Names.	             V. d.  Mol. Wt.  Wt. of O.	  Elem.        Symbol.
Carbon monoxide...    14       28	16	   12	          ?
Carbon dioxide....    22       44	32	   12	          ?
Hydrogen monoxide...   9       18	16	    2	          ?
Nitrogen monoxide...  22       44	16	   28	          ?
Nitrogen trioxide...  38       76	48	   28	          ?
Nitrogen pentoxide... 54      108	80	   28	          ?


176. Molecular Symbols.--From the vapor density of the gases--
column 2--we obtain their molecular weight-- column 3. To find
the proportion of O, it must be separated by chemical means from
its compounds and separately weighed. These relative weights are
given in column 4. Now the smallest weight of O which unites in
any case is its atomic weight. If any compound of O should in
future be found in which its combining weight is 8 or 4, that
would be called its atomic weight. By dividing the numbers in
column 4, wt. of O, by 16, the atomic weight of O, we obtain the
number of O atoms in the molecule. Subtracting the weights of O
from the molecular weights, we have the parts of the other
elements, column 5, and dividing these by the atomic weight of
the respective elements, we have the number of atoms of those
elements, these last, combined with the number of O atoms, give
the symbol. In this way complete the last column.

Show how to get the atomic weight of Cl from these compounds,
arranging them in tabular form, and completing as above: HCl,
KCl, NaCl, ZnCl2, MgCl2; the atomic weight of N in these: N2O,
NO, NH3.

177. Molecular and Atomic Volumes.--We thus see that vapor
density and atomic weight are obtained in two quite different
ways. In the case of elements the two are usually identical, i.e.
with the few whose vapor density is known; but this is not always
true, and it leads to interesting conclusions regarding atomic
volume. In O both vapor density and atomic weight are 16. This
gives 2 atoms of O to the molecule, i.e. the molecular weight /
the atomic weight. The size of an O atom is therefore half the
gaseous molecule, and is represented by one square. S has a vapor
density and an atomic weight of 32 each. Compute the number of
atoms in the molecule. Compute for I, in which the two are
identical, 127. P has an atomic weight of 31, while its vapor
density is 62. Its molecule must consist of 4 atoms, each half
the size of the H atom, The vapor density of As is 150, the
atomic weight 75. Compute the number of atoms in its molecule,
and represent their relative size. Hg has an atomic weight of
200, a vapor density of 100. Compute as before, and compare the
results with those on page 12. Ozone has an atomic weight of 16,
a vapor density 24. Compute.

Chapter XXXVI.

DIFFUSION AND CONDENSATION OF GASES.

178. Diffusion of Gases.--Oxygen is 16 times as heavy as H. If
the two gases were mixed, without combining, in a confined space,
it might be supposed that O would settle to the bottom and H rise
to the top. This would, in fact, take place at first, but only
for an instant, for all gases tend to diffuse or become
intimately mixed. The lighter the gas the more quickly it
diffuses.

179. Law of Diffusion of Gases.--The diffusibility of gases
varies inversely as the square roots of their vapor densities.
Compare the diffusibility of H with that of O. dif. H:dif. O::
sqrt(16): sqrt(1), or dif: H: dif. O:: 4: 1.

That is to say, if H and O be set free from separate receivers in
a room, the H will become intermingled with the atmosphere four
times as quickly as the O. Compare the diffusibility of O and N;
of Cl and H. Take the atomic weights of these, since they are the
same as the vapor densities. In case of a compound gas, half the
molecular weight must be taken for the vapor density; e.g. dif.
N20: dif. O.:: sqrt(16): sqrt(22).

180. Cause.--Diffusion is due to molecular motion; the lighter
the gas the more rapid the vibration of its molecules. Compare
the diffusibility of CO2 and that of Cl; of HCl and SO2; of HF
and I.

181. Liquefaction and Solidification of Gases.--Water boils at
100 degrees, under standard pressure, though evaporating at all
temperatures; it vaporizes at a lower point if the pressure be
less, as on a mountain, and at a higher temperature if the
pressure be greater, as at points below the sea level. Alcohol
boils at 78 degrees, standard pressure, and every liquid has a
point of temperature and pressure above which it must pass into
the gaseous state. Likewise every gas has a critical temperature
above which it cannot be liquefied at any pressure.

This condition was not recognized formerly, and before 1877, O,
H, N, C4, CO, NO, etc., had not been liquefied, though put under
a pressure of more than 2,000 atmospheres. They were called
permanent gases. In 1877 Cailletet and Pictet liquefied and
solidified these and others. The lowest temperature, about -225
degrees, was produced by suddenly releasing the pressure from
solid N to 4mm, which caused it rapidly to evaporate.
Evaporation, especially under diminished pressure, always lowers
the temperature by withdrawing heat.

