chemical substance, as a compound containing and consisting of hydrogen
and oxygen. What are these water constituents, hydrogen and oxygen? Each
of them is a gas, but each a gas having totally different properties. On
decomposing water and collecting the one of these two gases, the
hydrogen gas, in one vessel, and the other, the oxygen gas, in another
vessel, twice as large a volume of hydrogen gas is given off by the
decomposing water as of oxygen. You may now notice a certain meaning in
the formula assigned to water, H_{2}O: two volumes of hydrogen combined
with one of oxygen; and it may be added that when such combination takes
place, not three volumes of resulting water vapour (steam), but two
volumes are produced. This combination of the two gases, when mixed
together, is determined by heating to a high temperature, or by passing
an electric spark; it then takes place with the consequent sudden
condensation of three volumes of mixture to two of compound, so as to
cause an explosion. I may also mention that as regards the weights of
these bodies, oxygen and hydrogen, the first is sixteen times as heavy
as the second; and since we adopt hydrogen as the unit, we may consider
H to stand for hydrogen, and also to signify 1--the unit; whilst O
means oxygen, and also 16. Hence the compound atom or molecule of
water, H_{2}O, weighs 18. I must now show you that these two gases are
possessed of totally different properties. Some gases will extinguish a
flame; some will cause the flame to burn brilliantly, but will not burn
themselves; and some will take fire and burn themselves, though
extinguishing the flame which has ignited them. We say the first are
non-combustible, and will not support combustion; the second are
supporters of combustion, the third are combustible gases. Of course
these are, as the lawyers say, only _ex parte_ statements of the truth;
still they are usually accepted. Oxygen gas will ignite a red-hot match,
but hydrogen will extinguish an inflamed one, though it will itself
burn. You generally think of water as the great antithesis of, the
universal antidote for, fire. The truth is here again only of an _ex
parte_ character, as I will show you. If I can, by means of a substance
having a more intense affinity for oxygen than hydrogen has, rob water
of its oxygen, I necessarily set the hydrogen that was combined with
that oxygen free. If the heat caused by the chemical struggle, so to
say, is great, that hydrogen will be inflamed and burn. Thus we are
destroying that antithesis, we are causing the water to yield us fire. I
will do this by putting potassium on water, and even in the cold this
potassium will seize upon the oxygen of the water, and the hydrogen will
take fire.
_Specific Gravity._--We must now hasten to other considerations of
importance. Water is generally taken as the unit in specific gravities
assigned to liquids and solids. This simply means that when we desire to
express how heavy a thing is, we are compelled to say it is so many
times heavier or lighter than something. That something is generally
water, which is regarded, consequently, as unit or figure 1. A body of
specific gravity 1·5, or 1-1/2, means that that body is 1-1/2 or 1·5
times as heavy as water. As hat manufacturers, you will have mostly to
do with the specific gravities of liquids, aqueous solutions, and you
will hear more of Twaddell degrees. The Twaddell hydrometer, or
instrument for measuring the specific gravities of liquids, is so
constructed that when it stands in water, the water is just level with
its zero or 0° mark. Well, since in your reading of methods and new
processes, you will often meet with specific gravity numbers and desire
to convert these into Twaddell degrees, I will give you a simple means
of doing this. Add cyphers so as to make into a number of four figures,
then strike out the unit and decimal point farthest to the left, and
divide the residue by 5, and you get the corresponding Twaddell degrees.
If you have Twaddell degrees, simply multiply by 5, and add 1000 to the
result, and you get the specific gravity as usually taken, with water as
the unit, or in this case as 1000. An instrument much used on the
Continent is the Beaumé hydrometer. The degrees (_n_) indicated by this
instrument can be converted into specific gravity (_d_) by the
formula: _d_ = 144·3/(144·3 - n)
_Ebullition or Boiling of Water, Steam._--The atmosphere around us is
composed of a mixture of nitrogen and oxygen gases; not a compound of
these gases, as water is of hydrogen and oxygen, but a mixture more like
sand and water or smoke and air. This mass of gases has weight, and
presses upon objects at the surface of the earth to the extent of 15 lb.
