CHAPTER XIV_Acetylene, the Principles of Its Generation and Use A Practical Handbook on the Production, Purification, and Subsequent Treatment of Acetylene for the Development of Light, Heat, and Power

VALUATION AND ANALYSIS OF CARBIDE

Regulations of British Acetylene Association

Regulations o£ German Acetylene Association

Regulations of Austrian Acetylene Association

Sampling carbide

Yield of gas from small carbide

Correction of volumes for temperature and pressure

Estimation of impurities

Tabular numbers

APPENDIX

DESCRIPTIONS OP GENERATORS

America: Canada

America: United States

Austria-Hungary

Belgium

France

Germany

Great Britain and Ireland

INDEX

INDEX TO APPENDIX

ACETYLENE

CHAPTER I

INTRODUCTORY--THE COST AND ADVANTAGES OF ACETYLENE LIGHTING

Acetylene is a gas [Footnote: For this reason the expression, "acetylene

gas," which is frequently met with, would be objectionable on the ground

of tautology, even if it were not grammatically and technically

incorrect. "Acetylene-gas" is perhaps somewhat more permissible, but it

is equally redundant and unnecessary.] of which the most important

application at the present time is for illuminating purposes, for which

its properties render it specially well adapted. No other gas which can

be produced on a commercial scale is capable of giving, volume for

volume, so great a yield of light as acetylene. Hence, apart from the

advantages accruing to it from its mode of production and the nature of

the raw material from which it is produced, it possesses an inherent

advantage over other illuminating gases in the smaller storage

accommodation and smaller mains and service-pipes requisite for the

maintenance of a given supply of artificial light. For instance, if a

gasholder is required to contain sufficient gas for the lighting of an

establishment or district for twenty-four hours, its capacity need not be

nearly so great if acetylene is employed as if oil-gas, coal-gas, or

other illuminating gas is used. Consequently, for an acetylene supply the

gasholder can be erected on a smaller area and for considerably less

outlay than for other gas supplies. In this respect acetylene has an

unquestionable economical advantage as a competitor with other varieties

of illuminating gas for supplies which have generally been regarded as

lying peculiarly within their preserves. The extent of this advantage

will be referred to later.

The advantages that accrue to acetylene from its mode of production, and

the nature of the raw material from which it is obtained, are in reality

of more importance. Acetylene is readily and quickly produced from a raw

material--calcium carbide--which, relatively to the yield of light of the

gaseous product, is less bulky than the raw materials of other gases. In

comparison also with oils and candles, calcium carbide is capable of

yielding, through the acetylene obtainable from it, more light per unit

of space occupied by it. This higher light-yielding capacity of calcium

carbide, ready to be developed through acetylene, gives the latter gas a

great advantage over all other illuminants in respect of compactness for

transport or storage. Hence, where facilities for transport or storage

are bad or costly, acetylene may be the most convenient or cheapest

illuminant, notwithstanding its relatively high cost in many other cases.

For example, in a district to which coal and oil must be brought great

distances, the freight on them may be so heavy that--regarding the

question as simply one of obtaining light in the cheapest manner--it may

be more economical to bring calcium carbide an equal or even greater

distance and generate acetylene from it on the spot, than to use oil or

make coal-gas for lighting purposes, notwithstanding that acetylene may

not be able to compete on equal terms with oil--or coal-gas at the place

from which the carbide is brought. Likewise where storage accommodation

is limited, as in vehicles or in ships or lighthouses, calcium carbide

may be preferable to oil or other illuminants as a source of light.

Disregarding for the moment intrinsic advantages which the light

obtainable from acetylene has over other lights, there are many cases

where, owing to saving in cost of carriage, acetylene is the most

economical illuminant; and many other cases where, owing to limited space

for storage, acetylene far surpasses other illuminants in convenience,

and is practically indispensable.

The light of the acetylene flame has, however, some intrinsic advantages

over the light of other artificial illuminants. In the first place, the

light more closely resembles sunlight in composition or "colour." It is

more nearly a pure "white" light than is any other flame or incandescent

body in general use for illuminating purposes. The nature or composition

of the light of the acetylene flame will be dealt with more exhaustively

later, and compared with that afforded by other illuminants; but,

speaking generally, it may be said that the self-luminous acetylene light

is superior in tint, to all other artificial lights, for which reason it

is invaluable for colour-judging and shade-matching. In the second

place, when the gas issues from a suitable self-luminous burner under

proper pressure, the acetylene flame is perfectly steady; and in this

respect it in preferable to most types of electric light, to all self-

luminous coal-gas flames and candles, and to many varieties of oil-lamp.

In steadiness and freedom from flicker it is fully equal to incandescent

coal-gas light, but it in distinctly superior to the latter by virtue of

its complete freedom from noise. The incandescent acetylene flame emits a

slight roaring, but usually not more than that coming from an

atmospheric coal-gas burner. With the exception of the electric arc,

self-luminous acetylene yields a flame of unsurpassed intensity, and yet

its light is agreeably soft. In the third place, where electricity is

absent, a brilliancy of illumination which can readily be obtained from

self-luminous acetylene can otherwise only be procured by the employment

of the incandescent system applied either to coal-gas or to oil; and

there are numerous situations, such as factories, workshops, and the

like, where the vibration of the machinery or the prevalence of dust

renders the use of mantles troublesome if not impossible. Anticipating

what will be said later, in cases like these, the cost of lighting by

self-luminous acetylene may fairly be compared with self-luminous coal-

gas or oil only; although in other positions the economy of the Welsbach

mantle must be borne in mind.

Acetylene lighting presents also certain important hygienic advantages

over other forms of flame lighting, in that it exhausts, vitiates, and

heats the air of a room to a less degree, for a given yield of light,

than do either coal-gas, oils, or candles. This point in favour of

acetylene is referred to here only in general terms; the evidence on

which the foregoing statement is based will be recorded in a tabular

comparison of the cost and qualities of different illuminants. Exhaustion

of the air means, in this connexion, depletion of the oxygen normally

present in it. One volume of acetylene requires 2-1/2 volumes of oxygen

for its complete combustion, and since 21 volumes of oxygen are

associated in atmospheric air with 79 volumes of inert gases--chiefly

nitrogen--which do not actively participate in combustion, it follows

that about 11.90 volumes of air are wholly exhausted, or deprived of

oxygen, in the course of the combustion of one volume of acetylene. If

the light which may be developed by the acetylene is brought into

consideration, it will be found that, relatively to other illuminants,

acetylene causes less exhaustion of the air than any other illuminating

agent except electricity. For instance, coal-gas exhausts only about 6-

1/2 times its volume of air when it is burnt; but since, volume for

volume, acetylene ordinarily yields from three to fifteen times as much

light as coal-gas, it follows that the same illuminative value is

obtainable from acetylene by considerably less exhaustion of the air than

from coal-gas. The exact ratio depends on the degree of efficiency of the

burners, or of the methods by which light is obtained from the gases, as

will be realised by reference to the table which follows. Broadly

speaking, however, no illuminant which evolves light by combustion

(oxidation), and which therefore requires a supply of oxygen or air for

its maintenance, affords light with so little exhaustion of the air as

acetylene. Hence in confined, ill-ventilated, or crowded rooms, the air

will suffer less exhaustion, and accordingly be better for breathing, if

acetylene is chosen rather than any other illuminant, except electricity.

Next, in regard to vitiation of the air, by which is meant the alteration

in its composition resulting from the admixture of products of combustion

with it. Electric lighting is as superior to other modes of lighting in

respect of direct vitiation as of exhaustion of the air, because it does

not depend on combustion. Putting it aside, however, light is obtainable

by means of acetylene with less attendant vitiation of the air than by

means of any other gas or of oil or candles. The principal vitiating

factor in all cases is the carbonic acid produced by the combustion. Now

one volume of acetylene on combustion yields two volumes of carbonic

acid, whereas one volume of coal-gas yields about 0.6 volume of carbonic

acid. But even assuming that the incandescent system of lighting is

applied in the case of coal-gas and not of acetylene, the ratio of the

consumption of the two gases for the development of a given illuminative

effect will be such that no more carbonic acid will be produced by the

acetylene; and if the incandescent system is applied either in both cases

or in neither, the ratio will be greatly in favour of acetylene. The

other factors which determine the vitiation of the air of a room in which

the gas is burning are likewise under ordinary conditions more in favour

of acetylene. They are not, however, constant, since the so-called

"impurities," which on combustion cause vitiation of the air, vary

greatly in amount according to the extent to which the gases have been

purified. London coal-gas, which was formerly purified to the highest

degree practically attainable, used to contain on the average only 10 to

12 grains of sulphur per 100 cubic feet, and virtually no other impurity.

But now coal-gas, in London and most provincial towns, contains 40 to 50

grains of sulphur per 100 cubic foot. At least 5 grains of ammonia per

100 cubic foot in also present in coal-gas in some towns. Crude acetylene

also contains sulphur and ammonia, that coming from good quality calcium

carbide at the present day including about 31 grains of the former and

25 grains of the latter per 100 cubic feet. But crude acetylene is also

accompanied by a third impurity, viz., phosphoretted hydrogen or

phosphine, which in unknown in coal-gas, and which is considerably more

objectionable than either ammonia or sulphur. The formation, behaviour,

and removal of those various impurities will be discussed in Chapter V.;

but here it may be said that there is no reason why, if calcium carbide

of a fair degree of purity has been used, and if the gas has been

generated from it in a properly designed and smoothly working apparatus--

this being quite as important as, or even more important than, the purity

of the original carbide--the gas should not be freed from phosphorus,

sulphur, and ammonia to the utmost necessary or desirable extent, by

processes which are neither complicated nor expensive. And if this is

done, as it always should be whenever the acetylene is required for

domestic lighting, the vitiation of the air of a room due to the

"impurities" in the gas will become much less in the case of acetylene

than in that of even well-purified coal-gas; taking equal illuminating

effect as the basis for comparison.

Acetylene is similarly superior, speaking generally, to petroleum in

respect of impurities, though the sulphur present in petroleum oils, such

as are sold in this country for household use, though very variable, is

often quite small in amount, and seldom is responsible for serious

vitiation of the atmosphere.

Regarding somewhat more closely the relative convenience and safety of

acetylene and paraffin for the illumination of country residences, it may

be remarked that an extraordinarily great amount of care must be bestowed

upon each separate lamp if the whole house is to be kept free from an

odour which is very offensive to the nostrils; and the time occupied in

this process, which of itself is a disagreeable one, reaches several

hours every day. Habit has taught the country dweller to accept as

inevitable this waste of time, and largely to ignore the odour of

petroleum in his abode; but the use of acetylene entirely does away with

the daily cleaning of lamps, and, if the pipe-fitting work has been done

properly, yields light absolutely unaccompanied by smell. Again, unless

most carefully managed, the lamp-room of a large house, with its store of

combustible oil, and its collection of greasy rags, must unavoidably

prove a sensible addition to the risk of fire. The analogue of the lamp-

room when acetylene is employed is the generator-house, and this is a

separate building at some distance from the residence proper. There need

be no appreciable odour in the generator-house, except during the times

of charging the apparatus; but if there is, it passes into the open air

instead of percolating into the occupied apartments.

The amount of heat developed by the combustion of acetylene also is less

for a given yield of light than that developed by most other illuminants.

The gas, indeed, is a powerful heating gas, but owing to the amount

consumed being so small in proportion to the light developed, the heat

arising from acetylene lighting in a room is less than that from most

other illuminating agents, if the latter are employed to the extent

required to afford equally good illumination. The ratio of the heat

developed in acetylene lighting to that developed in, _e.g._,

lighting by ordinary coal-gas, varies considerably according to the

degree of efficiency of the burners, or, in other words, of the methods

by which light is obtained from the gases. Volume for volume, acetylene

yields on combustion about three and a half times as much heat as coal-

gas, yet, owing to its superior efficiency as an illuminant, any required

light may be obtained through it with no greater evolution of heat than

the best practicable (incandescent) burners for coal-gas produce. The

heat evolved by acetylene burners adequate to yield a certain light is

very much less than that evolved by ordinary flat-flame coal-gas burners

or by oil-lamps giving the same light, and is not more than about three

times as much as that from ordinary electric lamps used in numbers

sufficient to give the same light. More exact figures for the ratio

between the heat developed in acetylene lighting and that in other modes

of lighting are given in the table already referred to.