These low degrees are indicated by a H thermometer, or if too low
for that, by a "thermo-electric couple" of copper and German
silver.

The pupil can easily liquefy SO, by passing it through a U-tube
which is surrounded by a mixture of ice and salt in a large
receiver. At the meeting of the American Association for the
Advancement of Science in 1887, a solid brick of CO2 was seen and
handled by the members, Liquid H is steel blue.

A few results obtained under a pressure of one atmosphere are:--
Boiling Points: C2H4--102 degrees; CH4--184 degrees; O--181
degrees; N --194 degrees; CO--190 degrees; NO--154 degrees; Air--
191 degrees.

Solidifying Points: Cl -102 degrees; HCl -115 degrees; Ether -129
degrees; Alcohol -130 degrees.

Chapter XXXVII.

SULPHUR.

Examine brimstone, flowers of sulphur, pyrite, chalcopyrite,
sphalerite, galenite, gypsum, barite.

182. Separation.

Experiment 103.--To a solution of 2 g. of sodium sulphide,, Na2S2
in 10 cc. H2O add 3 or 4cc. HCl, and look for a ppt. Filter, and
examine the residue. It is lac sulphur, or milk of sulphur.

183. Crystals from Fusion.

Experiment 104.--In a beaker of 25 or 50 cc. capacity put 20 g.
brimstone. Place this over a flame with asbestos paper
interposed, and melt it slowly. Note the color of the liquid,
then let it cool, watching for crystals. When partly solidified
pour the liquid portion into an evapo- rating-dish of water, and
observe the crystals of S forming in the beaker (Fig. 42). The
hard mass may be separated from the glass by a little HNO3 and a
thin knife-blade, or by CS2.

184. Allotropy.

Experiment 105.--Place in a t.t. 15g of brimstone, then heat
slowly till it melts. Notice the thin amber-colored liquid. The
temperature is now a little above 100 degrees. As the heat
increases, notice that it grows darker till it becomes black and
so viscid that it cannot be poured out. It is now above 200
degrees. Still heat, and observe that it changes to a slightly
lighter color, and is again a thin liquid. At this time it is
above 300 degrees. Now pour a little into an evaporating dish
containing water. Examine this, noticing that it can be stretched
like rubber. Leave it in the water till it becomes hard. Continue
heating thebrimstone in the t.t. till it boils at about 450
degrees, and note the color of the escaping vapor. Just above
this point it takes fire. Cool the t.t., holding it in the light
meantime, and look for a sublimate of S on the sides.

185. Solution.

Experiment 106.--Place in an evaporating-dish a gram of powdered
brimstone, and add 5cc, CS2, carbon disulphide. Stir, and see
whether S is dissolved. Put this in a draft of air, and note the
evaporation of the liquid CS2, and the deposit of S crystals.
These crystals are different in form from those resulting from
cooling from fusion.

186. Theory of Allotropy.--The last three experiments well
illustrate allotropy. We found S to crystallize in two different
ways. Substances can crystallize in seven different systems, and
usually a given substance is found in one of these systems only;
e.g. galena is invariably cubical. An element having two such
forms is said to be dimorphous. If it crystallizes in three
systems, it is trimorphous. A crystal has a definite arrangement
of its molecules. If without crystalline form, a substance is
called amorphous. An illustration of amorphism was S after it had
been poured into water. Thus S has at least three allotropic
forms, and the gradations between these probably represent
others. Allotropy seems to be due to varied molecular structure.
We know but little of the molecular condition of solids and
liquids, since we have no law to guide us like Avogadro's in
gases; but, from the density of S vapor at different
temperatures, we infer that liquids and solids have their
molecules very differently made up from those of gases. The least
combining weight of S is 32. Its vapor density at 1,000 degrees
is 32; hence its molecular weight is 64, i.e. vapor density x 2;
and there are 2 atoms in its molecule at that temperature,
molecular weight / atomic weight. At 500 degrees, however, the
vapor density is 96and the molecular weight 192. At this degree
the molecule must contain 6 atoms. How many it has in the
allotropic forms, as a solid, is beyond our knowledge; but it
seems quite likely that allotropy is due to some change of
molecular structure.

The above experiments show two modes of obtaining crystals, by
fusion and by solution.

187. Occurrence and Purification.--Sulphur occurs both free and
combined, and is a very common element. It is found free in all
volcanic regions, but Sicily furnishes most of it. Great
quantities are thrown up from the interior of the earth during an
eruption. The heat of volcanic action probably separates it from
its compound, which may be CaSO4. Vast quantities of the
poisonous SO2 gas are also liberated during an eruption, this
being, in volume of gases evolved, next to H2O. S is crudely

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