The times., 1900-12-30


with many of the then received atomi
weights. The assumption that equal vol
umes of gases contain equal numbers o
particles, as they were termed by him
molecules integrantes. was made in 1si
by Avogadro, professor of physics a
Turin Q77e-1S56) This theory, which ha
proved of the utmost Importance to th
sciences both of physics and of chemistry
had no doubt occurred to Gay-Lussac. ant
had been rejected by him for the follow
ing reasons: A certain volume of hydro
gen, say one cubic inch. may be supposed
to contain an equal number of particle:
(atoms) as an equal volume of chlorine
| Now, these two gases unite in equal vol
umes. The deduction appears so far quit,
legitimate that one atom of hydrogen ha:
combined with one atom of chlorine. Bu
the resulting gas occupies two cubit
inches. and must therefore contain the
same number of particles Of hydrogen
chloride, the compound of the two ele-
ments, as one cubic inch originally con
tafned of hydrogen. or of chlorine. Thus
we have two cubic inches containing, OF
uncombined gases, twice as many par
ticles as is contained in that volume after
solved the difficulty. By promising two
different orders of particles, now termed
atoms and molecules, the solution was
plain. According to him, each particle or
molecule of hydrogen is complex, and
contains two atoms: the same is the ease
with chlorine. When these gases combine,
or rather react, TO form hydrogen chloride,
the phenomenon is one of a change of
partners; the molecule, the double atom.
| of hydrogen splits; the same is the case
with the moleeule of chlorine; and each
liberated atom of hydrogen unites with a
liberated atom of chlorine, forming a COM
pound, hydrogen chloride, which equally
consists of molecule, or double atom.
| Thus two cubic inches of hydrogen chlo-
ride consists of a definite number of mole
cules, equal in number to those contained
| in cubic inch of hydrogen plus those
contained in cubic inch of chlorine. The
case is precisely similar, if other COM
| pounds of gases be considered.
! Berzelius was at first inclined to adopt
| this theory, and indeed went SO far as to
| change many of his atomic weights to
make them fit 1t. But later he somewhat
withdrew from his position. for 1t ap-
! extend to liquids and solids theory
which could be held only of gases. Avo-
| gadro's suggestion, therefore, rested in
allegiance until the publication. in 1858. by
Cannizzaro. now professor of chemistry in
Rome, of an essay in which all the argu.
ments in favor of the hypothesis were col
lected, and stated in mastery manner.
1t will be advisable to revert to this n.y.
pothesis at a later point, and to consider
other guides for the determination of
atomic weights.
In 1919, Dulong d7S5-1s3s), director of the
| Ecole Polyteehnique at Paris. and Petit
Q791-1s2O professor OF physics there, made
the discovery that equal amounts of heat
are required to raise equally the tempera-
ture OF solid and liquid elements. provided
quantities are taken proportional to their
atomic weights. Thus to raise the temper
ature of Sc grammes of Iron through one
degree requires approximately the same
amount of heat as is required TO raise
through -one degree 80 grammes OF sui
phur, 63.5 grammes Of copper, and so on;
these numbers representing the atomic
weights of the elements named. In other
words, equal numbers OF atoms have equal
units or calories Zone calory is the amount
of heat required to raise the temperature
or 1 gramme of water through / degree
centigrade) which is necessary to raise the
atomic weight expressed in grammes of
any solid or liquid element through / de-
gree centigrade iS approximately G2; 1t
varies between 81 and &C in actual part
This affords means of determining the
true value of the atomic weight of an ele.
ment, as the following example will show:
The analysis of the only compound of zinc
and chlorine shows that it contains a.. 40
per cent OF zinc and 52.16 per cent of chic
rine. Now, one grain OF hydrogen COM
bines with 35.5 grains of chlorine to form
36.5 grains of hydrogen chloride: and as
already remarked, one volume of hydro
gen and one volume of chlorine combine
forming two volumes of hydrogen chlo-
ride. Applying Avogadro's hypothesis,
one molecule Of hydrogen and one mole.
cule of chlorine react to yield two mole
cules of hydrogen chloride; and as each
molecule is supposed to consist In this
case of two atoms. hydrogen-chloride con-
sists of one atom of each of its constitu.
ent elements. The amount of that ele-
ID
grains of chlorine may give the numerical
value of the atomic weight Of the element,
If the compound contains one atom of
each element; in that case the formula of
the above compound would be zinc and
the atomic weight Of zinc, 32.7 but if the
formula is ZuCls, the atomic weightJof zinc
would be 32,7,t2: If ZuCls. 32.7x3, and so on. !
The specific heat of metallic zinc enables this |
question TO be solved For 1t has been found |
experimentally to be about 0.O95: and |
0.0g5=e5.2, close approximation to |
32.7x2=65.4. The conclusion is therefore |
drawn that zinc chloride is composed of one |
atom of zinc in combination with two atoms of |
chlorine, that the atomic weight of zinc is 654 |
and that the molecular weight of zinc chloride |
is 65.4+ (35.5>2i Inasmuch as the rela-
tive weight OF mole lie of hydrogen is 2 |
Chat of an atom being D. zinc chloride in
the gaseous state should be 1885, 4-:9=:isa
times that of hydrogen, measured at the
same temperature and pressure This has
been found experimentally to be the case.
The methods of determining the vapor
densities or relative weights of vapors, are
thrae in number: the first method, due to
Dumas 0S2O consists in vaporizing the
substance in question in bulb of glass or
Of porcelain, at n known temperature.
closing the bulb while stiIl hot, and weigh
ing it after it is cold. knowing the ca.
pacity of the bulb, the weight of hydro
gen necessary to fill it at the desired tenn
perature can be calculated, and the densi-
ty of the vapor thus arrived at. A second
method was devised by Gay-Lussac and
perfected by a. W. Hofmann QssS); and
a third, preferable for its simplicity and