In connexion with the smaller amount of heat developed per unit of light

when acetylene is the illuminant, the frequently exaggerated claim that

acetylene does not blacken ceilings at all may be studied. Except it be a

carelessly manipulated petroleum-lamp, no form of artificial illuminant

employed nowadays ever emits black smoke, soot, or carbon, in spite of

the fact that all luminous flames commercially capable of utilisation do

contain free carbon in the elemental state. The black mark on a ceiling

over a source of light is caused by a rising current of hot air and

combustion products set up by the heat accompanying the light, which

current of hot gas carries with it the dust and dirt always present in

the atmosphere of an inhabited room. As this current of air and burnt gas

travels in a fairly concentrated vertical stream, and as the ceiling is

comparatively cool and exhibits a rough surface, that dust and dirt are

deposited on the ceiling above the flame, but the stain is seldom or

never composed of soot from the illuminant itself. Proof of this

statement may be found in the circumstance that a black mark is

eventually produced over an electric glow-lamp and above a pipe

delivering hot water. Clearly, therefore, the depth and extent of the

mark will depend on the volume and temperature of the hot gaseous

current; and since per unit of light acetylene emits a far smaller

quantity of combustion products and a far smaller amount of heat than any

other flame illuminant except incandescent coal-gas, the inevitable black

mark over its flame takes very much longer to appear. Quite roughly

speaking, as may be deduced from what has already been said on this

subject, the luminous flame of acetylene "blackens" a ceiling at about

the same rate as a coal-gas burner of the best Welsbach type.

There is one respect in which acetylene and other flame illuminants are

superior to electric lighting, viz., that they sterilise a larger volume

of air. All the air which is needed to support combustion, as well as the

excess of air which actually passes through the burner tube and flame in

incandescent burners, is obviously sterilised; but so also is the much

larger volume of air which, by virtue of the up-current due to the heat

of the flame, is brought into anything like close proximity with the

light. The electric glow-lamp, and the most popular and economical modern

enclosed electric arc-lamp, sterilise only the much smaller volume of air

which is brought into direct contact with their glass bulbs. Moreover,

when large numbers of persons are congregated in insufficiently

ventilated buildings--and many public rooms are insufficiently

ventilated--the air becomes nauseous to inspire and positively

detrimental to the health of delicate people, by reason of the human

effluvia which arise from soiled raiment and uncleansed or unhealthy

bodies, long before the proportion of carbonic acid by itself is high

enough to be objectionable. Thus a certain proportion of carbonic acid

coming from human lungs and skin is more harmful than the same proportion

of carbonic acid derived from the combustion of gas or oil. Hence

acetylene and flame illuminants generally have the valuable hygienic

advantages over electric lighting, not only of killing a far larger

number of the micro-organisms that may be present in the air, but, by

virtue of their naked flames, of burning up and destroying a considerable

quantity of the aforesaid odoriferous matter, thus relieving the nose and

materially assisting in the prevention of that lassitude and anæmia

occasionally follow the constant inspiration of air rendered foul by

human exhalations.

The more important advantages of acetylene as an illuminant have now been

indicated, and it remains to discuss the cost of acetylene lighting in

comparison with other modes of procuring artificial light. At the outset

it may be stated that a very much greater reduction in the price of

calcium carbide--from which acetylene is produced--than is likely to

ensue under the present methods and conditions of manufacture will be

required to make acetylene lighting as cheap as ordinary gas lighting in

towns in this country, provided incandescent burners are used for the

gas. On the score of cheapness (and of convenience, unless the acetylene

were delivered to the premises from some central generating station)

acetylene cannot compete as an illuminant with coal-gas where the latter

costs, say, not more than 5s. per 1000 cubic feet, if only

reasonable attention is given to the gas-burners, and at least a quarter

of them are on the incandescent system. If, on the other hand, coal-gas

is misused and wasted through the employment only of interior or worn-out

flat-flame burners, while the best types of burner are used for

acetylene, the latter gas may prove as cheap for lighting as coal-gas at,

say, 2s. 6d. per 1000 cubic feet (and be far better hygienically);

whereas, contrariwise, if coal-gas is used only with good and properly

maintained incandescent burners, it may cost over 10s. per 1000 cubic

feet, and be cheaper than acetylene burned in good burners (and as good

from the hygienic standpoint). More precise figures on the relative costs

of coal-gas lighting and acetylene lighting are given in the tabular

statement at the close of this chapter.

With regard to electric lighting it is somewhat difficult to lay down a

fair basis of comparison, owing to the wide variations in the cost of

current, and in the efficiency of lamps, and to the undoubted hygienic

and aesthetic claims of electric lighting to precedence. But in towns in

this country where there is a public electricity supply, electric

lighting will be used rather than acetylene for the same reasons that it

is preferred to coal-gas. Cost is only a secondary consideration in such

cases, and where coal-gas is reasonably cheap, and nevertheless gives

place to electric lighting, acetylene clearly cannot hope to supplant the

latter. [Footnote: Where, however, as is frequently the case with small

public electricity-supply works, the voltage of the supply varies

greatly, the fluctuations in the light of the lamps, and the frequent

destruction of fuses and lamps, are such manifest inconveniences that

acetylene is in fact now being generally preferred to electric lighting

in such circumstances.] But where current cannot be had from an

electricity-supply undertaking, and it is a question, in the event of

electric lighting being adopted, of generating current by driving a

dynamo, either by means of a gas-engine supplied from public gas-mains,

by means of a special boiler installation, or by means of an oil-engine

or of a power gas-plant and gas-engine, the claims of acetylene to

preference are very strong. An important factor in the estimation of the

relative advantages of electricity and acetylene in such cases is the

cost of labour in looking after the generating plant. Where a gas-engine

supplied from public gas-mains is used for driving the dynamo, electric

lighting can be had at a relatively small expenditure for attendance on

the generating plant. But the cost of the gas consumed will be high, and

actually light could be obtained directly from the gas by means of

incandescent mantles at far loss cost than by consuming the gas in a

motor for the indirect production of light by means of electric current.

Therefore electric lighting, if adopted under these conditions, must be

preferred to gas lighting from considerations which are deemed to

outweigh those of a much higher cost, and acetylene does not present so

great advantages over coal-gas as to affect the choice of electric

lighting. But in the cases where there is no public gas-supply, and

current must be generated from coal or coke or oil consumed on the spot,

the cost of the skilled labour required to look after either a boiler,

steam-engine and dynamo, or a power gas-plant and gas-engine or oil-

engine and dynamo, will be so heavy that unless the capacity of the

installation is very great, acetylene will almost certainly prove a

cheaper and more convenient method of obtaining light. The attention

required by an acetylene installation, such as a country house of upwards

of thirty rooms would want, is limited to one or two hours' labour per

diem at any convenient time during daylight. Moreover, the attendant need

not be highly paid, as he will not have required an engineman's training,

as will the attendant on an electric lighting plant. The latter, too,

must be present throughout the hours when light is wanted unless a heavy

expenditure has been incurred on accumulators. Furthermore, the capital

outlay on generating plant will be very much less for acetylene than for

electric lighting. General considerations such as these lead to the

conclusion that in almost all country districts in this country a house

or institution could be lighted more cheaply by means of acetylene than

by electricity. In the tabular statement of comparative costs of

different modes of lighting, electric lighting has been included only on

the basis of a fixed cost per unit, as owing to the very varied cost of

generating current by small installations in different parts of the

country it would be futile to attempt to give the cost of electric

lighting on any other basis, such as the prime cost of coal or coke in a

particular district. Where current is supplied by a public electricity-

supply undertaking, the cost per unit is known, and the comparative costs

of electric light and acetylene can be arrived at with tolerable

precision. It has not been thought necessary to include in the tabular

statement electric arc-lamps, as they are only suitable for the lighting

of large spaces, where the steadiness and uniformity of the illumination

are of secondary importance. Under such conditions, it may be stated

parenthetically, the electric arc-light is much less costly than

acetylene lighting would be, but it is now in many places being

superseded by high-pressure gas or oil incandescent lights, which are

steady and generally more economical than the arc light.

The illuminant which acetylene is best fitted to supersede on the score

of convenience, cleanliness, and hygienic advantages is oil. By oil is

meant, in this connection, the ordinary burning petroleum, kerosene, or

paraffin oil, obtained by distilling and refining various natural oils

and shales, found in many countries, of which the United States

(principally Pennsylvania), Russia (the Caucasus chiefly), and Scotland

are practically the only ones which supply considerable quantities for

use in Great Britain. Attempts are often made to claim superiority for

particular grades of these oils, but it may be at once stated that so for

as actual yield of light is concerned, the same weight of any of the

commercial oils will give practically the same result. Hence in the

comparative statement of the cost of different methods of lighting, oil

will be taken at the cheapest rate at which it could ordinarily be

obtained, including delivery charges, at a country house, when bought by

the barrel. This rate at the present time is about ninepence per gallon.

A higher price may be paid for grades of mineral oil reputed to be safer

or to give a "brighter" or "clearer" light; but as the quantity of light

depends mainly upon the care and attention bestowed on the burner and

glass fittings of the lamp, and partly upon the employment of a suitable

wick, while the safety of each lamp depends at least as much upon the

design of that lamp, and the accuracy with which the wick fits the burner

tube, as upon the temperature at which the oil "flashes," the extra

expense involved in burning fancy-priced oils will not be considered

here.

The efficiency (_i.e._, the light yielded per pint or other unit

volume consumed) of oil-lamps varies greatly, and, speaking broadly,

increases with the power of the lamp. But as large or high-power lamps

are not needed throughout a house, it is fairer to assume that the light

obtainable from oil in ordinary household use is the mean of that

afforded by large and that afforded by small lamps. A large oil-lamp as

commonly used in country houses will give a light of about 20 candle-

power, while a convenient small lamp will give a light of not more than

about 5 candle-power. The large lamp will burn about 55 hours for every

gallon of oil consumed, or give an illuminating duty of about 1100

candle-hours (_i.e._, the product of candle-power by burning-hours)

per gallon. The small lamp, on the other hand, will burn about 140 hours

for every gallon of oil consumed, or give an illuminating duty of about

700 candle-hours per gallon. Actually large lamps would in most country

houses be used only in the entrance hall, living-rooms, and kitchen,

while passages and minor rooms on the lower floors would be lighted by

small lamps. Hence, making due allowance for the lower rate of

consumption of the small lamps, it will be seen that, given equal numbers

of large and small lamps in use, the mean illuminating duty of a gallon

of oil as burnt in country houses will be 987, or, in round figures, 990

candle-hours. Usually candles are used in the bedrooms of country houses

where the lower floors are lighted by means of petroleum lamps; but when

acetylene is installed in such a house it will frequently be adopted in

the principal bed- and dressing-rooms as well as in the living-rooms, as,

unless candles are employed very lavishly, they are really totally

inadequate to meet the reasonable demands for light of, _e.g._, a

lady dressing for dinner. Where acetylene displaces candles as well as

lamps in a country house, it is necessary, in comparing the cost of the

new illuminant with that of the candles and oil, to bear in mind the

superior degree of illumination which is secured in all rooms, at least

where candles were formerly used.

