492 
THE BIRTH OF CHEMISTRY 
IX, 
Early Ideas concerning the Process of Combustion. —Association 
of Nitre with the Air, so far as the part they play in Combus- 
tion is concerned.—Hooke's Theory of Combustion. —Mayow's 
Lxperiments.—Early Pneumatic Chemistry.—Proof of the 
Analogy existing between Combustion and Respiration, 
AS in the history of matter we find molecules grouping them- 
selves around a common centre or a common line, thus 
constituting crystalline bodies, so in the history of sciences and 
of nations we may often observe well-defined axes, about which 
the facts of particular epochs congregate, Such axes are to be 
found in the history of chemistry. At the particular period of 
which we now write, the facts of the science mainly grouped 
themselves around theories connected with combustion, which 
involved as collateral matters conceptions regarding the nature 
of calcination, and of the air, 
Combustion was, and still is, the most prominent exhibition 
of chemical force, with which man ordinarily comes into con- 
tact. Itis a purely chemical action—the union of dissimilar 
bodies under the influence of chemical affinity, attended by the 
evolution of light and heat. Many attempts were made to ex- 
plain its cause. Fire, in common with earth, air, and water, 
as we have before seen, was regarded as an element, till almost 
within our own memory. Epicurus regarded heat as a congeries 
of minute spherical paficles possessing rapid motion, and 
readily insinuating themselves into the densest bodies. Fire was 
simply an intense form of heat. Cardanus speaks of flame as 
aer accensus, and of fire as heat immensely augmented. During 
the Middle Ages the existence of two kinds of fire was ad- 
mitted—the one pure celestial fire “sudtlis ignis,” ‘* calestis 
ignis,” the pricciple or essence of fire; the other ‘‘ gross earthly 
fire,” or ‘‘mundane fire.” The latter was the saveria, the 
former the forma, Celestial fire became mundane fire when it 
was associated with combustible bodies, that is, in ordinary 
combustion, Seneca tells us that the Egyptians divided each 
element into an active and & passive form; fire became 
active flame which burns, and comparatively passive warmth 
and light. The elemental nature of fire was nor universally ad 
mitted during the Middle Ages ; thus Francis Bacon asseris, in 
the Novum Organum, that fire is ‘‘ merely compounded of the 
conjunction of light and heat in any substance,” and he defines 
heat as a rapid motion of material particles, Athanasius 
Kircher, in his ponderous treatise, ** drs Magna Lucis et Um- 
ére,” affirms that fire is air which is causet to glow by the 
violent collision of bodies, by which means combustible bodies 
become flame. At an early date it was observed that fire cannot 
exist without air; the experiment of burning a canile in a 
closed vessel was well known. Some affirmed that ‘‘air is the 
food of fire,” some that ‘‘air nourishes fire.” The influence of 
a blast of air upon fire was well recognised ; we have seen that 
bellows were known at a very early date. When nitre—which 
for many centuries was one of the most important bodies in 
chemistry—came to be known, it was soon noti.ed that it pro- 
duces intense ignition; that, in fact, to direct a blast of air 
upon a red-hot coal, or to throw some nitre upon it, produced the 
same result, viz. greatly augmented combustion. Hence aro-e 
the idea that nitre and the air are in some way connected, for 
‘*things which are equal to the same are equal to each other.” 
This association of ideas may seem crude to us now, yet we 
must remember that nitre produces rapid cembustion simply 
because it contains a great quantity of that constituent of the 
air, oxygen gas, which ordinarily produces combustion. Tnus 
the old natural philosophers, wandering in the dim twilight of 
experimental kn »wledge, were not so far wrong in th-ir suppo- 
sition. The idea men ioned above was very prevalent two cen- 
turies ago : Robert Boyle speaks of the pre-ence of a ‘* volatile 
nitre” in the air; Lord Bacon says that nitre contains a ‘‘volatile, 
crude, and windy spirit” ; Clark attribu'ed thunder and light- 
ning to the presence of nitre in the air ; Gassendi imagined that 
minute particles of nitre are diffused throughout the atmosphere. 
