed between
the nitric oxide and the oxygen, but the solubility
of gases in water appears not to have struck him as
important. He notices, however, that the combina-
tion of the two gases is attended by rise of tem-
perature, and is in so far analogous to combustion.

It would lead us too far to consider in detail
Mayow's theories of fermentation and of respira-
tion. Suffice it to say that he ascribes the pro-
duction of animal heat to the consumption of his
fire-air particles by the animal, and remarks that
the pulse is heightened by respiration. This view
was in opposition to that held by his contem-
poraries, viz. that the purpose of respiration was to
cool the blood.

It is impossible to avoid being impressed with
the clearness and justice of Mayow's inferential
reasoning. All that was wanting was the dis-
covery of oxygen and carbon dioxide, and the
identification of the first with his fire-air, and of



i BOYLE, MAYOW, AND HALES 27

the second with one of the products of combus-
tion. But these discoveries were not made until a
century after his death. Had he lived, there can
be little doubt that, unless discouraged by the
want of appreciation with which his ideas were
received, he would have continued to labour in the
fruitful fields from which he had already reaped
so rich a harvest.

Before leaving the seventeenth century, it is
perhaps fitting to mention the name of Jean Key,
a French physician, who wrote in 1630 concerning
the gain in weight of tin and lead when calcined.
While Key exhibited some leaning to wards the modern
methods of experimentation, he still lay fettered in
the bonds of mediaeval scholasticism. In discuss-
ing the weight of air and fire, he finds occasion to
consider the question whether a vacuum can exist.
His words are so quaint that they are worth
quoting : " It is quite certain that in the bounds of
Nature a vacuum, which is nothing, can find no
place. There is no power in nature from which
nothing could have made the universe, and none
which could reduce the universe to nothing : that
requires the same virtue. Now the matter would
be otherwise if there could be a vacuum. For if it



28 THE GASES OF THE ATMOSPHERE CHAP.

could be here, it could also be there ; and being
here and there, why not elsewhere ? and why not
everywhere ? Thus the universe could reach anni-
hilation by its own forces ; but to Him alone who
could make it is the glory of being able to compass
its destruction." And since air cannot be drawn
down by a vacuum, it must descend by virtue of its
own weight when it fills a hole. And hence, as
air has weight, tin and lead gain in weight when
they combine with air. It will be admitted that
this is very inferior to the speculations and deduc-
tions of Boyle and Mayow.

The next stage in the history of our subject is
the consideration of the work of Stephen Hales
and of Joseph Priestley. Both of these philoso-
phers were essentially experimentalists. While
both discovered gases and prepared them in a more
or less pure state, Hales had no theory to guide
him, and concluded as the result of his researches
that air was possessed of " a chaotic nature " ; for
he did not recognise his gases as different kinds of
matter, but supposed them all to be modified air.
Priestley, on the other hand, was an adherent of
the theory of phlogiston, and interpreted all his
experiments by its help. Hales was a country



i BOYLE, MAYOW, AND HALES 29

clergyman, interested in botany, and undertook
researches on air in order to gain knowledge of the
growth and development of plants. Priestley was
also a divine, who amused himself with experi-
ments during the intervals of composing sermons
or writing controversial pamphlets on disputed
doctrines. Both possessed the experimental faculty,
and both employed it to good purpose.

Hales' chief work is entitled " Statical Essays,
containing Vegetable Staticks ; or an account of
Statical Experiments on the Sap in Vegetables,
being an Essay towards a Natural History of Vege-
tation : of use to those who are curious in the
Culture and Improvement of Gardening, etc. : Also,
a specimen of an attempt to analyse the air by a
great Variety of Chymiostatical Experiments, which
were read at several meetings before the Eoyal
Society. By Stephen Hales, D.D., F.R.S., Rector
of Farringdon, Hampshire, and Minister of Ted-
dington, Middlesex."

In his " Introduction" Hales reveals his method
of research. The determination of weight and
volume was at that date especially necessary ; for
want of numerical data the experimental researches
of the time were of a somewhat vague character,



30 THE GASES OF THE ATMOSPHERE CHAP.

and it often happened that the conclusions drawn
from them were incorrect. Hence it is with a feel-
ing of satisfaction that we read (vol. i. p. 2) :

"And since we are assured that the all- wise
Creator has observed the most exact proportions of
number, weight, and measure in the make of all
things, the most likely way, therefore, to get any
insight into the nature of those parts of the crea-
tion which come within our observation must in all
reason be to number, weigh, and measure. And
we have much encouragement to pursue this method
of searching into the nature of things, from the
great success which has attended any attempts of
this kind." For God has " comprehended the dust
of the earth in a measure, and weighed the mount-
ains in scales, and the hills in a balance."