In regard to exhaustion and vitiation of the air, and to heat evolved,

self-luminous petroleum lamps stand on much the same footing as coal-gas

when the latter is burned in flat-flame burners, if the comparison is

based on a given yield of light. A large lamp, owing to its higher

illuminating efficiency, is better in this respect than a small one--

light for light, it is more hygienic than ordinary flat-flame coal-gas

burners, while a small lamp is less hygienic. It will therefore be

understood at once, from what has already been said about the superiority

on hygienic grounds of acetylene to flat-flame coal-gas lighting, that

acetylene is in this respect far superior to petroleum lamps. The degree

of its superiority is indicated more precisely by the figures quoted in

the tabular statement which concludes this chapter.

Before giving the tabular statement, however, it is necessary to say a

few words in regard to one method of lighting which, may possibly develop

into a more serious competitor with acetylene for the lighting of the

better class of country house than any of the illuminating agents and

modes of lighting so far referred to. The method in question is lighting

by so-called air-gas used for raising mantles to incandescence in

upturned or inverted burners of the Welsbach-Kern type. "Air-gas" is

ordinary atmospheric air, more or less completely saturated with the

vapour of some highly volatile hydrocarbon. The hydrocarbons practically

applied have so far been only "petroleum spirit" or "carburine," and

"benzol." "Petroleum spirit" or "carburine" consists of the more highly

volatile portion of petroleum, which is removed by distillation before

the kerosene or burning oil is recovered from the crude oil. Several

grades of this highly volatile petroleum distillate are distinguished in

commerce; they differ in the temperature at which they begin to distil

and the range of temperature covered by their distillation, and, speaking

more generally, in their degree of volatility, uniformity, and density.

If the petroleum distillate is sufficiently volatile and fairly uniform

in character, good air-gas may be produced merely by allowing air to pass

over an extended surface of the liquid. The vapour of the petroleum

spirit is of greater density than air, and hence, if the course of the

air-gas is downward from the apparatus at which it is produced, the flow

of air into the apparatus and over the surface of the spirit will be

automatically maintained by the "pull" of the descending air-gas when

once the flow has been started until the outlet for the air-gas is

stopped or the spirit in the apparatus is exhausted. Hence, if the

apparatus for saturating air with the vapour of the light petroleum is

placed well above all the points at which the air-gas is to be burnt--

_e.g._, on the roof of the house--the production of the air-gas may

by simple devices become automatic, and the only attention the apparatus

will require will be the replenishing of its reservoir from time to time

with light petroleum. But a number of precautions are required to make

this simple process operate without interruption or difficulty. For

instance, the evaporation of the spirit must not be so rapid relatively

to its total bulk as to lower its temperature, and thereby that of the

overflowing air, too much; the reservoir must be protected from extreme

cold and extreme heat; and the risk of fire from the presence of a highly

volatile and highly inflammable liquid on or near the roof of the house

must be met. This risk is one to which fire insurance companies take

exception.

More commonly, however, air-gas is made non-automatically, or more or

less automatically by the employment of some mechanical means. The light

petroleum, benzol, or other suitable volatile hydrocarbon is volatilised,

where necessary, by the application of gentle heat, while air is driven

over or through it by means of a small motor, which in some cases is a

hot-air engine operated by heat supplied by a flame of the air-gas

produced. These air-gas producers, or at least the reservoir of volatile

hydrocarbon, may be placed in an outbuilding, so that the risk of fire in

the house itself is minimised. They require, however, as much attention

as an acetylene generator, usually more. It is difficult to give reliable

data as to the cost of air-gas, inclusive of the expenses of production.

It varies considerably with the description of hydrocarbon employed, and

its market price. Air-gas is only slightly inferior hygienically to

acetylene, and the colour of its light is that of the incandescent light

as produced by coal-gas or acetylene. Air-gas of a certain grade may be

used for lighting by flat-flame burners, but it has been available thus

for very many years, and has failed to achieve even moderate success. But

the advent of the incandescent burner has completely changed its position

relatively to most other illuminants, and under certain conditions it

seems likely to be the most formidable competitor with acetylene. Since

air-gas, and the numerous chemically identical products offered under

different proprietary names, is simply atmospheric air more or less

loaded with the vapour of a volatile hydrocarbon which is normally

liquid, it possesses no definite chemical constitution, but varies in

composition according to the design of the generating plant, the

atmospheric temperature at the time of preparation, the original degree

of volatility of the hydrocarbon, the remaining degree of volatility

after the more volatile portions have been vaporised, and the speed at

which the air is passed through the carburettor. The illuminating power

and the calorific value of air-gas, unless the manufacture is very

precisely controlled, are apt to be variable, and the amount of light,

emitted, either in self-luminous or in incandescent burners, is somewhat

indeterminate. The generating plant must be so constructed that the air

cannot at any time be mixed with as much hydrocarbon vapour as

constitutes an explosive mixture with it, otherwise the pipes and

apparatus will contain a gas which will forthwith explode if it is

ignited, _i.e._, if an attempt is made to consume it otherwise than

in burners with specially small orifices. The safely permissible mixtures

are (1) air with less hydrocarbon vapour than constitutes an explosive

mixture, and (2) air with more hydrocarbon vapour than constitutes an

explosive mixture. The first of these two mixtures is available for

illuminating purposes only with incandescent mantles, and to ensure a

reasonable margin of safety the mixing apparatus must be so devised that

the proportion of hydrocarbon vapour in the air-gas can never exceed 2

per cent. From Chapter VI. it will be evident that a little more than 2

per cent. of benzene, pentane or benzoline vapour in air forms an

explosive mixture. What is the lowest proportion of such vapours in

admixture with air which will serve on combustion to maintain a mantle in

a state of incandescence, or even to afford a flame at all, does not

appear to have been precisely determined, but it cannot be much below 1-

1/2 per cent. Hence the apparatus for producing air-gas of this first

class must be provided with controlling or governing devices of such

nicety that the proportion of hydrocarbon vapour in the air-gas is

maintained between about 1-1/2 and 2 per cent. It is fair to say that in

normal working conditions a number of devices appear to fulfil this

requirement satisfactorily. The second of the two mixtures referred to

above, viz., air with more hydrocarbon vapour than constitutes an

explosive mixture, is primarily suitable for combustion in self-luminous

burners, but may also be consumed in properly designed incandescent

burners. But the generating apparatus for such air-gas must be equipped

with some governing or controlling device which will ensure the

proportion of hydrocarbon vapour in the mixture never falling below, say,

7 per cent. On the other hand, if saturation of the air with the vapour

is practically attained, should the temperature of the gas fall before it

arrives at the point of combustion, part of the spirit will condense out,

and the product will thus lose part of its illuminating or calorific

intensity, besides partially filling the pipes with liquid products of

condensation. The loss of intensity in the gas during cold weather may or

may not be inconvenient according to circumstances; but the removal of

part of the combustible material brings the residual air-gas nearer to

its limit of explosibility--for it is simply a mixture of combustible

vapour with air, which, normally, is not explosive because the proportion

of spirit is too high--and thus, when led into an atmospheric burner, the

extra amount of air introduced at the injector jets may cause the mixture

to be an explosive mixture of air and spirit, so that it will take fire

within the burner tube instead of burning quietly at the proper orifice.

This matter will be made clearer on studying what is said about explosive

limits in Chapter VI., and what is stated about incandescent acetylene

(carburetted or not) in Chapters IX. and X. Clearly, however, high-grade

air-gas is only suitable for preparation at the immediate spot where it

is to be consumed; it cannot be supplied to a complete district unless it

is intentionally made of such lower intensity that the proportion of

spirit is too small ever to allow of partial deposition in the mains

during the winter.

It is perhaps necessary to refer to the more extended use of candles for

lighting in some few houses in which lamps are disliked on aesthetic, or,

in some cases, ostensibly on hygienic grounds. Candle lighting, speaking

broadly, is either very inadequate so far as ordinary living-rooms are

concerned, or, if adequate, is very costly. Tests specially carried out

by one of the authors to determine some of the figures required in the

ensuing table show that ordinary paraffin or "wax" candles usually emit

about 20 per cent. more light than that given by the standard spermaceti

candle, whose luminosity is the unit by which the intensity of other

lights is reckoned in Great Britain; and also that the light so emitted

by domestic candles is practically unaffected by the sizes--"sixes,"

"eights," or "twelves"--burnt. In the sizes examined the light evolved

has varied between 1.145 and 1.298 "candles," perhaps tending to increase

slightly with the diameter of the candle tested. Hence, to obtain

illumination in a room equal on the average to that afforded by 100

standard candles, or some other light or lights aggregating 100 candle-

power, would require the use of only 80 to 85 ordinary paraffin,

ozokerite, or wax candles. But actually the essential objects in a room

could be equally well illuminated by, say, 30 candles well distributed,

as by two or three incandescent gas-burners, or four or five large oil-

lamps. Lights of high intensity, such as powerful gas-burners or oil-

lamps, must give a higher degree of illumination in their immediate

vicinity than is really necessary, if they are to illuminate adequately

the more distant objects. The dissemination and diffusion of their light

can be greatly aided by suitable colouring of ceilings, walls and

drapings; but unless the illumination by means of lights of relatively

high intensity is made almost wholly indirect, candles or other lights of

low intensity, such as small electric glow-lamps, can, by proper

distribution, be made to give more uniform or more suitably apportioned

illumination. In this respect candles have an economical and, in some

measure, a material advantage over acetylene also. (But when the method

of lighting is by flames--candle or other--the multiplication of the

number of units which is involved when they are of low intensity,

seriously increases the risk of fire through accidental contact of

inflammable material with any one of the flames. This risk is much

greater with naked flames, such as candles, than with, say, inverted

incandescent gas flames, which are to all intents and purposes fully

protected by a closed glass globe.) Hence, in the tabular statement which

follows of the comparative cost, &c., of different illuminants, it will

be assumed that 30 good candles would in practice be equally efficient in

regard to the illumination of a room as large oil-lamps, acetylene

flames, or incandescent gas-burners aggregating 100 candle-power.

For the same reason it will be assumed that electric glow-lamps of low

intensity (nominally of 8 candle-power or less), aggregating 70-80

candle-power, will practically serve, if suitably distributed, equally as

well as 100 candle-power obtained from more powerful sources of light.

Electric glow-lamps of a nominal intensity of 16 candles or thereabouts,

and good flat-flame gas-burners, aggregating 90-95 candle-power, will

similarly be taken as equivalent, if suitably distributed, to 100 candle-

power from more powerful sources of light. Of the latter it will be

assumed that each source has an intensity between 20 and 30 candle-power,

such as is afforded by a large oil-lamp, a No. 1 Welsbach-Kern upturned,

or a "Bijou" inverted incandescent gas-burner, or a 0.70-cubic-foot-per-

hour acetylene burner. Either of these sources of light, when used in

sufficient numbers, so that with proper distribution they light a room

adequately, will be taken in the tabular statement which follows as

affording, per candle-power evolved, the standard illuminating effect

required in that room. The same illuminating effect will be regarded as

attainable by means of candles aggregating only 35 per cent., or small

electric glow-lamps aggregating 77 per cent., or large electric glow-

lamps and flat-flame gas-burners aggregating 90 to 95 per cent. of this

candle-power; while if sources of light of higher intensity are used,

such as Osram or Tantalum electric lamps, or the larger incandescent gas-

burners (the Welsbach "C" or "York," or the Nos. 3 or 4 Welsbach-Kern

upturned, or the No. 1 or larger size inverted burners) or incandescent

acetylene burners, it will be assumed that their aggregate candle-power

must be in excess by about 15 per cent., in order to compensate for the

impossibility of obtaining equally well distributed illumination. These

assumptions are based on general considerations and data as to the effect

of sources of light of different intensities in giving practically the

same degree of illumination in a room; it would occupy too much space

here to discuss more fully the grounds on which they have been made. It

must suffice to say that they have been adopted with the object of being

perfectly fair to each means of illumination.

