although the illumination yielded by, say, the Veritas 60 line is very good, yet the heat given off would be absolutely insupportable in a dwelling room.

Taking the composition of lamp oil as being 86 per cent. carbon and 14 per cent. hydrogen, the combustion of one pound would give 47,056 B.T.U.'s, while 50 grains, yielding one candle of illumination, would give 336 B.T.U.'s.

One cubic foot of coal gas gives on an average 620 B.T.U.'s, and the following results are obtained on calculating the amount of heat emitted per candle by various forms of burner:

The advantages to be gained by the use of oil as an illuminant are most evident in country districts, where, as a rule, coal gas is dear, whilst its popularity would be even greater than it is were it not for the number of accidents that have occurred in the use of lamps.

The risk of accidents could be reduced to a minimum if certain types of lamp could be done away with and people educated up to the proper use of oil for lighting purposes. All the cheap lamps, and those with side feeds and glass reservoirs, should be rigorously banned, and when burning it should be remembered that a lamp must not be moved from its place, whilst the flames should be extinguished, not by blowing down the chimney, but by turning the wick down and then blowing across the chimney.

We have seen that the introduction of the incandescent mantle has had an effect on the gas industry of the world that cannot be overrated, and it was natural therefore that its application to the flame produced by the gas of oil instead of the gas of coal should soon be attempted. It is clear that, inasmuch as oil gas and oil vapours only differ from coal gas in the larger proportion and greater complexity of the hydrocarbon molecules present, it only needs a sufficiently large proportion of air to be mingled with the oil gas or vapour before combustion to make the flame available for the same method of developing light.

The difficulties which exist in attaining this end are, however, very great, as the very richness of the hydrocarbons renders it a matter of considerable difficulty to get the

necessary amount of air thoroughly mixed with the gas.

I

A non-luminous flame can be obtained by burning a mixture of 1 volume of coal gas with 2:27 volumes of air; but if the flame be superheated, a certain proportion of luminosity will re-appear, as the increased temperature causes the decomposition into carbon and hydrogen of the hydrocarbon molecules in the gas, which could not take place when the gas was cool, owing to their separation and dilution with air. If a mantle be placed on such a flame, the superheating caused by the mantle itself will soon tend to the decomposition of the hydrocarbons in the gas, with the result that the mantle is blackened and rapidly destroyed. An increased proportion of air, however, entirely alters the flame, and the hydrocarbon molecules being burnt up before impact with the heated surface of the mantle, all chance of blackening is avoided.

With the flame yielded by gasified or vaporised oil this trouble proved to be a serious one, as although it was an easy matter to obtain a non-luminous flame, yet blackening made its appearance directly the mantle was placed over it, whilst if a further supply of air was brought into the flame, the flame required constant attention, a factor which was quite sufficient to prevent its adoption in daily practice.

Another source of trouble was found in the wick, the slightest irregularity of which seriously interfered with the correct working of the lamp, and it is by no means an easy matter to obtain a wick that is perfectly symmetrical, so that the wick was relegated to the duty of drawing the oil up into a vapourising chamber where the oil gas was produced, which, on then being mixed with air, gave the flame.

In the earlier types of incandescent oil lamps, a circular wick was generally used to bring the oil to within a short distance below the burner head, and at this point the oil was vapourised by the heat conducted from the flame by the metal of the burner. A central tube furnished one air supply, whilst a second was so arranged as to discharge itself almost horizontally on to the burning gas at the base of the flame, resulting in a non-luminous and very hot flame. The necessity for careful and constant attention, however, and the irregu larity of their performance prevented these lamps from attaining commercial success.

An ingenious lamp was devised, in which oil and water were vaporised by the heat of a

little oil lamp in a lower and separate chamber, and the mixture of oil gas and steam was then burnt in a burner head with a special arrangement of air supply, heating a mantle suspended above the burner head.

The perfect petroleum incandescent lamp, however, on any of these systems has not yet been made, but with one lamp with which I experimented it was easy to obtain 3,500 candle hours per gallon of oil, or three times the amount obtainable from the oil when burnt under ordinary conditions.

