400 
, COM 
COM 
C O M 
liable, if wereco'lect that the quantity which 
appears during conibustion depends altoge- 
ther upou toe combustible. Phosphorus 
emits a vast quantity, charcoal a smaller, and 
hydrogen the smallest of all; ■set the quan- 
tity of oxygen which combines with the com- 
bu.dible during these processes, is greatest in 
those cases "where the light is smallest. Be- 
sides, the colour ot the light depends in all 
cases upon the combustible that burns ; a 
circumstance which could scarcely be sup- 
posed to take place, unless the light was se- 
arated from the combustible, It is well 
nown too, that when vegetables are made to 
grow in the dark, no combustible substances 
are formed in them ; the presence of light 
being absolutely necessary for the formation 
of these substances. These facts, and several 
others which might be enumerated, give a 
onsiderable degree of probability to the 
opinion that light constitutes a component 
part of all combustible substances ; but they 
by no means amount to a decisive proof. 
IS or indeed would it be easy to answer all 
the objections which might be started against 
this opinion. At the same time, it will be 
allowed that none of these objections amount 
to a positive proof of the falsehood of the 
hypothesis. It is always a proof of the diffi- 
culty of an investigation, and of the little pro- 
gress which has been made in it, when plau- 
sible arguments can be brought forward on 
bot h sides of the question. 
W ere we to suppose that the oxygen em- 
ployed in combustion contains caloric as a 
component part, while combustibles contain 
light, it would not be difficult to explain what 
takes place during combustion. The com- 
ponent parts of the oxygen so employed are 
two; namely, 1. a base; 2. caloric. The 
component parts of combustibles are like- 
wise two; namely, 1. a base; 2. light. Dur- 
ing combustion the base of the oxygen com- 
bines with the base of the combustible, and 
forms the product ; while at the same time 
the caloric of the oxygen combines with the 
light of the combustible, and the compound 
flies off in the form of lire. Thus combustion 
is a double decomposition ; the oxygen and 
combustible divide themselves each into two 
portions, which combine in pairs : the one 
compound is the product, and the other the 
fire which escapes. 
COMET, an opake, spherical, and solid 
body, like a planet, performing revolutions 
about the sun in elliptical Orbits, which have 
the sun in one of the foci. See Astrono- 
my, p. 177. 
COMETARIUM, a curious machine ex- 
hibiting an idea of the revolution of a comet 
about the sun. It is contrived in such a 
manner, as by elliptical wheels to shew the 
unequal motion of a comet in every part of 
its orbit. See Instruments Astrono- 
mical. 
COMETES, a genus of the pentandria 
monogvnia class and order. The involucrum 
is four-leaved, three-flowered ; calyx four- 
le tvedjVaps. tricoccous. There is one species, 
a native of Suratte. 
COMITIUM, in Roman antiquity, a large 
hall in the forum, where the comitia, or as- 
semblies of the people, were ordinarily held. 
This hall was a long time open at top, for 
which reason the assemblies were often in- 
terrupted by bad weather. It was first co- 
vered in the time of the second Punic war ; 
and according to Rosinus, the consuls and 
tiibuneswere not created in the cbmitium, 
but in the Campus Martius. 
COMMA, among grammarians, a point 
or character marked thus (,) ; serving to de- 
note a short stop, and to divide the members 
of a period. 
COMMANDING ground, in the mili- 
tary art, an eminence overlooking any post 
or strong place. A commanding ground is 
of three kinds : first, a front ground, lacing 
an height opposite to the face of some post, 
which plays upon its front. Secondly, a re- 
verse ground, being an eminence that can 
play upon the back of aiiy post. Thirdly, 
an enfilade commanding ground, or curtin 
commanding ground, being an eminence that 
with its shot can scour all the length of a 
straight line. 
COM MEN DAM. When a benefice is 
void, or to prevent its becoming void, it is 
commended to the charge of some sufficient 
clerk, to be supplied till it may be conve- 
niently provided with a pastor. When a 
parson is made bishop, there is a cession of 
his benefice by the promotion ; but if the 
king give him power to retain his benefice, 
lie shall continue parson, and is said to hold 
it in commendam; but it must be always be- 
fore consecration, for afterwards it conies too 
late, because the benefice is then absolutely 
void. 