COST PER HOUR AND HYGIENIC EFFECT OF LIGHTING BY DIFFERENT MEANS

The data (except in the column headed "cost per 100 candle-hours") refer

to the illumination afforded by medium-sized (0.5 to 0.7 cubic foot per

hour) acetylene burners yielding together a light of about 100 candle-

power, and to the approximately equivalent illumination as afforded by

other means of illumination, when the lighting-units or sources of light

are rationally distributed.

Interest and depreciation charges on the outlay on piping or wiring a

house, on brackets, fittings, lamps, candelabra, and storage

accommodation (for carbide and oil) have been taken as equivalent for all

modes of lighting, and omitted in computing the total cost. The cost of

labour for attendance on acetylene plant, oil lamps, and candles is an

uncertain and variable item--approximately equal for all these modes of

lighting, but saved in coal-gas and electric lighting from public supply

mains.

______________________________________________________________________

| | | | | | |

| | |Power of| of | Candle- | per |

|____________|____________________|________|_________|_________|_______|

| | | | | | |

| | cubic foot per hour| 18 | 5 | 90 | 1.11 |

| Acetylene | cubic foot per hour| 27 | 4 | 108 | 1.02 |

| | cubic foot per hour| 45.5 | 3 | 136 | 0.85 |

| | cubic foot per hour| 50 | 3 | 150 | 0.49 |

|____________|____________________|________|_________|_________|_______|

| | | | | | |

| Petroleum | Large lamp . . . . | 20 | 5 | 100 | 0.84 |

| oil) | Small lamp . . . . | 5 | 14 | 70 | 1.31 |

|____________|____________________|________|_________|_________|_______|

| | | | | | |

| | cubic feet per hour| 8 | 10 | 80 | 3.75 |

| Coal Gas | cubic feet per hour| 16 | 6 | 96 | 2.25 |

| | Kern or Bijou In- | 25 | 4 | 100 | 0.38 |

|____________|____________________|________|_________|_________|_______|

| | | | | | |

| Candles |"Wax" (so-called) . | 1.2 | 30 | 35 | 6.14 |

|____________|____________________|________|_________|_________|_______|

| | | | | | |

| | Small glow . . . . | 7 | 11 | 77 | 2.81 |

| | Large glow . . . . | 13 | 7 | 91 | 2.90 |

| | Tantalum . . . . . | 19 | 5 | 95 | 1.52 |

| | Osram . . . . . . | 14 | 7 | 98 | 1.00 |

|____________|____________________|________|_________|_________|_______|

___________________________________________________________________

| | | | |

| | | | |

|____________|____________________|____________________|____________|

| | | | |

|____________|____________________|____________________|____________|

| | | | |

| oil) | Small lamp . . . . | delivery charges. | 0.92 |

|____________|____________________|____________________|____________|

| | | | |

|____________|____________________|____________________|____________|

| | | | |

|____________|____________________|____________________|____________|

| | | | |

|____________|____________________|____________________|____________|

_______________________________________________________________________

| | | | | | |

|____________|____________________|______|_________|__________|_________|

| | | | | | |

| | cubic foot per hour| [1] | 29.8 | 5.0 | 900 |

| Acetylene | cubic foot per hour| | 33.3 | 5.6 | 1010 |

| | cubic foot per hour| | 35.7 | 6.0 | 1000 |

| | cubic foot per hour| [2] | 17.9 | 3.0 | 545 |

|____________|____________________|______|_________|__________|_________|

| | | | | | |

| Petroleum | Large lamp . . . . | | 140.0 | 19.6 | 3630 |

| (paraffin | | [3] | | | |

| oil) | Small lamp . . . . | | 154.0 | 21.6 | 4000 |

|____________|____________________|______|_________|__________|_________|

| | | | | | |

| | cubic feet per hour| Nil | 270.0 | 27.0 | 7750 |

| Coal Gas | cubic feet per hour| Nil | 195.0 | 19.5 | 5580 |

| | Kern or Bijou In- | [4] | 27.0 | 2.7 | 775 |

|____________|____________________|______|_________|__________|_________|

| | | | | | |

| Candles |"Wax" (so-called) . | Nil | 100.5 | 13.7 | 2700 |

|____________|____________________|______|_________|__________|_________|

| | | | | | |

| | Small glow . . . . |2s.6d.| Nil | Nil | 285 |

| | Large glow . . . . |2s.6d.| " | " | 360 |

| Electricity| | [5] | | | |

| | Tantalum . . . . . |7s.6d.| " | " | 172 |

| | Osram . . . . . . | 6s. | " | " | 96 |

|____________|____________________|______|_________|__________|_________|

[Footnote 1: Interest and depreciation charges on generating and

purifying plant = 0.15 penny. Purifying material and burner renewals =

0.05 penny.]

[Footnote 2: Mantle renewals as for coal-gas.]

[Footnote 3: Renewals of wicks and chimneys = 0.02 penny.]

[Footnote 4: Renewals and mantles (and chimneys) at contract rate of 3s.

per burner per annum.]

[Footnote 5: Renewals of lamps and fuses, at price indicated per lamp per

annum.]

The conventional method of making pecuniary comparisons between different

sources of artificial light consists in simply calculating the cost of

developing a certain number of candle-hours of light--_i.e._, a

certain amount of standard candle-power for a given number of hours--on

the assumption that as many separate sources of light are employed as may

be required to bring the combined illuminating power up to the total

amount wanted. In view of the facts as to dissemination and diffusion, or

the difference between sheer illuminating power and useful illuminating

effect, which have just been elaborated, and in view of the different

intensities of the different unit sources of light (which range from the

single candle to a powerful large incandescent gas-burner or a metallic

filament electric lamp), such a method of calculation is wholly illusory.

The plan adopted in the following table may also appear unnecessarily

complicated; but it is not so to the reader if he remembers that the

apparently various amount of illumination is corrected by the different

numbers of illuminating units until the amount of simple candle-power

developed, whatever illuminant be employed, suffices to light a room

having an area of about 300 square feet (_i.e._, a room, 17-1/2 feet

square, or one 20 feet long by 15 feet wide), so that ordinary print may

be read comfortably in any part of the room, and the titles of books,

engravings, &c., in any position on the walls up to a height of 8 feet

from the ground may be distinguished with ease. The difference in cost,

&c., of a greater or less degree of illumination, or of lighting a larger

or smaller room by acetylene or any other of the illuminants named, will

be almost directly proportional to the cost given for the stated

conditions. Nevertheless, it should be recollected that when the

conventional system is retained--useful illuminating effect being

sacrificed to absolute illuminating power--acetylene is made to appear

cheaper in comparison with all weaker unit sources of light, and dearer

in comparison with all stronger unit sources of light than the

accompanying table indicates it to be. In using the comparative figures

given in the table, it should be borne in mind that they refer to more

general and more brilliant illumination of a room than is commonly in

vogue where the lighting is by means of electric light, candles, or oil-

lamps. The standard of illumination adopted for the table is one which is

only gaining general recognition where incandescent gas or acetylene

lighting is available, though in exceptional cases it has doubtless been

attained by means of oil-lamps or flat-flame gas-burners, but very rarely

if ever by means of carbon-filament electric glow-lamps, or candles. It

assumes that the occupants of a room do not wish to be troubled to bring

work or book "to the light," but wish to be able to work or read

wheresoever in the room they will, without consideration of the

whereabouts of the light or lights.

It should, perhaps, be added that so high a price as 5s. per 1000

cubic feet for coal-gas rarely prevails in Great Britain, except in small

outlying towns, whereas the price of 6d. per Board of Trade unit

for electricity is not uncommonly exceeded in the few similar country

places in which there is a public electricity supply.

CHAPTER II

THE PHYSICS AND CHEMISTRY OF THE REACTION BETWEEN CARBIDE AND WATER

THE NATURE OF CALCIUM CARBIDE.--The raw material from which, by

interaction with water, acetylene is obtained, is a solid body called

calcium carbide or carbide of calcium. Inasmuch as this substance can at

present only be made on a commercial scale in the electric furnace--and

so far as may be foreseen will never be made on a large scale except by

means of electricity--inasmuch as an electric furnace can only be worked

remuneratively in large factories supplied with cheap coal or water

power; and inasmuch as there is no possibility of the ordinary consumer

of acetylene ever being able to prepare his own carbide, all descriptions

of this latter substance, all methods of winning it, and all its

properties except those which concern the acetylene-generator builder or

the gas consumer have been omitted from the present book. Hitherto

calcium carbide has found but few applications beyond that of evolving

acetylene on treatment with water or some aqueous liquid, hygroscopic

solid, or salt containing water of crystallisation; but it has

possibilities of further employment, should its price become suitable,

and a few words will be devoted to this branch of the subject in Chapter

XII. Setting these minor uses aside, calcium carbide has no intrinsic

value except as a producer of acetylene, and therefore all its

characteristics which interest the consumer of acetylene are developed

incidentally throughout this volume as the necessity for dealing with

them arises.

It is desirable, however, now to discuss one point connected with solid

carbide about which some misconception prevails. Calcium carbide is a

body which evolves an inflammable, or on occasion an explosive, gas when

treated with water; and therefore its presence in a building has been

said to cause a sensible increase in the fire risk because attempts to

extinguish a fire in the ordinary manner with water may cause evolution

of acetylene which should determine a further production of flame and

heat. In the absence of water, calcium carbide is absolutely inert as

regards fire; and on several occasions drums of it have been recovered

uninjured from the basement of a house which has been totally destroyed

by fire. With the exception of small 1-lb. tins of carbide, used only by

cyclists, &c., the material is always put into drums of stout sheet-iron

with riveted or folded seams. Provided the original lid has not been

removed, the drums are air- and water-tight, so that the fireman's hose

may be directed upon them with impunity. When a drum has once been

opened, and not all of its contents have been put into the generator,

ordinary caution--not merely as regards fire, but as regards the

deterioration of carbide when exposed to the atmosphere--suggests either

that the lid must be made air-tight again (not by soldering it),

[Footnote: Carbide drums are not uncommonly fitted with self-sealing or

lever-top lids, which are readily replaced hermetically tight after

opening and partial removal of the contents of the drum.] or preferably

that the rest of the carbide shall be transferred to some convenient

receptacle which can be perfectly closed. [Footnote: It would be a

refinement of caution, though hardly necessary in practice, to fit such a

receptacle with a safety-valve. If then the vessel were subjected to

sudden or severe heating, the expansion of the air and acetylene in it

could not possibly exert a disruptive effect upon the walls of the

receptacle, which, in the absence of the safety-valve, is imaginable.]

Now, assuming this done, the drums are not dependent upon soft solder to

keep them sound, and so they cannot open with heat. Fire and water,

accordingly, cannot affect them, and only two risks remain: if stored in

the basement of a tall building, falling girders, beams or brickwork may

burst them; or if stored on an upper floor, they may fall into the

basement and be burst with the shock--in either event water then having

free access to the contents. But drums of carbide would never be stored

in such positions: a single one would be kept in the generator-house;

several would be stored in a separate room therein, or in some similar

isolated shed. The generator-house or shed would be of one story only;

the drums could neither fall nor have heavy weights fall on them during a

fire; and therefore there is no reason why, if a fire should occur, the

firemen should not be permitted to use their hose in the ordinary

fashion. Very similar remarks apply to an active acetylene generator.

