thus forms a part of the plant, and from this much of the I of
commerce is obtained. Algae are collected in the spring, on the
coasts of Ireland, Scotland, and Normandy, where rough weather
throws them up. They are dried, and finally burned or distilled;
the ashes are leached to dissolve I salts; the water is nearly
evaporated, and the residue is treated with H2SO4, and MnO2, as
in the case of Br and Cl. I also occurs in Chili, as NaI and
NaIO3, mixed with NaNO3. This is an important source of the I
supply.

167. Uses.--I is much used in medicine, and was formerly employed
in taking daguerreotypes and photographs. Its solution in alcohol
or in ether is known as tincture of iodine.

168. Fluorine.--F, Cl, Br, I, are called halogens or haloids, and
exist in compounds--salts--in sea water. F is the most active of
all elements, combining with every element except O. Until
recently it has never been isolated, for as soon as set free from
one compound it attacks the nearest substance, and seems to be as
much averse to combining with itself, or to existing in the
elementary state, as to uniting with O. It is supposed to be a
gas, and, as is claimed, has lately been isolated by electrolysis
from HF in a Pt U-tube. Fluorite (CaF2) and cryolite (Al2F6 + 6
NaF) are its two principal mineral sources. The enamel of the
teeth contains F in composition.

CHAPTER XXXIII.

THE HALOGENS.

169. Halogens Compared.--The elements F, Cl, Br, I, form a
natural group. Their properties, as well as those of their
compounds, vary in a step-by-step way, as seen below. F is
sometimes an exception. They are best remembered by comparing
them with one another. Notice:

1. Similarity of name-ending. Each name ends in ine.

2. Similarity of origin. Salt water is the ultimate source of
all, except F.

3. Similarity of valence. Each is usually a monad.

4. Similarity of preparation. Cl, Br, I, are obtained from their
salts by means of MnO2 end H2SO4.

5. Variation in occurrence. Cl occurs in sea-salt, Br in sea-
water, I in sea-weed.

6. Variation in color; F being colorless, Cl green, Br red, I
violet.

7. Gradation in sp. gr.; F 19, Cl 35.5, Br 80, I 127.

8. Gradation in state, corresponding to sp. gr.; F being a light
gas, Cl a heavy gas, Br a liquid, I a solid.

9. Corresponding gradation in their usual chemical activity; F
being most active, then Cl, Br, and I.

10. Corresponding gradation in the strength of the H acids; the
strongest being HF, the next, HCl, etc.

11. Corresponding gradation in the explosibility of their N
compounds; the strongest NCl3, the next, NBr3, etc.

12. Corresponding gradation in the number of H and O acids; Cl 4,
Br 3, I 2.

170. Compounds.--The following are some of the oxides, acids, and
salts of the halogens. Name them.


CI2O (+H2O=) 2 HClO. The salts are hypochlorites, as Ca(ClO)2.
Cl2O3 (+H20=) 2 HClO2. The salts are chlorites, as KClO2.
Cl2O4
-- HClO3 The salts are chlorates, as KClO3.
-- HClO4 The salts are perchlorates, as KClO4,
-- HBrO	The salts are ? KBrO,
-- -- The salts are wanting.
-- HBrO3.	The salts are ? KBrO3,
-- HBrO4.	The salts are ? KBrO4,
-- -- The salts are wanting.
-- -- The salts are wanting.
I2O5 (+H2O=) 2 HIO3. The salts are ? KIO3.
-- HIO4. The salts are ? KIO4.


F forms no oxides, and no acids except HF. HF, HCl, HBr, HI, are
striking illustrations of acids with no O. HClO4 is a very strong
oxidizing agent. A drop of it will set paper on fire, or with
powdered charcoal explode violently. This is owing to the ease
with which it gives up 0. Notice why its molecule is broken up
more readily than HC103. The higher the molecular tower, or the
more atoms it contains, the greater its liability to fall. Some
organic compounds contain hundreds of atoms, and hence are easily
broken down, or, as we say, are unstable. Inorganic compounds
are, as a rule, much more stable than organic ones. It is not
always true, however, that the compound with the least number of
atoms is the most stable. SO2 is more stable than SO3, but H2SO3
is less so than H2SO4.
Chapter XXXIV.

VAPOR DENSITY AND MOLECULAR WEIGHT.

Examine a liter measure, in the form of a cube,--cubic decimeter,
--and a cubic centimeter.

171. Gaseous Weights and Volumes.--A liter of  H, at 0 degrees
and 760 mm., weighs nearly 0.09 g. This weight is called a crith.
Find the weight of H in the following, in criths and in grams: 15
1., 0.07 1., 50.3 1., 0.035 1., 0.6 1..

It has been estimated that there are (10) 24. molecules of H in a
liter. Does the number vary for different gases? The weight of a
molecule of H in parts of a crith is 1/(10) 24.; in parts of a
gram .09/(10) 24.. If the H molecule is composed of 2 atoms, what
is the weight of its atom in fractions of a crith? What in
fractions of a gram? The weight of the H atom is a microcrith.
What part of a crith is a microcrith?

