There are also small quantities of other gases the principal of which is carbon dioxide CO2, which is present to the extent of only 0.0004, and may be neglected for present purposes.

By weight the atmosphere contains

23.15 parts of oxygen)

nitrogen) ratio 1 to 3-3196

The mean atmospheric pressure at sea level is assumed by Rankine to be 14-704 pounds per square inch, at a temperature of 32°F.=0°C. The mercury barometer then stands at 29-922 inches. At this pressure water boils at 212° F. = 100°C. The metrical atmosphere also measured at 0°C. is 760 mm. of mercury column=29.922 inches. At the ordinary temperature of 57° 8F. the mercury barometer of 30"=1 atmosphere, and at all ordinary temperatures and for purposes of steam engineering it may be called 30 inches.

Expressed in metric measures, one atmosphere is 1.0333 kilos per square centimetre at Paris.

A mercury column giving 14-704 at London will give 14.6967=1.0333 kilos at Paris and 14.686 at New York.

The barometric height varies slightly with the latitude.

To reduce for any other latitude the formula is as follows

(1+0.00531 Sin2 48° 50′)

H=760 x

(1+0.00531 Sin L.)

; where

48° 50' is the latitude of Paris.

The pressure and density of the atmosphere varies with the elevation above sea level, and may be thus calculated—

H=60,000 (1-477-log R); where

R is the elevation in feet above sea level;

H is the barometric height in inches at elevation R. and 1.477=log 30.

High elevation requires consideration in regard to the relative volume of air for furnace supply.

gas.

Air at all temperatures for purposes of furnace work behaves as a perfect

The weight of a cubic foot of dry air at 62°F. is 532-5 grains. If saturated with moisture the weight is 529 grains. The specific gravity of air is 819 times less than water and one pound of air measures 13'146 cubic feet at 62°F.

The standard barometric pressure of 1 atmosphere or 14-6967 pound per square inch at Paris 1.0333 k. per cm. is curiously approximate to 1 k. per cm,. or to 14-21 per square inch.

=

Approximately 1 atmosphere is equal to a pressure of 1 k. per square centi

metre.

The density of air relative to hydrogen is 14-44, its specific heat is 0-2375 at constant pressure, and 0.1686 at constant volume. One pound of air measures 12.385 cubic feet at 0°C. =32°F., and 1 cubic foot weighs 0-08073 pound. It liquefies at 81.6°C. absol., and remains liquid up to 133°C. absol. under a pressure of 39 atmospheres. 1 litre of air weighs 1.292743 grams at 0°C. and 760 mm.

Oxygen.

Oxygen is the active constituent of the atmosphere in promoting combustion. It combines with most elements to form oxides with evolution of heat. The atomic weight of oxygen is 16 and it forms one stable oxide with hydrogen—=H ̧0 (see water) and two oxides with carbon, viz.—

(1) Carbon monoxide or carbonic oxide =CO, which contains 12 by weight of carbon and 16 by weight of oxygen, and

(2) Carbonic acid or carbon dioxide =CO,, containing 12 by weight of carbon to 32 of oxygen.

The density of oxygen is 16; its weight per cubic foot is 0.08926 pound at 0°C. =32°F. and 11·203 cubic feet weigh one pound.

Its specific heat at constant pressure is 0.217 and at constant volume 0-1548. It can be liquefied at 92° absol. C, and under a pressure of 50 atmospheres remains liquid up to 160°C. absol. One litre of oxygen at 0°C. and 760 mm. weighs 1-4293 grams. Oxygen boils at 92°C. absol. under atmospheric pressure.

Nitrogen.

This gas constitutes about four-fifths of the atmosphere. It is a colourless gas and very inert. It does not support combustion, but acts by dilution to restrain its intensity and to reduce the temperature.

Its density is 14, specific heat=0.244 at constant pressure, and 0-173 at constant volume. It weighs 0.07845 per cubic foot and 1 pound equals 12-763 cubic feet. It liquefies at 80°C. absol. and remains liquid up to 128°C. absol., under 33-6 atmospheres. One litre of nitrogen weighs 1.2505 grams at 0°C. and

960 mm.

The weight of nitrogen in the atmosphere is 3-32 times that of oxygen. It is, therefore, the cause of much dilution of the products of a furnace and reduces the theoretical temperature of combustion to a figure much below that of combustion in oxygen.

It has been claimed that at high temperatures the nitrogen unites with oxygen in the furnace, but there is no good reason to suppose that such is the case and every reason to hope it never may be.

Chapter IX

WATER

TEAM is produced by heating water to such a temperature that the elasticity of the water vapour becomes greater than the superincumbent air pressure

STEA

of about 14-7 pounds per square inch at the level of the sea. (See Air).

Pure water is not found in nature but is closely approximated in rain caught on hill tops distant from towns, and in streams which flow off the barren country associated with granitic rocks, the Millstone Grits, and certain other geological strata. Water is an oxide of hydrogen and its chemical formula is H2O. It con

sists of 2 parts by weight of hydrogen to 16 parts of oxygen, and it is produced when hydrogen is burned, the combustion setting free a large amount of heat. (See Hydrogen).