As early as 1885 Mr. Arthur Kitson attempted to make a burner for heating purposes on the principle of injecting oil under pressure from a fine tube into a chamber where it would be heated by the waste heat escaping from the flame below. The vapour so produced was then made to issue from a small jet under the pressure caused by the initial air pressure and the expansion in the gasifying tube. This jet of gas was then led into what was practically an atmospheric burner, and it drew in with it sufficient air to ensure its combustion with a non-luminous blue flame of great heating power.

At this date the Welsbach mantle was yet in its infancy, so that a mantle of platinum wire was at first used to obtain the light from this flame, but like all other platinum mantles, although the light at first was very fine, yet the illuminating power rapidly fell off owing to the action of the flame gases on the platinum. The perfecting of the Welsbach mantle, however, gave the finishing touch to this method of consuming the oil, and the Kitson lamp has now attained an assured success, the only drawback being that its use demands a certain amount of intelligent care. Where this, however, has been forthcoming, the lamp has fully answered the high expectations formed for it.

In this lamp, as used for street lighting, the oil reservoir is in the base of the pillar; it is made of steel and tested to a high pressure in order to ensure its being absolutely safe at the pressure at which it is used. A small pump enables the oil to be put under a pressure of air amounting to 50 to 60 lbs., by which it is forced up through a small capillary tube to the burner head. Here it passes through a small cross tube containing filtering material, for removing any solid particles in it, and it is then ejected through a small aperture into a lower cross tube placed immediately above the top of the mantle, the heat from which, passing upwards, causes vaporisation and partial gasification of the oil. The mixed gas and vapour rush out under considerable pressure

from a small aperture in the side of the far end of the tube down what is practically an inverted bunsen tube, through the holes in the side of which it draws in sufficient air to render the flame at the burner head not only non-luminous, but sufficiently oxidising in its character to prevent any deposition of carbon on the mantle. By the time the burner head is reached, the gas is practically down to atmospheric pressure again, so that the wear and tear to the mantle is not excessive.

The difficulties that were found with the forms of this lamp in use down to six months ago were the occasional clogging of the small vapour hole with carbon, and the risk that the oil might be turned on accidentally when the lamp was not in use. Another difficulty was that a defective valve might cause a slight leakage of oil through the vapour tube into the burner, which might, under certain circumstances, have resulted in the auxiliary flame turned on to heat the vapour tube causing a slight flare.

These objections have now, however, been surmounted, and the vapour tube is made with a needle holder and needle running longitudinally through it, so that the small vapour hole can be cleared in case of any stoppage by simply pulling a small chain attached to a lever arm working the needle, whilst any chance of oil escaping, either from a leaky valve, or turning on the oil supply to the cold lamp, is prevented by a thermostatic valve, which prevents any access of oil to the vapour tube until it is at a sufficient temperature to vaporise the oil properly. This valve operates by the difference in expansion of the two metals employed in its construction, and as long as the arm is cold it entirely cuts off the oil, and the valve being above the vapour tube, it cannot get hot enough to release the oil until the vapour tube has got to the proper temperature.

These modifications are a distinct improvement. Other steps in the right direction are heating the air supply before it enters the mixing tube, and so doing away with any danger of condensation of the oil vapour, and preventing a good deal of the hissing sound which was an objectionable feature of the older type.

It is satisfactory to find that the safety of the system is vouched for by the fact that among all the installations that have been put up in various parts of the world, no fire has originated from its use, and the progress during the past few years in new installations has been very satis

factory. One of the great advantages which it has over the intensified systems of incandescent mantle lighting adopted to obtain big units of illumination, is that it does not necessarily disturb the streets, as the whole installation is self-contained.

Careful experiments that I have made with this lamp show, that with a large single burner, it is perfectly easy to obtain illuminating values of from 1,000 to over 1,200 candles; and that with large installations 1,000 candle power can readily be obtained for one penny, which is only one-third the cost of the same amount of light from the electric arc. The recent success of its adoption for the illumination of large open spaces in London and other cities, points to the fact that it is destined to play an important part in the street illumination of the future.