172. Vapor Density.--Vapor density, or specific gravity referred
to H as the standard, (Physics) is the ratio of the weight of a
given volume of a gas or vapor to the weight of the same volume
of H. A liter of steam weighs nine times as much as a liter of H.
Its vapor density is therefore nine. For convenience, a definite
volume of H is usually taken as the standard, viz., the H atom.
The volume of the H atom and that of the half-molecule of H2O, or
of any gas are identical, each being represented by one square.
If, then, the standard of vapor density is the H atom, half the
molecular weight of a gas must be its vapor density, since it is
evident that we thus compare the weights of equal volumes. The
vapor density of H2O, steam, is found from the symbol as follows:
(2 + 16) / 2 = 9. To obtain the vapor density of any compound
from the formula, we have only to divide its molecular weight by
two. Find the vapor density of HCl, N2O, NO, C12H22O11, Cl, CO2,
HF, SO2. Explain each case.

The half-molecule, instead of the whole, is taken; because our
standard is the hydrogen atom, the smallest portion of matter, by
weight, known to science.

How many criths in a liter of HCl? How many grams? Compute the
number of criths and of grams in one liter of the compounds whose
symbols appear above.

PROBLEMS.

(1) A certain volume of H weighs 0.36 g. at standard temperature
and pressure. How many liters does it contain? If one liter
weighs 0.09 g., to weigh 0.36 g. it will take 0.36 / 0.09 = 4
liters.

(2) How many liters, or criths, of H in 63 g.? 2.7 g.? 1 g.? 5
g.? 250 g.? Explain each.

(3) Suppose the gas to be twice as heavy as H, how many liters in
0.36 g.? A liter of the gas will weigh 0.18 g. (0.09 X 2). In
0.36 g. there will be 0.36 / 0.18 = 2. Answer the question for 63
g., 2.7 g., etc.

(4) How many liters of Cl in each of the above numbers of grams?

(5) How many of HCl? H2O (steam)? CO2? Explain fully every case.

Vapor density is very easily determined from the formula by the
method given above. But in practice the formula is obtained from
the vapor density, and hence the method there given has to be
reversed.

173. Vapor Density of Oxygen.--Suppose we were to obtain the
vapor density of O. We should carefully seal and weigh a given
volume, say a liter, at a noted temperature and barometric
pressure, which are reducedto 0 degrees and 760 mm, and compare
it with the weight of the same volume of H. This has been done
repeatedly, and O has been found to weigh 16 times as much as H,
volume for volume, or, more exactly, 15.96+. Now a liter of each
gas has the same number of molecules, therefore the O molecule
weighs 16 times the H molecule. The half-molecule of each has the
same proportion, and the vapor density of O is 16. Atomic weight
is obtained in a very different way.

PROBLEMS.

(1) A liter of Cl is found to weigh 3.195 g. Compute its vapor
density, and explain fully.

(2) A liter of Hg vapor, under standard conditions, weighs 9 g.
Find its vapor density, and explain.

The vapor density of only a few elements has been satisfactorily
determined. See page 12. Some cannot be vaporized; others can be,
but only under conditions which prevent weighing them. The vapor
density of very many compounds also is unknown.

(3) A liter of CO2 weighs 1.98 g. Find the vapor density, and
from that the molecular weight, remembering that the latter is
twice the former. See whether it corresponds to that obtained
from the formula, CO2. This is,in fact, the way a formula is
ascertained, if the atomic weights of its elements are known.

(4) A liter of a compound gas weighs 2.88 g. Analysis shows that
its weight is half S and half O. As the atomic weight of S is 32,
and that of O is 16, what is the symbol for the gas?

Solution. Its molecular weight is 64, i.e. (2.88=0.09) X 2, of
which 32 is S and 32 O. The atomic weight of S is 32, hence there
is one atom of S, while of O there are two atoms. The formula is
SO2.

(5) A liter of a compound gas, which is found to contain 1 C and
3 O by weight, weighs 1.26 g. What is its formula? Atomic weights
are taken from page 12. Prove your answer.

(6) A liter of a compound of N and O weighs 1.98 g. The N is
7/11; and the O 4/11. What is the gas?

(7) A compound of N and H gas weighs 0.765 g. to the liter. The N
is 14/17 of the whole, the H 3/17. What gas is it? CHAPTER XXXV.

ATOMIC WEIGHT.

174. Definition.--We have seen that the molecular weight of a
compound, as well as of most elements, is obtained from the vapor
density by doubling the latter. It remains to explain how atomic
weights are obtained. The term is rather misleading. The atomic
weight of an element is its least combining weight, the smallest
portion that enters into chemical union, which is, of course, the
weight of an atom.

175. Atomic Weight of Oxygen.--Suppose we wish to find the atomic
weight of oxygen. We must find the smallest proportion by weight
in which it occurs in any compound. This can only be done by
analyzing all the compounds of O that can be vaporized. As
illustrative of these compounds take the six following:--


                                    	       Wt. of other

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