on the square inch. Now some liquids, such as water, were it not for
this atmospheric pressure, would not remain liquids at all, but would
become gases. The pressure thus tends to squeeze gases together and
convert them into liquids. Any force that causes gases to contract will
do the same thing, of course--for example, cold; and _ceteris paribus_
removal of pressure and expansion by heat will act so as to gasify
liquids. When in the expansion of liquids a certain stage or degree is
reached, different for different liquids, gas begins to escape so
quickly from the liquid that bubbles of vapour are continually formed
and escape. This is called ebullition or boiling. A certain removal of
pressure, or expansion by heat, is necessary to produce this, _i.e._ to
reach the boiling-point of the liquid. As regards the heat necessary for
the boiling of water at the surface of the earth, _i.e._ under the
atmospheric pressure of 15 lb. on the square inch, this is shown on the
thermometer of Fahrenheit as 212°, and on the simpler centigrade one, as
100°, water freezing at 0° C. But if what I have said is true, when we
remove some of the atmospheric pressure, the water should boil with a
less heat than will cause the mercury in the thermometer to rise to 100°
C., and if we take off all the pressure, the water ought to boil and
freeze at the same time. This actually happens in the Carré ice-making
machine. The question now arises, "Why does the water freeze in the
Carré machine?" All substances require certain amounts of heat to enable
them to take and to maintain the liquid state if they are ordinarily
solid, and the gaseous state if ordinarily liquid or solid, and the
greater the change of state the greater the heat needed. Moreover, this
heat does not make them warm, it is simply absorbed or swallowed up, and
becomes latent, and is merely necessary to maintain the new condition
assumed. In the case of the Carré machine, liquid water is, by removal
of the atmospheric pressure, coerced, as it were, to take the gaseous
form. But to do so it needs to absorb the requisite amount of heat to
aid it in taking that form, and this heat it must take up from all
surrounding warm objects. It absorbs quickly all it can get out of
itself as liquid water, out of the glass vessel containing it, and from
the surrounding air. But the process of gasification with ebullition
goes on so quickly that the temperature of the water thus robbed of heat
quickly falls to 0° C., and the remaining water freezes. Thus, then, by
pumping out the air from a vessel, _i.e._ working in a vacuum, we can
boil a liquid in such exhausted vessel far below its ordinary boiling
temperature in the open air. This fact is of the utmost industrial
importance. But touching this question of latent heat, you may ask me
for my proof that there is latent heat, and a large amount of it, in a
substance that feels perfectly cold. I have told you that a gasified
liquid, or a liquefied solid, or most of all a gasified solid, contains
such heat, and if reconverted into liquid and solid forms respectively,
that heat is evolved, or becomes sensible heat, and then it can be
decidedly felt and indicated by the thermometer. Take the case of a
liquid suddenly solidifying. The heat latent in that liquid, and
necessary to keep it a liquid, is no longer necessary and comes out, and
the substance appears to become hot. Quicklime is a cold, white, solid
substance, but there is a compound of water and lime--slaked lime--which
is also a solid powdery substance, called by the chemist, hydrate of
lime. The water used to slake the quicklime is a liquid, and it may be
ice-cold water, but to form hydrate of lime it must assume a solid form,
and hence can and does dispense with its heat of liquefaction in the
change of state. You all know how hot lime becomes on slaking with
water. Of course we have heat of chemical combination here as well as
evolution of latent heat. As another example, we may take a solution of
acetate of soda, so strong that it is just on the point of
crystallising. If it crystallises it solidifies, and the liquid
consequently gives up its latent heat of liquefaction. We will make it
crystallise, first connecting the tube containing it to another one
containing a coloured liquid and closed by a cork carrying a narrow tube
dipping into the coloured liquid. On crystallising, the solution gives
off heat, as is shown by the expansion of the air in the corked tube,
and the consequent forcing of the coloured liquid up the narrow tube.
Consequently in your works you never dissolve a salt or crystal in water
or other liquid without rendering heat latent, or consuming heat; you
never allow steam to condense in the steam pipes about the premises
without losing vastly more heat than possibly many are aware of. Let us
inquire as to the latent heat of water and of steam.
_Latent Heats of Water and Steam._--If we mix 1 kilogram (about 2 lb.)
of ice (of course at zero or 0° C.) with 1 kilogram of water at 79° C.,
and stir well till the ice is melted, _i.e._ has changed its state from
solid to liquid, we find, on putting a thermometer in, the temperature
is only 0° C. This simply means that 79° of heat (centigrade degrees)
have become latent, and represent the heat of liquefaction of 1 kilogram
of ice. Had we mixed 1 kilogram of water at 0° C. with 1 kilogram of
water at 79° C. there would have been no change of state, and the
temperature of the mixture might be represented as a distribution of the
79° C. through the whole mass of the 2 kilograms, and so would be
39-1/2° C. We say, therefore, the latent heat of water is the heat which
is absorbed or rendered latent when a unit of weight, say 1 kilogram of
water as ice, melts and liquefies to a unit of water at zero, or it is
79 heat units. These 79 units of heat would raise 79 units of weight of
liquid water through 1° C., or one unit of liquid water through 79°.
Let us now inquire what the latent heat of steam is. If we take 1
kilogram of water at 0° C. and blow steam from boiling water at 100° C.
into it until the water just boils, and then stop and weigh the
resulting water, we shall find it amounts to 1·187 kilograms, so that
0·187 kilogram of water which was in the gaseous steam form, and had
besides a sensible heat of 100° C., has changed its state to that of
liquid water. This liquid water, being at the boiling-point, has still
the 100° C. of sensible heat, and hence the water in the gaseous steam
form can have given up to the water at 0° C. into which it was blown,
only the latent heat of gasification which was not sensible, but by
virtue of which it was enabled to assume the gaseous form. But if 0·187
kilogram of steam at 100° C. can heat 1 kilogram of water through 100
degrees, then 1 kilogram of steam can raise 5·36 kilograms of ice-cold
water through 100 degrees, or 536 kilograms through 1 degree, and thus
the latent heat of steam is 536 heat units.