Well built, such plant will stand much heat and fire without failure; if

it is non-automatic, and of combustible materials contains nothing but

gas in the holder, the worst that could happen in times of fire would be

the unsealing of the bell or its fracture, and this would be followed,

not at all by any explosion, but by a fairly quiet burning of the

escaping gas, which would be over in a very short time, and would not add

to the severity of the conflagration unless the generator-house were so

close to the residence that the large flame of burning gas could ignite

part of the main building. Even if the heat were so great near the holder

that the gas dissociated, it is scarcely conceivable that a dangerous

explosion should arise. But it is well to remember, that if the

generator-house is properly isolated from the residence, if it is

constructed of non-inflammable materials, if the attendant obeys

instructions and refrains from taking a naked light into the

neighbourhood of the plant, and if the plant itself is properly designed

and constructed, a fire at or near an acetylene generator is extremely

unlikely to occur. At the same time, before the erection of plant to

supply any insured premises is undertaken, the policy or the company

should be consulted to ascertain whether the adoption of acetylene

lighting is possibly still regarded by the insurers as adding an extra

risk or even as vitiating the whole insurance.

REGULATIONS FOR THE STORAGE OF CARBIDE: BRITISH.--There are also certain

regulations imposed by many local authorities respecting the storage of

carbide, and usually a licence for storage has to be obtained if more

than 5 lb. is kept at a time. The idea of the rule is perfectly

justifiable, and it is generally enforced in a sensible spirit. As the

rules may vary in different localities, the intending consumer of

acetylene must make the necessary inquiries, for failure to comply with

the regulations may obviously be followed by unpleasantness.

Having regard to the fact that, in virtue of an Order in Council dated

July 7, 1897, carbide may be stored without a licence only in separate

substantial hermetically closed metal vessels containing not more than 1

lb. apiece and in quantities not exceeding 5 lb. in the aggregate, and

having regard also to the fact that regulations are issued by local

authorities, the Fire Offices' Committee of the United Kingdom has not up

to the present deemed it necessary to issue special rules with reference

to the storage of carbide of calcium.

The following is a copy of the rules issued by the National Board of Fire

Underwriters of the UNITED STATES OF AMERICA for the storage of calcium

carbide on insured premises:

RULES FOR THE STORAGE OF CALCIUM CARBIDE.

(_a_) Calcium carbide in quantities not to exceed six hundred (600)

pounds may be stored, when contained in approved metal packages not to

exceed one hundred (100) pounds each, inside insured property, provided

that the place of storage be dry, waterproof and well ventilated, and

also provided that all but one of the packages in any one building shall

be sealed and the seals shall not be broken so long as there is carbide

in excess of one (1) pound in any other unsealed package in the building.

(_b_) Calcium carbide in quantities in excess of six hundred (600)

pounds must be stored above ground in detached buildings, used

exclusively for the storage of calcium carbide, in approved metal

packages, and such buildings shall be constructed to be dry, waterproof

and well ventilated.

(_c_) Packages to be approved must be made of metal of sufficient

strength to insure handling the package without rupture, and be provided

with a screwed top or its equivalent.

They must be constructed so as to be water- and air-tight without the use

of solder, and conspicuously marked "CALCIUM CARBIDE--DANGEROUS IF NOT

KEPT DRY."

The following is a summary of the AUSTRIAN GOVERNMENT rules relating to

the storage and handling of carbide:

(1) It must be sold and stored only in closed water-tight vessels, which,

if the contents exceed 10 kilos., must be marked in plain letters

"CALCIUM CARBIDE--TO BE KEPT CLOSED AND DRY." They must not be of copper

and if soldered must be opened by mechanical means and not by

unsoldering. They must be stored out of the reach of water.

(2) Quantities not exceeding 300 kilos. may be stored in occupied houses,

provided the single drums do not exceed 100 kilos. nominal capacity. The

storage-place must be dry and not underground.

(3) The limits specified in Rule 2 apply also to generator-rooms, with

the proviso also that in general the amount stored shall not exceed five

days' consumption.

(4) Quantities ranging from 300 to 1000 kilos. must be stored in special

well-ventilated uninhabited non-basement rooms in which lights and

smoking are not allowed.

(5) Quantities exceeding 1000 kilos. must be stored in isolated fireproof

magazines with light water-tight roofs. The floors must be at least 8

inches above ground-level.

(6) Carbide in water-tight drums may be stored in the open in a fenced

enclosure at least 30 feet from buildings, adjoining property, or

inflammable materials. The drums must be protected from wet by a light

roof.

(7) The breaking of carbide must be done by men provided with respirators

and goggles, and care taken to avoid the formation of dust.

(8) Local or other authorities will issue from time to time special

regulations in regard to carbide trade premises.

The ITALIAN GOVERNMENT rules relating to the storage and transport of

carbide follow in the main those of the Austrian Government, but for

quantities between 300 and 2000 kilos sanction is required from the local

authorities, and for larger quantities from superior authorities. The

storage of quantities ranging from 300 to 2000 kilos is forbidden in

dwelling-houses and above the latter quantity the storage-place must be

isolated and specially selected. No special permit is required for the

storage of quantities not exceeding 300 kilos. Workmen exposed to carbide

dust arising from the breaking of carbide or otherwise must have their

eyes and respiratory organs suitably protected.

THE PURCHASE OF CARBIDE.--Since calcium carbide is only useful as a means

of preparing acetylene, it should be bought under a guarantee (1) that it

contains less impurities than suffice to render the crude gas dangerous

in respect of spontaneous inflammability, or objectionable in a manner to

be explained later on, when consumed; and (2) that it is capable of

evolving a fixed minimum quantity of acetylene when decomposed by water.

Such determination, however, cannot be carried out by the ordinary

consumer for himself. A generator which is perfectly satisfactory in

general behaviour, and which evolves a sufficient proportion of the

possible total make of gas to be economical, does not of necessity

decompose the carbide quantitatively; nor is it constructed in a fashion

to render an exact measurement of the gas liberated at standard

temperature and pressure easy to obtain. For obvious reasons the careful

consumer of acetylene will keep a record of the carbide decomposed and of

the acetylene generated--the latter perhaps only in terms of burner-

hours, or the like; but in the event of serious dispute as to the gas-

making capacity of his raw material, he must have a proper analysis made

by a qualified chemist.

Calcium carbide is crushed by the makers into several different sizes, in

each of which all the lumps exceed a certain size and are smaller than

another size. It is necessary to find out by experiment, or from the

maker, what particular size suits the generator best, for different types

of apparatus require different sizes of carbide. Carbide cannot well be

crushed by the consumer of acetylene. It is a difficult operation, and

fraught with the production of dust which is harmful to the eyes and

throat, and if done in open vessels the carbide deteriorates in gas-

making power by its exposure to the moisture of the atmosphere. True dust

in carbide is objectionable, and practically useless for the generation

of acetylene in any form of apparatus, but carbide exceeding 1 inch in

mesh is usually sold to satisfy the suggestions of the British Acetylene

Association, which prescribes 5 per cent, of dust as the maximum. Some

grades of carbide are softer than others, and therefore tend to yield

more dust if exposed to a long journey with frequent unloadings.

There are certain varieties of ordinary carbide known as "treated

carbide," the value of which is more particularly discussed in Chapter

III. The treatment is of two kinds, or of a combination of both. In one

process the lumps are coated with a strong solution of glucose, with the

object of assisting in the removal of spent lime from their surface when

the carbide is immersed in water. Lime is comparatively much more soluble

in solutions of sugar (to which class of substances glucose belongs) than

in plain water; so that carbide treated with glucose is not so likely to

be covered with a closely adherent skin of spent lime when decomposed by

the addition of water to it. In the other process, the carbide is coated

with or immersed in some oil or grease to protect it from premature

decomposition. The latter idea, at least, fulfils its promises, and does

keep the carbide to a large extent unchanged if the lumps are exposed to

damp air, while solving certain troubles otherwise met with in some

generators (cf. Chapter III.); but both operations involve additional

expense, and since ordinary carbide can be used satisfactorily in a good

fixed generator, and can be preserved without serious deterioration by

the exercise of reasonable care, treated carbide is only to be

recommended for employment in holderless generators, of which table-lamps

are the most conspicuous forms. A third variant of plain carbide is

occasionally heard of, which is termed "scented" carbide. It is difficult

to regard this material seriously. In all probability calcium carbide is

odourless, but as it begins to evolve traces of gas immediately

atmospheric moisture reaches it, a lump of carbide has always the

unpleasant smell of crude acetylene. As the material is not to be stored

in occupied rooms, and as all odour is lost to the senses directly the

carbide is put into the generator, scented carbide may be said to be

devoid of all utility.

THE REACTION BETWEEN CARBIDE AND WATER.--The reaction which occurs when

calcium carbide and water are brought into contact belongs to the class

that chemists usually term double decompositions. Calcium carbide is a

chemical compound of the metal calcium with carbon, containing one

chemical "part," or atomic weight, of the former united to two chemical

parts, or atomic weights, of the latter; its composition expressed in

symbols being CaC_2. Similarly, water is a compound of two chemical parts

of hydrogen with one of oxygen, its formula being H_2O. When those two

substances are mixed together the hydrogen of the water leaves its

original partner, oxygen, and the carbon of the calcium carbide leaves

the calcium, uniting together to form that particular compound of

hydrogen and carbon, or hydrocarbon, which is known as acetylene, whose

formula is C_2H_2; while the residual calcium and oxygen join together to

produce calcium oxide or lime, CaO. Put into the usual form of an

equation, the reaction proceeds thus--

(1) CaC_2 + H_2O = C_2H_2 + CaO.

This equation not only means that calcium carbide and water combine to

yield acetylene and lime, it also means that one chemical part of carbide

reacts with one chemical part of water to produce one chemical part of

acetylene and one of lime. But these four chemical parts, or molecules,

which are all equal chemically, are not equal in weight; although,

according to a common law of chemistry, they each bear a fixed proportion

to one another. Reference to the table of "Atomic Weights" contained in

any text-book of chemistry will show that while the symbol Ca is used,

for convenience, as a contraction or sign for the element calcium simply,

it bears a more important quantitative significance, for to it will be

found assigned the number 40. Against carbon will be seen the number 12;

against oxygen, 16; and against hydrogen, 1. These numbers indicate that

if the smallest weight of hydrogen ever found in a chemical compound is

called 1 as a unit of comparison, the smallest weights of calcium,

carbon, and oxygen, similarly taking part in chemical reactions are 40,

12, and 16 respectively. Thus the symbol CaC_2, comes to convoy three

separate ideas: (_a_) that the substance referred to is a compound

of calcium and carbon only, and that it is therefore a carbide of

calcium; (_b_) that it is composed of one chemical part or atom of

calcium and two atoms of carbon; and (_c_) that it contains 40 parts

by weight of calcium combined with twice twelve, or 24, parts of carbon.

It follows from (_c_) that the weight of one chemical part, now

termed a molecule as the substance is a compound, of calcium carbide is

(40 + 2 x 12) = 64. By identical methods of calculation it will be found

that the weight of one molecule of water is 18; that of acetylene, 26;

and that of lime, 56. The general equation (1) given above, therefore,

states in chemical shorthand that 64 parts by weight of calcium carbide

react with 18 parts of water to give 26 parts by weight of acetylene and

56 parts of lime; and it is very important to observe that just as there

are the same number of chemical parts, viz., 2, on each side, so there

are the same number of parts by weight, for 64 + 18 = 56 + 26 = 82. Put

into other words equation (1) shows that if 64 grammes, lb., or cwts. of

calcium carbide are treated with 18 grammes, lb., or cwts. of water, the

whole mass will be converted into acetylene and lime, and the residue

will not contain any unaltered calcium carbide or any water; whence it

may be inferred, as is the fact, that if the weights of carbide and water

originally taken do not stand to one another in the ratio 64 : 18, both

substances cannot be entirely decomposed, but a certain quantity of the

one which was in excess will be left unattacked, and that quantity will

be in exact accordance with the amount of the said excess--indifferently

whether the superabundant substance be carbide or water.