Water being so universal in nature, and being a liquid, is used as the unit point in many physical data. The specific gravity of all other substances is referred to that of water as unity. So also is the specific heat of all other substances, and excepting its constituent hydrogen alone, the specific heat of water is the highest of any known body. The amount of heat necessary to raise the temperature of 1 kilogram of water from 0°C. to 1oC. is denominated the great calorie or simply the calorie, the little calorie having reference to the weight of one gram only and being employed by chemists and physicists.

Similarly the heat necessary to raise the temperature of one pound of water from 32°F. to 33°F. is called the British Thermal Unit or B. Th. U. calorie 3.9683 B. Th. U. and 1 B. Th. U. =0.252 calorie.

Weight.

Thus 1

One gallon of pure distilled water at 62°F. weighs 10 pounds by Act of Parliament. The American or old wine gallon weighs 8 pounds and measures 231 cubic inches, as compared with the British Imperial 10 lb. gallon of 277-479 cubic inches (Chaney). One cubic decimetre of water or 1 litre weighs, by law, 1 kilogram, the kilo. being 2-204 pounds. Thus 1,000 k. weigh very nearly 1 ton.

A column of water 1 foot high exerts a pressure at the base of 0.434 pounds. per square inch. Thus a pressure of 1 pound per square inch represents a column of 2-3 feet. Hence an atmosphere of pressure is equivalent to 33-8 feet of water column.

Compressibility.

Water is nearly incompressible, the co-efficient at 0°C. =32F. being 000052, and at nearly 53°C. =127°F. =0.0000441. It is thus negligible:

Expansion.

Water changes its volume with change of temperature, but not to an amount that is of serious account in steam engineering.

The foregoing table gives the weight per cubic foot of water at various temperatures, showing that the maximum expansion in the open air does not reach 5 per cent.

Water attains its maximum density at 4°C. =39°1F.

It becomes solid at a temperature of 0°C. =32°F., the freezing point of water being employed in fact as the 0° of the Centigrade thermometers.

=

Ice has a specific gravity of 0.922 and a specific heat of 0.504. To reduce 1 pound of ice at 32°F. 0°C. to water also at 32°F. requires 142 B.Th.U. = 35.78 calories. The latent heat of water is thus said to be 35.78 calories or 142 B.Th.U. per pound or 78.86 calories per kilogram.

Specific Heat.

The specific heat of water called 1.00 at 0°C. =32°F. is not uniform, but increases slightly with increase of temperature, as per the following table:—

As at the above temperatures the bulk of water is increased in a much greater ratio than the specific heat, the total heat per cubic foot will decrease somewhat with rise of temperature.

Solubility.

Water is a solvent of many substances. Most gases are soluble to a greater or less extent, and usually more so at low temperatures. Hydrogen forms a notable exception to this, being equally soluble at various temperatures. There is, however, no known exception to the rule that the solubility is proportional to the pressure. The co-efficient of absorption of gas in water is the relative volume absorbed

by one volume of water at 0°C. =32°F. and 760 mm. pressure =1 atmosphere= 147 pounds per square inch.

The following table is given by Bunsen for the solubility of gases

As the total heat contained in one pound of steam is nearly 1,200 B. Th. U., this amount of heat is more or less thrown away when steam is used to atomize liquid fuel. The gases never leave a furnace below 212°F., and every pound of steam carries off its load of 966 units of latent heat to the chimney. Air being already a gas, and being necessary to combustion, causes no loss in this manner, but it requires power to compress air and some steam is thereby used, but, especially at sea, such steam can be condensed and does not therefore lead to a loss of fresh water. No extra work is thrown upon the evaporation plant. Water may be split up by heat into its two constituent gases. In this process of dissociation or decomposition exactly as much heat is absorbed as was produced by the combination of the gases when the water was formed. This plain chemical fact is quite ignored by those who dream about the use of steam for fuel and who imagine that steam jets introduced into a furnace will decompose and burn with any effect in increasing the total heat production of the furnace. Steam thus employed is useful as a mechanical draught producer only, or there may be some truth that hydrocarbons burn better in the presence of moisture. But no further claim is in any sense tenable.

Sources of Water.

Though pure water does not exist in Nature, there are certain natural waters which are practically pure, as for example the water of Loch Katrine or the supply of the city of Manchester, which is practically pure and non-scaling, and only calls for a little alkali to correct slight corrosive effect due to possibly peaty acids.

Water from chalk and limestone areas, and from the Keuper Marl, is hard and produces much incrustation. Water from wells under London, though from the chalk, is barely one-third the hardness of water from wells where the chalk extends to the surface. The presence of the lower tertiary beds above the chalk appears to account for this. Water from tidal estuaries is now apt to be very corrosive owing probably to the decomposition of the chloride salts into acids by the high temperature of modern boilers. The presence of sewage is not per se