_Effect of Increase of Pressure on the Boiling of Water._--Now we have
referred to diminution of pressure and its effect on the boiling-point
of water, and I may point out that by increasing the pressure, such,
_e.g._, as boiling water under a high pressure of steam, you raise the
boiling-point. There are some industrial operations in which the action
of certain boiling solutions is unavailing to effect certain
decompositions or other ends when the boiling is carried on under the
ordinary atmospheric pressure, and boiling in closed and strong vessels
under pressure must be resorted to. Take as an example the wood-pulp
process for making paper from wood shavings. Boiling in open pans with
caustic soda lye is insufficient to reduce the wood to pulp, and so
boiling in strong vessels under pressure is adopted. The temperature of
the solution rises far above 212° F. (100° C.). Let us see what may
result chemically from the attainment of such high temperatures of water
in our steam boilers working under high pressures. If you blow ordinary
steam at 212° F. or 100° C., into fats or oils, the fats and oils remain
undecomposed; but suppose you let fatty and oily matters of animal or
vegetable origin, such as lubricants, get into your boiler feed-water
and so into your boiler, what will happen? I have only to tell you that
a process is patented for decomposing fats with superheated steam, to
drive or distil over the admixed fatty acids and glycerin, in order to
show you that in your boilers such greasy matters will be more or less
decomposed. Fats are neutral as fats, and will not injure the iron of
the boilers; but once decompose them and they are split up into an acid
called a fat acid, and glycerin. That fat acid at the high temperature
soon attacks your boilers and pipes, and eats away the iron. That is one
of the curious results that may follow at such high temperatures.
Mineral or hydrocarbon oils do not contain these fat acids, and so
cannot possibly, even with high-pressure steam, corrode the boiler
metal.
_Effect of Dissolved Salts on the Boiling of Water._--Let us inquire
what this effect is? Suppose we dissolve a quantity of a salt in water,
and then blow steam at 100° C. (212° F.) into that water, the latter
will boil not at 212° F., but at a higher temperature. There is a
certain industrial process I know of, in course of which it is necessary
first to maintain a vessel containing water, by means of a heated closed
steam coil, at 212° F. (100° C.), and at a certain stage to raise the
temperature to about 327° F. (164° C.). The pressure on the boiler
connected with the steam coil is raised to nearly seven atmospheres, and
thus the heat of the high-pressure steam rises to 327° F. (164° C.), and
then a considerable quantity of nitrate of ammonium, a crystallised
salt, is thrown into the water, in which it dissolves. Strange to say,
although the water alone would boil at 212° F., a strong solution in
water of the ammonium nitrate only boils at 327° F., so that the effect
of dissolving that salt in the water is the same as if the pressure were
raised to seven atmospheres. Now let us, as hat manufacturers, learn a
practical lesson from this fact. We have observed that wool and fur
fibres are injured by boiling in pure water, and the heat has much to do
with this damage; but if the boiling take place in bichrome liquors or
similar solutions, that boiling will, according to the strength of the
solution in dissolved matters, take place at a temperature more or less
elevated above the boiling-point of water, and so the damage done will
be the more serious the more concentrated the liquors are, quite
independently of the nature of the substances dissolved in those
liquors.
_Solution._--We have already seen that when a salt of any kind dissolves
in water, heat is absorbed, and becomes latent; in other words, cold is
produced. I will describe a remarkable example or experiment, well
illustrating this fact. If you take some Glauber's salt, crystallised
sulphate of soda, and mix it with some hydrochloric acid (or spirits of
salt), then so rapidly will the solution proceed, and consequently so
great will be the demand for heat, that if a vessel containing water be
put in amongst the dissolving salt, the heat residing in that vessel and
its water will be rapidly extracted, and the water will freeze. As
regards solubility, some salts and substances are much more quickly and
easily dissolved than others. We are generally accustomed to think that
to dissolve a substance quickly we cannot do better than build a fire
under the containing vessel, and heat the liquid. This is often the
correct method of proceeding, but not always. Thus it would mean simply
loss of fuel, and so waste of heat, to do this in dissolving ordinary
table salt or rock salt in water, for salt is as soluble in cold water
as in hot. Some salts are, incredible though it may appear, less soluble
in boiling water than in cold. Water just above the freezing-point
dissolves nearly twice as much lime as it does when boiling. You see,
then, that a knowledge of certain important facts like these may be so
used as to considerably mitigate your coal bills, under given
circumstances and conditions.
LECTURE IV
WATER: ITS CHEMISTRY AND PROPERTIES; IMPURITIES AND THEIR ACTION; TESTS
OF PURITY--_Continued_