Dictionary of Chemistry [6th Ed.]

merly, carbynes were called methyli-

dynes.

carcinogen Any agent that pro-

duces cancer, e.g. tobacco smoke,

certain industrial chemicals, and

ionizing radiation (such as X-rays

and ultraviolet rays).

Carius methodA method of deter-

mining the amount of sulphur and

halogens in an organic compound, by

heating the compound in a sealed

tube with silver nitrate in concen-

trated nitric acid. The compound is

decomposed and silver sulphide and

halides are precipitated, separated,

and weighed.

carnallite A mineral consisting of a

hydrated mixed chloride of potas-

sium and magnesium, KCl.MgCl 2.

6H 2 O.

carnauba wax A natural wax ob-

tained from the leaves of the copaiba

palm of South America. It is ex-

tremely hard and brittle and is used

in varnishes and to add hardness and

lustre to other waxes in polishes.

Carnot, Nicolas Léonard Sadi

(1796–1832) French physicist, who

Ürst worked as a military engineer.

He then turned to scientiÜc research

and in 1824 published his analysis of

the efÜciency of heat engines. The

key to this analysis is the thermody-

namic *Carnot cycle and the even-

tual introduction of the idea of

*entropy in thermodynamics. He

died at an early age from cholera.

Carnot cycle The most efÜcient

cycle of operations for a reversible

*heat engine. Published in 1824 by

Nicolas *Carnot, it consists of four

operations on the working substance

in the engine (see illustration):

a. Isothermal expansion at thermody-

namic temperature T 1 with heat q 1

taken in.

b. Adiabatic expansion with a fall of

temperature to T 2.

c. Isothermal compression at temper-

ature T 2 with heat q 2 given out.

d. Adiabatic compression with a rise

of temperature back to T 1.

According to the Carnot principle, the

efÜciency of any reversible heat en-

gine depends only on the tempera-

ture range through which it works,

rather than the properties of the

working substances. In any reversible

engine, the efÜciency (η) is the ratio

of the work done (W) to the heat

input (q 1 ), i.e. η= W/q 1. As, according

to theÜrst law of *thermodynamics,

W = q 1 – q 2 , it follows that η=

(q 1 – q 2 )/q 1. For the Kelvin tempera-

ture scale, q 1 /q 2 = T 1 /T 2 and η=

(T 1 – T 2 )/T 1. For maximum efÜciency

T 1 should be as high as possible and

T 2 as low as possible.

carcinogen 104

c

adiabatic

compression

isothermal

compression

adiabatic

expansion

isothermal

expansion

T 1 q 1

T 2 q 2

V

P

Carnot cycle

carnotite A radioactive mineral

consisting of hydrated uranium

potassium vanadate,

K 2 (UO 2 ) 2 (VO 4 ) 2 .nH 2 O. It varies in col-

our from bright yellow to lemon- or

greenish-yellow. It is a source of ura-

nium, radium, and vanadium. The

chief occurrences are in the Colorado

Plateau, USA; Radium Hill, Australia;

and Katanga, Zaïre.

Carnot principle See carnot

cycle.