Hitherto, for the sake of simplicity, the by-product in the preparation

of acetylene has been described as calcium oxide or quicklime. It is,

however, one of the leading characteristics of this body to be

hygroscopic, or greedy of moisture; so that if it is brought into the

presence of water, either in the form of liquid or as vapour, it

immediately combines therewith to yield calcium hydroxide, or slaked

lime, whose chemical formula is Ca(OH)_2. Accordingly, in actual

practice, when calcium carbide is mixed with an excess of water, a

secondary reaction takes place over and above that indicated by equation

(1), the quicklime produced combining with one chemical part or molecule

of water, thus--

CaO + H_2O = Ca(OH)_2.

As these two actions occur simultaneously, it is more usual, and more in

agreement with the phenomena of an acetylene generator, to represent the

decomposition of calcium carbide by the combined equation--

(2) CaC_2 + 2H_2O = C_2H_2 + Ca(OH)_2.

By the aid of calculations analogous to those employed in the preceding

paragraph, it will be noticed that equation (2) states that 1 molecule of

calcium carbide, or 64 parts by weight, combines with 2 molecules of

water, or 36 parts by weight, to yield 1 molecule, or 26 parts by weight

of acetylene, and 1 molecule, or 74 parts by weight of calcium hydroxide

(slaked lime). Here again, if more than 36 parts of water are taken for

every 64 parts of calcium carbide, the excess of water over those 36

parts is left undecomposed; and in the same fashion, if less than 36

parts of water are taken for every 64 parts of calcium carbide, some of

the latter must remain unattacked, whilst, obviously, the amount of

acetylene liberated cannot exceed that which corresponds with the

quantity of substance suffering complete decomposition. If, for example,

the quantity of water present in a generator is more than chemically

sufficient to attack all the carbide added, however largo or small that

excess may be, no more, and, theoretically speaking, no less, acetylene

can ever be evolved than 26 parts by weight of gas for every 64 parts by

weight of calcium carbide consumed. It is, however, not correct to invert

the proposition, and to say that if the carbide is in excess of the water

added, no more, and, theoretically speaking, no less, acetylene can ever

be evolved than 26 parts by weight of gas for every 36 parts of water

consumed, as might be gathered from equation (2); because equation (1)

shows that 26 parts of acetylene may, on occasion, be produced by the

decomposition of 18 parts by weight of water. From the purely chemical

point of view this apparent anomaly is explained by the circumstance that

of the 36 parts of water present on the left-hand aide of equation (2),

only one-half, _i.e._, 18 parts by weight, are actually decomposed

into hydrogen and oxygen, the other 18 parts remaining unattacked, and

merely attaching themselves as "water of hydration" to the 56 parts of

calcium oxide in equation (1) so as to produce the 74 parts of calcium

hydroxide appearing on the right-hand side of equation (2). The matter is

perhaps rendered more intelligible by employing the old name for calcium

hydroxide or slaked lime, viz., hydrated oxide of calcium, and by writing

its formula in the corresponding form, when equation (2) becomes

CaC_2 + 2H_2O = C_2H_2 + CaO.H_2O.

It is, therefore, absolutely correct to state that if the amount of

calcium carbide present in an acetylene generator is more than chemically

sufficient to decompose all the water introduced, no more, and

theoretically speaking no less, acetylene can ever be liberated than 26

parts by weight of gas for every 18 parts by weight of water attacked.

This, it must be distinctly understood, is the condition of affairs

obtaining in the ideal acetylene generator only; since, for reasons which

will be immediately explained, when the output of gas is measured in

terms of the water decomposed, in no commercial apparatus, and indeed in

no generator which can be imagined fit for actual employment, does that

output of gas ever approach the quantitative amount; but the volume of

water used, if not actually disappearing, is always vastly in excess of

the requirements of equation (2). On the contrary, when the make of gas

is measured in terms of the calcium carbide consumed, the said make may,

and frequently does, reach 80, 90, or even 99 per cent. of what is

theoretically possible. Inasmuch as calcium carbide is the one costly

ingredient in the manufacture of acetylene, so long as it is not wasted--

so long, that is to say, as nearly the theoretical yield of gas is

obtained from it--an acetylene generator is satisfactory or efficient in

this particular; and except for the matter of solubility discussed in the

following chapter, the quantity of water consumed is of no importance

whatever.

HEAT EVOLVED IN THE REACTION.--The chemical reaction between calcium

carbide and water is accompanied by a large evolution of heat, which,

unless due precautions are taken to prevent it, raises the temperature of

the substances employed, and of the apparatus containing them, to a

serious and often inconvenient extent. This phenomenon is the most

important of all in connexion with acetylene manufacture; for upon a

proper recognition of it, and upon the character of the precautions taken

to avoid its numerous evil effects, depend the actual value and capacity

for smooth working of any acetylene generator. Just as, by an immutable

law of chemistry, a given weight of calcium carbide yields a given weight

of acetylene, and by no amount of ingenuity can be made to produce either

more or less; so, by an equally immutable law of physics, the

decomposition of a given weight of calcium carbide by water, or the

decomposition of a given weight of water by calcium carbide, yields a

perfectly definite quantity of heat--a quantity of heat which cannot be

reduced or increased by any artifice whatever. The result of a production

of heat is usually to raise the temperature of the material in which it

is produced; but this is not always the case, and indeed there is no

necessary connexion or ratio between the quantity of heat liberated in

any form of chemical reaction--of which ordinary combustion is the

commonest type--and the temperature attained by the substances concerned.

This matter has so weighty a bearing upon acetylene generation, and

appears to be so frequently misunderstood, that a couple of illustrations

may with advantage be studied. If a vessel full of cold water, and

containing also a thermometer, is placed over a lighted gas-burner, at

first the temperature of the liquid rises steadily, and there is clearly

a ratio between the size of the flame and the speed at which the mercury

mounts up the scale. Finally, however, the thermometer indicates a

certain point, viz., 100° C, and the water begins to boil; yet although

the burner is untouched, and consequently, although heat must be passing

into the vessel at the same rate as before, the mercury refuses to move

as long as any liquid water is left. By the use of a gas meter it might

be shown that the same volume of gas is always consumed (_a_) in

raising the temperature of a given quantity of cold water to the boiling-

point, and another equally constant volume of gas is always consumed

(_b_) in causing the boiling water to disappear as steam. Hence, as

coal-gas is assumed for the present purpose to possess invariably the

same heating power, it appears that the same quantity of heat is always

needed to convert a given amount of cold water at a certain temperature

into steam; but inasmuch as reference to the meter would show that about

5 times the volume of gas is consumed in changing the boiling water into

steam as is used in heating the cold water to the boiling-point, it will

be evident that the temperature of the mass is raised as high by the heat

evolved during the combustion of one part of gas as it is by that

liberated on the combustion of 6 times that amount.

A further example of the difference between quantity of heat and sensible

temperature may be seen in the combustion of coal, for (say) one

hundredweight of that fuel might be consumed in a very few minutes in a

furnace fitted with a powerful blast of air, the operation might be

spread over a considerable number of hours in a domestic grate, or the

coal might be allowed to oxidise by exposure to warm air for a year or

more. In the last case the temperature might not attain that of boiling

water, in the second it would be about that of dull redness, and in the

first it would be that of dazzling whiteness; but in all three cases the

total quantity of heat produced by the time the coal was entirely

consumed would be absolutely identical. The former experiment with water

and a gas-burner, too, might easily be modified to throw light upon

another problem in acetylene generation, for it would be found that if

almost any other liquid than water were taken, less gas (_i.e._, a

smaller quantity of heat) would be required to raise a given weight of it

from a certain low to a certain high temperature than in the case of

water itself; while if it were possible similarly to treat the same

weight of iron (of which acetylene generators are constructed), or of

calcium carbide, the quantity of heat used to raise it through a given

number of thermometric degrees would hardly exceed one-tenth or one-

quarter of that needed by water itself. In technical language this

difference is due to the different specific heats of the substances

mentioned; the specific heat of a body being the relative quantity of

heat consumed in raising a certain weight of it a certain number of

degrees when the quantity of heat needed to produce the same effect on

the same weight of water is called unity. Thus, the specific heat of

water being termed 1.0, that of iron or steel is 0.1138, and that of

calcium carbide 0.247, [Footnote: This is Carlson's figure. Morel has

taken the value 0.103 in certain calculations.] both measured at

temperatures where water is a liquid. Putting the foregoing facts in

another shape, for a given rise in temperature that substance will absorb

the most heat which has the highest specific heat, and therefore, in this

respect, 1 part by weight of water will do the work of roughly 9 parts by

weight of iron, and of about 4 parts by weight of calcium carbide.

From the practical aspect what has been said amounts to this: During the

operation of an acetylene generator a large amount of heat is produced,

the quantity of which is beyond human control. It is desirable, for

various reasons, that the temperature shall be kept as low as possible.

There are three substances present to which the heat may be compelled to

transfer itself until it has opportunity to pass into the surrounding

atmosphere: the material of which the apparatus is constructed, the gas

which is in process of evolution, and whichever of the two bodies--

calcium carbide or water--is in excess in the generator. Of these, the

specific heat at constant pressure of acetylene has unfortunately not yet

been determined, but its relative capacity for absorbing heat is

undoubtedly small; moreover the gas could not be permitted to become

sufficiently hot to carry off the heat without grave disadvantages. The

specific heat of calcium carbide is also comparatively small, and there

are similar disadvantages in allowing it to become hot; moreover it is

deficient in heat-conducting power, so that heat communicated to one

portion of the mass does not extend rapidly throughout, but remains

concentrated in one spot, causing the temperature to rise objectionably.

Steel has a sufficient amount of heat-conducting power to prevent undue

concentration in one place; but, as has been stated, its specific heat is

only one-ninth that of water. Water is clearly, therefore, the proper

substance to employ for the dissipation of the heat generated, although

it is strictly speaking almost devoid of heat-conducting power; for not

only is the specific heat of water much greater than that of any other

material present, but it possesses in a high degree the faculty of

absorbing heat throughout its mass, by virtue of the action known as

convection, provided that heat is communicated to it at or near the

bottom, and not too near its upper surface. Moreover, water is a much

more valuable substance for dissipating heat than appears from the

foregoing explanation; for reference to the experiment with the gas-

burner will show that six and a quarter times as much heat can be

absorbed by a given weight of water if it is permitted to change into

steam, as if it is merely raised to the boiling-point; and since by no

urging of the gas-burner can the temperature be raised above 100° C. as

long as any liquid water remains unevaporated, if an excess of water is

employed in an acetylene generator, the temperature inside can never--

except quite locally--exceed 100° C., however fast the carbide be

decomposed. An indefinitely large consumption of water by evaporation in

a generator matters nothing, for the liquid may be considered of no

pecuniary value, and it can all be recovered by condensation in a

subsequent portion of the plant.