When we heat lead or tin in a current of air, these metals are 
respectively converted into a powder, or ca/x, and calcination 
was one of the most important processes in old chemistry. Cal- 
cination seemed to be due more or less directly to tie air; and 
metals could also be calcined by heating them with nitre, or with 
the spirit of nitre—nitric acid; hence arose another bond of 
connection between nitre and the air; at least, they had some- 
thing in common. Lemery in his ‘‘ Cour de Chimie,”’ published 
NATURE 
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in 1675, affirms that the acid of nitre contains a number of — 
‘*corpuscules ignées” locked up in it, and he defines these latter 
as ‘a sub le matter, which having been thrown into a very rapid — 
motion, still retains the power of moving with impetuosity, even 
when it is enclosed in grosser matter; and when it finds some 
bodies which by their texture or figure are apt to be put into 
motion, it drives them about so strongly that, their parts rub- 
bing violently against each other, heat is thereby produced.” 
Thus recognising the causes which had Id to the association 
of the air with nitre, at least so far as they are both concerned 
in the production of combustion, we are prepared to examine 
Robert Hooke’s theory of combustion, The announcement of 
this theory marks an important history in the theory of chemis- 
try ; it was the first chemical theory worthy of the name, and it 
gave a far more just and accurate explanation of combustion 
than the crude and over-belauded theory of Phlogiston, of 
Beccher and Stahl. Hooke'’s theory was, moreover, founded — 
upon experiment, and although unfortunately he does not describe 
the experiments, we see at a glance that it could not have been 
constructed without such means. ‘* This hypothesis,”’ he writes, 
“T have endeavoured to raise from an infinity of observations — 
and experiments,” and all who know Hooke’s writings, are well 
aware how good an experimenter he was. The theory was pub- 
lished in 1665 in Hooke’s ‘‘ Micrographia ;” it is there found 
(Observation 16) buried ina mass of irrelevant matter, and to 
this cause may, perhaps, to some extent be attributed the fact 
that it has been so little recognised and known. ‘The theory is 
stated in twelve propositions, the principal of which are as 
follows :— 
1. That the air is the “ universal dissolvent of all sulphureous 
bodies.” 
Sulphur was long regarded as the type of combustible bodies, 
on account of its ready inflammability ; some even derive the 
name trom sa/, wip, the silt of fire. By sulphureous bodies, 
Hooke simply meant combustible bodies, viz. bodies that can burn 
in a supporter of combustion. By air being the ‘‘ universal dis- 
solvent,” he meant that through the agency of air combustible 
bodies are caused to become transformed into similarly invisible — 
substances, [For instance, we burn a pound of wood, and a 
few grains of ash remain, the rest has disappeared into air; 
as we say now, it has been converted mto carbonic anhydride © 
gas ; as Hooke sad then, it has been dissolved by the air. re 
2. ‘* That this action it (the air) performs not until the body — 
be sufficiently heated,” ‘ ‘ o- 
In mvre modern phraseology, every combustible possesses its — 
special igniting point, phosphorus 92° F., sulphur 482° F., and — 
so On. 
3. ‘‘ That this action of dissolution produces or generates a 
very great hea\, and that which we call Five.” aed : 
4. ‘That thi. action is performed with so great a violence, — 
and does so minutely act, and rapidly agitate the smallest parts — 
of the combustible matter, that it produces in the diaphanous 
medium of the air the action, or pulse, of Lighe. 
This would seem to iniicate that Ho»ke considered light to 
be an intensified form of heat, and to be generated in the same 
manner, and to be a kid of very rapid motions 
5. ‘* That the dissolution of suphurous bodies is made by a 
substance inherent and mixed w th the air, that is like, if mot the 
very same with, that which is fixed in saltpe re.” 
Hooke had evidently traced the connection between certain 
actions produced by the air and by saltpe:re or nitre; and he 
says it may be readily demonstrated that combustion is effected 
by that coustituent of the air which is fixed in saltpetre. This 
is a remarkable assertion, because oxygen gas was not discovered 
until more than a century after tue proposition of Hooke's . 
theory ; and we now know that nitre contains ‘‘ fixed” init the 
same substance—uxygen gas—which causes air to “dissolve” . 
combusti ‘le bodies It is probable that the connection between 
air and nitre may have been rendered the more probable in the 3 
minds of Hooke and his contemporaries by the knowledge that | 
guopowder will burn in a space devoid of air; thus, if sulphar 
and charcoal burn in air, and consume air in burning, and if 
nitre will cause them to burn out of con act with air it would 
surely appear that nitre must contain ar, or one of its com- 
ponents. 
1o. ‘* That the dissolving parts of the air are but few, .. . 4 
whereas saltpetre is a menstruum, when melted and rd hot, 
that abounds more with these dissolvent particles, and therelore — 
as a small quantity of it will dissolve a great sulphureous body, 
so Will the dissolution be very quick eid valet “a 