From experiments on the rise of sap in plants,
many of them very ingenious and well adapted to
secure their end, and which are still regarded by
botanists as classic, Hales noticed that a quan-
tity of air was inspired by plants. In order to
ascertain the composition and amount of this
air, the process of distillation was resorted to ; for
Hales remarks : " That elasticity is no immutable
property of air is further evident from these ex-



I BOYLE, MAYOW, AND HALES 31

periments ; because it were impossible for such
great quantities of it to be confined in the sub-
stances of animals and vegetables, in an elastick
state, without rending their constituent parts
with a vast explosion" (Preface, p. viii). Hence,
concluding that the air absorbed by plants and
animals could be recovered by their distillation,
Hales proceeded to distil a great number of sub-
stances of animal and vegetable origin, such as
hogs' blood, tallow, a fallow-deer's horn, oyster-
shell, oak, wheat, peas, amber, tobacco, camphor,
aniseed oil, honey, bees'-wax, sugar, Newcastle coal,
earth, chalk, pyrites, a mixture of salt and bone-
ash, of nitre and bone-ash, tartar, compound aqua-
fortis, and a number of other substances. He col-
lected the " air " in each case over water, and gave
numerical data to show what proportion the air
bore by weight to the substance from which it had
been obtained. He even tried to compare the
weight of ordinary air with that of air from
distilled tartar ; but his experiment led to no posi-
tive conclusion, because of the crudeness of his
appliances. The compressibility or " elasticity " of
the air from tartar, however, was found to be iden-
tical with that of common air.



32 THE GASES OF THE ATMOSPHERE CHAP.

Hales does not appear to have made any special
experiments on the properties of his various airs,
by trying whether they supported combustion,
whether they were themselves combustible, etc.
We see from this list that he had under his hands
mixtures of hydrocarbons, carbon dioxide, probably
sulphur dioxide, hydrochloric acid and ammonia
(both, however, dissolving in water as they were
formed), oxides of nitrogen, possibly chlorine, and,
as minium or red-lead was one of the substances he
tried, oxygen in a more or less pure state. It must
be remembered that in all cases the gas obtained was
mixed with the air originally present in the retort.
He next proceeded to produce "air" by the fermenta-
tion of grain, of raisins, and of other fruits ; this "air"
obviously was carbon dioxide more or less pure.

It is curious to note here that he anticipated
Lord Kelvin in devising a sounding -lead which
should register the depth of the sea by the com-
pression of air, the distance to which the air
had receded along the tube being shown by the
entry of treacle. He successfully carried out a
sounding by means of his apparatus.

The next series of experiments related to the
generation of " air " by the action of acids on



i BOYLE, MAYOW, AND HALES 33

metals. Aqua-regia and gold, aqua-regia and
antimony, aquafortis and iron, dilute oil-of-vitriol
and iron, yielded gases which contracted on stand-
ing in contact with water. This, in the case of the
oxides of nitrogen, is to be ascribed to their reacting
with the oxygen of the air accidentally present in the
receiver ; but in the last case Hales noticed that the
gas absorbed in cold weather was re-evolved on rise
of temperature, as one would expect with hydrogen.

These experiments led him to investigate the
action of certain mixtures on ordinary air. Thus a
mixture of spirits of hartshorn (or ammonia) with
iron filings absorbed lj cubic inches of air, and
one with copper filings twice as much. Further,
a mixture of iron filings and brimstone absorbed in
two days no less than 19 cubic inches of air.

But it is disappointing to find that, in spite of
all the experimental facts which Hales accumulated,
he was unable to make use of them. The prejudice
in favour of the unity and identity of all these
" airs " was too great for him to overcome. True,
he sometimes theorises a little, as for example when
he remarks (p. 285) : " If fire was a particular kind
of body inherent in sulphur (i.e. combustible matter
of all kinds), as Mr. Homberg, Mr. Lemery, and



34 THE GASES OF THE ATMOSPHERE CHAP.

some others imagine, then such sulphureous bodies,
when ignited, should rarefy and dilute all the
circumambient air ; whereas it is found by many
of the preceding experiments, that acid sulphureous
fuel constantly attracts and condenses a considerable
part of the circumambient elastick air : an argument
that there is no fire endued with peculiar pro-
perties inherent in sulphur ; and also that the heat
of fire consists principally in the brisk vibrating
action and re-action between the elastick repelling
air and the strongly attracting acid sulphur, which
sulphur in its Analysis is found to contain an in-
flammable oil, an acid salt, a very fix'd earth, and
a little metal."

Enough has now been said to give a fair idea
of Stephen Hales' researches. It will suffice if his
conclusions be stated in his own words (p. 314) :

" Thus, upon the whole, we see that air abounds
in animal, vegetable, and mineral substances ; in
all which it bears a considerable part; if all the
parts of matter were only endued with a strongly
attracting power, whole nature would then imme-
diately become one unactive cohering lump ; where-
fore it was absolutely necessary, in order to the
actuating and enlivening this vast mass of attracting



I BOYLE, MAYOW, AND HALES 35

matter, that there should be everywhere intermix'd
with it a due proportion of strongly repelling
elastick particles, which might enliven the whole
mass, by the incessant action between them and
the attracting particles ; and since these elastick
particles are continually in great abundance reduced
by the power of the strong attracters, from an
elastick to a fixt state, it was therefore necessary
that these particles should be endued with a pro-
perty of resuming their elastick state, whenever
they were disengaged from that mass in which
they were fixt, that thereby this beautiful frame of
things might be maintained in a continual round
of the production and dissolution of animal and
vegetable bodies.

" The air is very instrumental in the production
and growth of animals and vegetables, both by
invigorating their several juices while in an elastick
active state, and also by greatly contributing in a
fix'd state to the union and firm connection of several
constituent parts of those bodies, viz. their water,
salt, sulphur, and earth. This band of union, in
conjunction with the external air, is also a very
powerful agent in the dissolution and corruption of
the same bodies ; for it makes one in every ferment-


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