It has been said that the quantity of heat liberated when a certain

amount of carbide suffers decomposition is fixed; it remains now to

consider what that quantity is. Quantities of heat are always measured in

terms of the amount needed to raise a certain weight of water a certain

number of degrees on the thermometric scale. There are several units in

use, but the one which will be employed throughout this book is the

"Large Calorie"; a large calorie being the amount of heat absorbed in

raising 1 kilogramme of water 1° C. Referring for a moment to what has

been said about specific heats, it will be apparent that if 1 large

calorie is sufficient to heat 1 kilo, of water through 1° C. the same

quantity will heat 1 kilo. of steel, whose specific heat is roughly 0.11,

through (10/011) = 9° C., or, which comes to the same thing, will heat 9

kilos, of steel through 1° C.; and similarly, 1 large calorie will raise

4 kilos. of calcium carbide 1° C. in temperature, or 1 kilo. 4° C. The

fact that a definite quantity of heat is manifested when a known weight

of calcium carbide is decomposed by water is only typical; for in every

chemical process some disturbance of heat, though not necessarily of

sensible (or thermometric) character, occurs, heat being either absorbed

or set free. Moreover, if when given weights of two or more substances

unite to form a given weight of another substance, a certain quantity of

heat is set free, precisely the same amount of heat is absorbed, or

disappears, when the latter substance is decomposed to form the same

quantities of the original substances; and, _per contra_, if the

combination is attended by a disappearance of heat, exactly the same

amount is liberated when the compound is broken up into its first

constituents. Compounds are therefore of two kinds: those which absorb

heat during their preparation, and consequently liberate heat when they

are decomposed--such being termed endothermic; and those which evolve

heat during their preparation, and consequently absorb heat when they are

decomposed--such being called exothermic. If a substance absorbs heat

during its formation, it cannot be produced unless that heat is supplied

to it; and since heat, being a form of motion, is equally a form of

energy, energy must be supplied, or work must be done, before that

substance can be obtained. Conversely, if a substance evolves heat during

its formation, its component parts evolve energy when the said substance

is being produced; and therefore the mere act of combination is

accompanied by a facility for doing work, which work may be applied in

assisting some other reaction that requires heat, or may be usefully

employed in any other fashion, or wasted if necessary. Seeing that there

is a tendency in nature for the steady dissipation of energy, it follows

that an exothermic substance is stable, for it tends to remain as it is

unless heat is supplied to it, or work is done upon it; whereas,

according to its degree of endothermicity, an endothermic substance is

more or less unstable, for it is always ready to emit heat, or to do

work, as soon as an opportunity is given to it to decompose. The

theoretical and practical results of this circumstance will be elaborated

in Chapter VI., when the endothermic nature of acetylene is more fully

discussed.

A very simple experiment will show that a notable quantity of heat is set

free when calcium carbide is brought into contact with water, and by

arranging the details of the apparatus in a suitable manner, the quantity

of heat manifested may be measured with considerable accuracy. A lengthy

description of the method of performing this operation, however, scarcely

comes within the province of the present book, and it must be sufficient

to say that the heat is estimated by decomposing a known weight of

carbide by means of water in a small vessel surrounded on all sides by a

carefully jacketed receptacle full of water and provided with a sensitive

thermometer. The quantity of water contained in the outer vessel being

known, and its temperature having been noted before the reaction

commences, an observation of the thermometer after the decomposition is

finished, and when the mercury has reached its highest point, gives data

which show that the reaction between water and a known weight of calcium

carbide produces heat sufficient in amount to raise a known weight of

water through a known thermometric distance; and from these figures the

corresponding number of large calories may easily be calculated. A

determination of this quantity of heat has been made experimentally by

several investigators, including Lewes, who has found that the heat

evolved on decomposing 1 gramme of ordinary commercial carbide with water

is 0.406 large calorie. [Footnote: Lewes returns his result as 406

calories, because he employs the "small calorie." The small calorie is

the quantity of heat needed to raise 1 gramme of water 1° C.; but as

there are 1000 grammes in 1 kilogramme, the large calorie is equal to

1000 small calories. In many respects the former unit is to be

preferred.] As the material operated upon contained only 91.3 per cent.

of true calcium carbide, he estimates the heat corresponding with the

decomposition of 1 gramme of pure carbide to be 0.4446 large calorie. As,

however, it is better, and more in accordance with modern practice, to

quote such data in terms of the atomic or molecular weight of the

substance concerned, and as the molecular weight of calcium carbide is

64, it is preferable to multiply these figures by 64, stating that,

according to Lewes' researches, the heat of decomposition of "1 gramme-

molecule" (_i.e._, 64 grammes) of a calcium carbide having a purity

of 91.3 per cent. is just under 26 calories, or that of 1 gramme-molecule

of pure carbide 28.454 calories. It is customary now to omit the phrase

"one gramme-molecule" in giving similar figures, physicists saying simply

that the heat of decomposition of calcium carbide by water when calcium

hydroxide is the by-product, is 28.454 large calories.

Assuming all the necessary data known, as happens to be the case in the

present instance, it is also possible to calculate theoretically the heat

which should be evolved on decomposing calcium carbide by means of water.

Equation (2), given on page 24, shows that of the substances taking part

in the reaction 1 molecular weight of calcium carbide is decomposed, and

1 molecular weight of acetylene is formed. Of the two molecules of water,

only one is decomposed, the other passing to the calcium hydroxide

unchanged; and the 1 molecule of calcium hydroxide is formed by the

combination of 1 atom of free calcium, 1 atom of free oxygen, and 1

molecule of water already existing as such. Calcium hydroxide and water

are both exothermic substances, absorbing heat when they are decomposed,

liberating it when they are formed. Acetylene is endothermic, liberating

heat when it is decomposed, absorbing it when it is produced.

Unfortunately there is still some doubt about the heat of formation of

calcium carbide, De Forcrand returning it as -0.65 calorie, and Gin as

+3.9 calories. De Forcrand's figure means, as before explained, that 64

grammes of carbide should absorb 0.65 large calorie when they are

produced by the combination of 40 grammes of calcium with 24 grammes of

carbon; the minus sign calling attention to the belief that calcium

carbide is endothermic, heat being liberated when it suffers

decomposition. On the contrary, Gin's figure expresses the idea that

calcium carbide is exothermic, liberating 3.9 calories when it is

produced, and absorbing them when it is decomposed. In the absence of

corroborative evidence one way or the other, Gin's determination will be

accepted for the ensuing calculation. In equation (2), therefore, calcium

carbide is decomposed and absorbs heat; water is decomposed and absorbs

heat; acetylene is produced and absorbs heat; and calcium hydroxide is

produced liberating heat. On consulting the tables of thermo-chemical

data given in the various text-books on physical chemistry, all the other

constants needed for the present purpose will be found; and it will

appear that the heat of formation of water is +69 calories, that of

acetylene -58.1 calories, and that of calcium hydroxide, when 1 atom of

calcium, 1 atom of oxygen, and 1 molecule of water unite together, is

+160.1 calories. [Footnote: When 1 atom of calcium, 2 atoms of oxygen,

and 2 atoms of hydrogen unite to form solid calcium hydroxide, the heat

of formation of the latter is 229.1 (cf. _infra_). This value is

simply 160.1 + 69.0 = 229.1; 69.0 being the heat of formation of water.]

Collecting the results into the form of a balance-sheet, the effect of

decomposing calcium carbide with water is this:

_Heat liberated._ | _Heat absorbed._

Formation of Ca(OH)_2 16O.1 | Formation of acetylene 58.1

| Decomposition of water 69.0

| Decomposition of carbide 3.9

| Balance 29.1

_____ | _____

Total 160.1 | Total 160.1

Therefore when 64 grammes of calcium carbide are decomposed by water, or

when 18 grammes of water are decomposed by calcium carbide (the by-

product in each case being calcium hydroxide or slaked lime, for the

formation of which a further 18 grammes of water must be present in the

second instance), 29.1 large calories are set free. It is not possible

yet to determine thermo-chemical data with extreme accuracy, especially

on such a material as calcium carbide, which is hardly to be procured in

a state of chemical purity; and so the value 28.454 calories

experimentally found by Lewes agrees very satisfactorily, considering all

things, with the calculated value 29.1 calories. It is to be noticed,

however, that the above calculated value has been deduced on the

assumption that the calcium hydroxide is obtained as a dry powder; but as

slaked lime is somewhat soluble in water, and as it evolves 3 calories in

so dissolving, if sufficient water is present to take up the calcium

hydroxide entirely into the liquid form (_i.e._, that of a

solution), the amount of heat set free will be greater by those 3

calories, _i.e._, 32.1 large calories altogether.

THE PROCESS OF GENERATION.--Taking 28 as the number of large calories

developed when 64 grammes of ordinary commercial calcium carbide are

decomposed with sufficient water to leave dry solid calcium hydroxide as

the by-product in acetylene generation, this quantity of heat is capable

of exerting any of the following effects. It is sufficient (1) to raise

1000 grammes of water through 28° C., say from 10° C. (50° F., which is

roughly the temperature of ordinary cold water) to 38° C. It is

sufficient (2) to raise 64 grammes of water (a weight equal to that of

the carbide decomposed) through 438° C., if that were possible. It would

raise (3) 311 grammes of water through 90° C., _i.e._, from 10° C.

to the boiling-point. If, however, instead of remaining in the liquid

state, the water were converted into vapour, the same quantity of heat

would suffice (4) to change 44.7 grammes of water at 10° C. into steam at

100° C.; or (5) to change 46.7 grammes of water at 10° C. into vapour at

the same temperature. It is an action of the last character which takes

place in acetylene generators of the most modern and usual pattern, some

of the surplus water being evaporated and carried away as vapour at a

comparatively low temperature with the escaping gas; for it must be

remembered that although steam, as such, condenses into liquid water

immediately the surrounding temperature falls below 100° C., the vapour

of water remains uncondensed, even at temperatures below the freezing-

point, when that vapour is distributed among some permanent gas--the

precise quantity of vapour so remaining being a function of the

temperature and barometric height. Thus it appears that if the heat

evolved during the decomposition of calcium carbide is not otherwise

consumed, it is sufficient in amount to vaporise almost exactly 3 parts

by weight of water for every 4 parts of carbide attacked; but if it were

expended upon some substance such as acetylene, calcium carbide, or

steel, which, unlike water, could not absorb an extra amount by changing

its physical state (from solid to liquid, or from liquid to gas), the

heat generated during the decomposition of a given weight of carbide

would suffice to raise an equal weight of the particular substance under

consideration to a temperature vastly exceeding 438° C. The temperature

attained, indeed, measured in Centigrade degrees, would be 438 multiplied

by the quotient obtained on dividing the specific heat of water by the

specific heat of the substance considered: which quotient, obviously, is

the "reciprocal" of the specific heat of the said substance.

The analogy to the combustion of coal mentioned on a previous page shows

that although the quantity of heat evolved during a certain chemical

reaction is strictly fixed, the temperature attained is dependent on the

time over which the reaction is spread, being higher as the process is

more rapid. This is due to the fact that throughout the whole period of

reaction heat is escaping from the mass, and passing into the atmosphere

at a fairly constant speed; so that, clearly, the more slowly heat is

produced, the better opportunity has it to pass away, and the less of it

is left to collect in the material under consideration. During the action

of an acetylene generator, there is a current of gas constantly

travelling away from the carbide, there is vapour of water constantly

escaping with the gas, there are the walls of the generator itself

constantly exposed to the cooling action of the atmosphere, and there is

either a mass of calcium carbide or of water within the generator. It is

essential for good working that the temperature of both the acetylene and

the carbide shall be prevented from rising to any noteworthy extent;

while the amount of heat capable of being dissipated into the air through

the walls of the apparatus in a given time is narrowly limited, depending

upon the size and shape of the generator, and the temperature of the

surrounding air. If, then, a small, suitably designed generator is

working quite slowly, the loss of heat through the external walls of the

apparatus may easily be rapid enough to prevent the internal temperature

from rising objectionably high; but the larger the generator, and the

more rapidly it is evolving gas, the less does this become possible.

Since of the substances in or about a generator water is the one which

has by far the largest capacity for absorbing heat, and since it is the

only substance to which any necessary quantity of heat can be safely or

conveniently transmitted, it follows that the larger in size an acetylene

generator is, or the more rapidly that generator is made to deliver gas,

the more desirable is it to use water as the means for dissipating the

surplus heat, and the more necessary is it to employ an apparatus in

which water is in large chemical excess at the actual place of

decomposition.

The argument is sometimes advanced that an acetylene generator containing

carbide in excess will work satisfactorily without exhibiting an

undesirable rise in internal temperature, if the vessel holding the

carbide is merely surrounded by a large quantity of cold water. The idea

is that the heat evolved in that particular portion of the charge which

is suffering decomposition will be communicated with sufficient speed

throughout the whole mass of calcium carbide present, whence it will pass

through the walls of the containing vessel into the water all round.