COMMELINA, in botany, a genus of the 
triandria monogvnia class of plants, whose 
corolla consists of six petals ; the exterior 
three of which are small, oval, and concave, 
of the dimensions of die perianthium ; the 
three interior and alternate petals are large, 
roundish, and coloured. The fruit is a nak- 
ed, roundish capsule, containing three cells, 
find divided by three valves; the seeds, being 
two only, are angulated. There are 12 spe- 
cies. 
COMMENSURABLE, among geometri- 
cians, an appellation given to such quantities 
as are measured by one and the same com- 
mon measure : thus if a line A a be equal to 
8, and a line Bb equal to 4 inches, these 
two lines will be commensurable, since the 
same common measure 2, measures them 
both. 
Commensurable numbers, whether in- 
tegers or fractions, are such as can be mea- 
sured or divided bv some other number, 
without any remainder. Such are 12 and 18, 
as being measured by 6 or 3. 
Commensurable in pnzver, is said of 
right lines, when their squares are measured 
by one and the same space, or superficies. 
Commensurable surds, those that be- 
ing reduced to their least terms, become true 
figurative quantities -of their kind, and are 
therefore as a rational quantity to a rational 
one. 
COMMERCE, the exchange of commo- 
dities for other articles, or for some repre- 
sentative of value for which other commodi- 
ties can be procured. At first, commerce 
consisted merely in exchanging the surplus 
of some particular article of produce which 
an individual possessed beyond what he found 
necessary for his own use, for the similar 
surplus of some other article possessed by 
another individual, which kind of traffic is 
now generally called barter. By the intro- 
duction of metallic and paper money, the 
operations of commerce were much facili- 
tated, while the increase of population ren- 
dered them of greater magnitude: improve* 
ments in the art of navigation extended thei|i 
boundaries, the advancement of science and 
increase of luxury multiplied the objects of 
trade, and the various duties and regulation* 
established by different governments, renJ 
dered the subject more complicated to indil 
viduals, but at the same time of increase® 
importance to the state. 
The advantages of commerce began to en-j 
gage the attention of mankind a a very early 
period. About 1300 years after the Flood] 
the commerce of the Phoenicians had attained 
to a considerable extent: they possessed ma- 
nufactures, entered into commercial partner- 
ships, made long voyages, had resident agents 
in foreign countries, and had adopted the 
practice of lending money on bottomry. 
The Greeks and Romans, particularly the 
latter, carried on an extensive trade,* con- 
sidering the imperfect state of navigation. 
The reign of Augustus was particularly faJ 
vourable to commerce, as the general peace 
which then prevailed enabled the merchants 
to pursue their concerns unmolested. At 
this period, Malta w ; as famous for its number 
of workmen and its various manufactures, 
particularly of fine cloth. The inhabitants 
were the descendants of the antient Phoeni- 
cians, and, like them, pursued commerce 
with avidity. Under Tiberius we find the 
Romans extending their protection to the 
north ; and the town of Havern, the most 
antient in Friesland, founded. The culti- 
vation of vines in France, Spain, and Portu- 
gal, seems to have increased about this time ; 
and from the inhabitants’ neglecting to culti- 
vate corn, and turning much of their arable' 
land into vineyards, we may presume the, 
latter afforded considerable profit. About 
the year 50, the capital of England was first 
described as a place famous for merchants! 
and plenty of merchandize. 
The establishment of Christianity in the, 
northern parts of Germany, in consequence® 
of the conquests of Charlemagne, produced] 
a connection between those parts and the] 
southern parts of Europe, which naturally] 
led to an interchange ot produce, and gradu-j 
ally introduced foreign commerce, in the’: 
north. The Germans had, however, pre-1 
viousl y to this period, cultivated a commercial 
intercourse with the neighbouring states ; * 
which w T as afterwards much increased by the ] 
discovery of valuable mines in Saxony, and ] 
the establishment of some manufactures inj 
Flanders. 
In the 9th century, the Venetians carried 1 
on a very beneficial commerce with the Le- j 
vant ; whence they brought spices, silks, 1 
drugs, and fruits from the East in great abun- 
dance, and supplied the greater part of Eu- ] 
rope with these commodities. 
The crusades contributed materially to the 
revival of commerce during the 12th and 
13th centuries, by introducing an acquaint- 
ance with the luxuries of the East, which I 
multiplied the objects of foreign trade. About 
this time, the commercial towns bordering 
on the coasts of the Baltic, finding their 
trade much interrupted by pirates, entered j 
into an agreement for mutual support and | 
assistance. This union, which became very I 
celebrated under the title of the Hanseatic j 
league, at first consisted of only twelve j 
towns, but finally of upwards of seventy. 