Provided the generator is quite small, provided the carbide container is

so constructed as to possess the maximum of superficial area with the

minimum of cubical capacity (a geometrical form to which the sphere, and

in one direction the cylinder, are diametrically opposed), and provided

the walls of the container do not become coated internally or externally

with a coating of lime or water scale so as to diminish in heat-

transmitting power, an apparatus designed in the manner indicated is

undoubtedly free from grave objection; but immediately any of those

provisions is neglected, trouble is likely to ensue, for the heat will

not disappear from the place of actual reaction at the necessary speed.

Apparent proof that heat is not accumulating unduly in a water-jacketed

carbide container even when the generator is evolving gas at a fair speed

is easy to obtain; for if, as usually happens, the end of the container

through which the carbide is inserted is exposed to the air, the hand may

be placed upon it, and it will be found to be only slightly warm to the

touch. Such a test, however, is inconclusive, and frequently misleading,

because if more than a pound or two of carbide is present as an undivided

mass, and if water is allowed to attack one portion of it, that

particular portion may attain a high temperature while the rest is

comparatively cool: and if the bulk of the carbide is comparatively cool,

naturally the walls of the containing vessel themselves remain

practically unheated. Three causes work together to prevent this heat

being dissipated through the walls of the carbide vessel with sufficient

rapidity. In the first place, calcium carbide itself is a very bad

conductor of heat. So deficient in heat-conducting power is it that a

lump a few inches in diameter may be raised to redness in a gas flame at

one spot, and kept hot for some minutes, while the rest of the mass

remains sufficiently cool to be held comfortably in the fingers. In the

second place, commercial carbide exists in masses of highly irregular

shape, so that when they are packed into any vessel they only touch at

their angles and edges; and accordingly, even if the material were a

fairly good heat conductor of itself, the air or gas present between each

lump would act as an insulator, protecting the second piece from the heat

generated in the first. In the third place, the calcium hydroxide

produced as the by-product when calcium carbide is decomposed by water

occupies considerably more space than the original carbide--usually two

or three times as much space, the exact figures depending upon the

conditions in which it is formed--and therefore a carbide container

cannot advisedly be charged with more than one-third the quantity of

solid which it is apparently capable of holding. The remaining two-thirds

of the space is naturally full of air when the container is first put

into the generator, but the air is displaced by acetylene as soon as gas

production begins. Whether that space, however, is occupied by air, by

acetylene, or by a gradually growing loose mass of slaked lime, each

separate lump of hot carbide is isolated from its neighbours by a

material which is also a very bad heat conductor; and the heat has but

little opportunity of distributing itself evenly. Moreover, although iron

or steel is a notably better conductor of heat than any of the other

substances present in the carbide vessel, it is, as a metal, only a poor

conductor, being considerably inferior in this respect to copper. If heat

dissipation were the only point to be studied in the construction of an

acetylene apparatus, far better results might be obtained by the

employment of copper for the walls of the carbide container; and possibly

in that case a generator of considerable size, fitted with a water-

jacketed decomposing vessel, might be free from the trouble of

overheating. Nevertheless it will be seen in Chapter VI. that the use of

copper is not permissible for such purposes, its advantages as a good

conductor of heat being neutralised by its more important defects.

When suitable precautions are not taken to remove the heat liberated in

an acetylene apparatus, the temperature of the calcium carbide

occasionally rises to a remarkable degree. Investigating this point, Caro

has studied the phenomena of heat production in a "dipping" generator--

_i.e._, an apparatus in which a cage of carbide is alternately

immersed in and lifted out of a vessel containing water. Using a

generator designed to supply five burners, he has found a maximum

recording thermometer placed in the gas space of the apparatus to give

readings generally between 60° and 100° C.; but in two tests out of ten

he obtained temperatures of about 160° C. To determine the actual

temperature of the calcium carbide itself, he scattered amongst the

carbide charge fragments of different fusible metallic alloys which were

known to melt or soften at certain different temperatures. In all his ten

tests the alloys melting at 120° C. were fused completely; in two tests

other alloys melting at 216° and 240° C. showed signs of fusion; and in

one test an alloy melting at 280° C. began to soften. Working with an

experimental apparatus constructed on the "dripping" principle--

_i.e._, a generator in which water is allowed to fall in single

drops or as a fine stream upon a mass of carbide--with the deliberate

object of ascertaining the highest temperatures capable of production

when calcium carbide is decomposed in this particular fashion, and

employing for the measurement of the heat a Le Chatelier thermo-couple,

with its sensitive wires lying among the carbide lumps, Lewes has

observed a maximum temperature of 674° C. to be reached in 19 minutes

when water was dripped upon 227 grammes of carbide at a speed of about 8

grammes per minute. In other experiments he used a laboratory apparatus

designed upon the "dipping" principle, and found maximum temperatures, in

four different trials, of 703°, 734°, 754°, and 807° C., which were

reached in periods of time ranging from 12 to 17 minutes. Even allowing

for the greater delicacy of the instrument adopted by Lewes for measuring

the temperature in comparison with the device employed by Caro, there

still remains an astonishing difference between Caro's maximum of 280°

and Lewes' maximum of 807° C. The explanation of this discrepancy is to

be inferred from what has just been said. The generator used by Caro was

properly made of metal, was quite small in size, was properly designed

with some skill to prevent overheating as much as possible, and was

worked at the speed for which it was intended--in a word, it was as good

an apparatus as could be made of this particular type. Lewes' generator

was simply a piece of glass and metal, in which provisions to avoid

overheating were absent; and therefore the wide difference between the

temperatures noted does not suggest any inaccuracy of observation or

experiment, but shows what can be done to assist in the dissipation of

heat by careful arrangement of parts. The difference in temperature

between the acetylene and the carbide in Caro's test accentuates the

difficulty of gauging the heat in a carbide vessel by mere external

touch, and supplies experimental proof of the previous assertions as to

the low heat-conducting power of calcium carbide and of the gases of the

decomposing vessel. It must not be supposed that temperatures such as

Lewes has found ever occur in any commercial generator of reasonably good

design and careful construction; they must be regarded rather as

indications of what may happen in an acetylene apparatus when the

phenomena accompanying the evolution of gas are not understood by the

maker, and when all the precautions which can easily be taken to avoid

excessive heating have been omitted, either by building a generator with

carbide in excess too large in size, or by working it too rapidly, or

more generally by adopting a system of construction unsuited to the ends

in view. The fact, however, that Lewes has noted the production of a

temperature of 807° C. is important; because this figure is appreciably

above the point 780° C., at which acetylene decomposes into its elements

in the absence of air.

Nevertheless the production of a temperature somewhat exceeding 100° C.

among the lumps of carbide actually undergoing decomposition can hardly

be avoided in any practical generator. Based on a suggestion in the

"Report of the Committee on Acetylene Generators" which was issued by the

British Home Office in 1902, Fouché has proposed that 130° C., as

measured with the aid of fusible metallic rods, [Footnote: An alloy made

by melting together 55 parts by weight of commercial bismuth and 45 parts

of lead fuses at 127° C., and should be useful in performing the tests.]

should be considered the maximum permissible temperature in any part of a

generator working at full speed for a prolonged period of time. Fouché

adopts this figure on the ground that 130° C. sensibly corresponds with

the temperature at which a yellow substance is formed in a generator by a

process of polymerisation; and, referring to French conditions, states

that few actual apparatus permit the development of so high a

temperature. As a matter of fact, however, a fairly high temperature

among the carbide is less important than in the gas, and perhaps it would

be better to say that the temperature in any part of a generator occupied

by acetylene should not exceed 100° C. Fraenkel has carried out some

experiments upon the temperature of the acetylene immediately after

evolution in a water-to-carbide apparatus containing the carbide in a

subdivided receptacle, using an apparatus now frequently described as

belonging to the "drawer" system of construction. When a quantity of

about 7 lb. of carbide was distributed between 7 different cells of the

receptacle, each cell of which had a capacity of 25 fluid oz., and the

apparatus was caused to develop acetylene at the rate of 7 cubic feet per

hour, maximum thermometers placed immediately over the carbide in the

different cells gave readings of from 70° to 90° C., the average maximum

temperature being about 80° C. Hence the Austrian code of rules issued in

1905 governing the construction of acetylene apparatus contains a clause

to the effect that the temperature in the gas space of a generator must

never exceed 80° C.; whereas the corresponding Italian code contains a

similar stipulation, but quotes the maximum temperature as 100° C.

(_vide_ Chapter IV.).

It is now necessary to see why the production of an excessively high

temperature in an acetylene generator has to be avoided. It must be

avoided, because whenever the temperature in the immediate neighbourhood

of a mass of calcium carbide which is evolving acetylene under the attack

of water rises materially above the boiling-point of water, one or more

of three several objectionable effects is produced--(_a_) upon the

gas generated, (_b_) upon the carbide decomposed, and (_c_)

upon the general chemical reaction taking place.

It has been stated above that in moat generators when the action between

the carbide and the water is proceeding smoothly, it occurs according to

equation (2)--

(2) CaC_2 + 2H_2O = C_2H_2 + Ca(OH)_2

rather than in accordance with equation (1)--

(1) CaC_2 + H_2O = C_2H_2 + CaO.

This is because calcium oxide, or quicklime, the by-product in (1), has

considerable affinity for water, evolving a noteworthy quantity of heat

when it combines with one molecule of water to form one molecule of

calcium hydroxide, or slaked lime, the by-product in (2). If, then, a

small amount of water is added to a large amount of calcium carbide, the

corresponding quantity of acetylene may be liberated on the lines of

equation (1), and there will remain behind a mixture of unaltered calcium

carbide, together with a certain amount of calcium oxide. Inasmuch as

both these substances possess an affinity for water (setting heat free

when they combine with it), when a further limited amount of water is

introduced into the mixture some of it will probably be attracted to the

oxide instead of to the carbide present. It is well known that at

ordinary temperatures quicklime absorbs moisture, or combines with water,

to produce slaked lime; but it is equally well known that in a furnace,

at about a red heat, slaked lime gives up water and changes into

quicklime. The reaction, in fact, between calcium oxide and water is

reversible, and whether those substances combine or dissociate is simply

a question of temperature. In other words, as the temperature rises, the

heat of hydration of calcium oxide diminishes, and calcium hydroxide

becomes constantly a less stable material. If now it should happen that

the affinity between calcium carbide and water should not diminish, or

should diminish in a lower ratio than the affinity between calcium oxide

and water as the temperature of the mass rises from one cause or other,

it is conceivable that at a certain temperature calcium carbide might be

capable of withdrawing the water of hydration from the molecule of slaked

lime, converting the latter into quicklime, and liberating one molecule

of acetylene, thus--

(3) CaC_2 + Ca_2(OH) = C_2H_2 + 2CaO.

It has been proved that a reaction of this character does occur, the

temperature necessary to determine it being given by Lewes as from 420°

to 430° C., which is not much more than half that which he found in a

generator having carbide in excess, albeit one of extremely bad design.

Treating this reaction in the manner previously adopted, the thermo-

chemical phenomena of equation (3) are:

_Heat liberated._ | _Heat liberated._

Formation of 2CaO 290.0 | Formation of acetylene 58.1

| Decomposition of Ca(OH)_2 [1] 229.1

| Decomposition of carbide 3.9

Balance 1.1 |

______ | _____

291.1 | 291.1

[1 Footnote: Into its elements, Ca, O_2, and H_2; _cf._ footnote,

p: 31.]

Or, since the calcium hydroxide is only dehydrated without being

entirely decomposed, and only one molecule of water is broken up, it may

be written:

Formation of CaO 145.0 | Formation of acetylene 58.1

| Decomposition of Ca(OH)_2 15.1

| Decomposition of water 69.0

Balance 1.1 | Decomposition of carbide 3.9

_____ | _____

146.1 | 146.1

Acetylene, the Principles of Its Generation and Use A Practical Handbook on the Production, Purification, and Subsequent Treatment of Acetylene for the Development of Light, Heat, and Power · 第 14 / 22 章