Air carburetted with the vapour of light petroleum yields an illuminating gas of considerable value, and several attempts have been made to utilise this for incandescent lighting. A successful form which gives an idea of the systems most in use is that devised by Mr. Van Vriesland, which is used in several places under the name of "Aerogene gas."

In this arrangement a revolving coil of pipes continually dips into petroleum spirit contained in a cylinder, and the air passed into the cylinder through the coil of pipes becomes highly carburetted by the time it reaches the outlet at the far end of the cylinder. The resulting gas when burnt in an ordinary burner gives a luminous flame, and can be burnt in atmospheric burners differing but little from those of the ordinary type. With an ordinary Welsbach C burner it gives a duty of about 30 candles per foot of gas consumed, the high illuminating power being due to the fact that the gas is under a pressure of from 6 to 8 inches.

A complete installation of this plant is to be found at the village of Breukelen in the Pays-bas, where it is used for street as well as for house lighting. There are about two miles of mains which in one place pass under a canal, and although one would expect in cold weather that there would be a serious deposition of the vapour, in practice this does not appear to be the case. It was found, however, that the evaporation of the carburetting material was much higher in the summer than in the winter, and this necessitated an adjustment of the air supply to the burners; when once this had been made, the lighting power of the installation was as high as before and worked in a perfectly satisfactory manner.

I made tests of the light given by the mantles close to the works and also at a distance of half a mile away, after the gas had passed below the canal, and found that there was a loss of only about I candle per cubic foot in illuminating power during distribution.

In considering the effect to be produced with the incandescent mantle by the combustion of gas in the atmospheric burner, we saw the great advantage of mixing the air with the gas before combustion at a point some distance from the burner head, and although this is of necessity done in using carburetted air, as in the Van Vriesland and kindred processes, yet a further extension of the principle gives greatly enhanced results, and gets over the difficulty which hampers all other processes due to overcarburation in hot weather.

In the Hooker process which I have examined in conjunction with Dr. Boverton Redwood, the air is only carburetted to the point at which it gives a non-luminous flame, that is, nearly the whole of the air needed for the combustion of the petroleum vapour is mixed with it before combustion, and this mixture is then consumed in burners of special construction, which eliminate any risk of flashing back, with the result that a great gain in illuminating value is obtained. This process promises results of the highest importance.

A very ingenious table lamp for burning petroleum vapour with the incandescent mantle has been devised by Mr. Legge, in which a reservoir filled with sponge or other absorbent material, is saturated with petroleum spirit containing volatile hydrocarbons in solution. The vapour from this, descending by gravity to the burner head, draws in the required volume of air for its non-luminous combustion, and gives a light of about 40 candles when used with a mantle, such a lamp having many points of convenience and value where coal gas is unattainable.

Yet another allied illuminant which has been steadily and quietly making headway during the past few years is acetylene, which, although the youngest member of the family of illuminants, is one that has already shown itself an important factor in illumination where coal gas cannot be obtained. I have so lately dealt with the subject in a previous course of Cantor lectures that there is no need now to go into details of its rise to favour, or the methods by which it is produced from the electrically-born calcium carbide. Although much hampered in its progress by the determined booming of improper forms of apparatus

that gave rise to much trouble and waste, it has gone forward without a check, and improvements have been made in all directions.

There are several generators now on the market, which leave little or nothing to be desired in their construction, as they are both simple and effective, and for country house lighting the beauty of the light has led to its firmly establishing itself as a favourite.

In the early days had generators and smoking of the burners proved real difficulties, but the improvements in burners as well as generators have been of a marked character. Besides the well-known ones of Continental origin, Bray is now making some excellent burners, which show an improvement on the older forms, in that they can be turned down without any smoking. This will do away with the inconvenience which was inseparable from the older forms of burners, of either having to turn out the jet or leaving it to burn at its full size. The possibility of bye-passing the gas and so keeping the burner low when not required will undoubtedly prove a great incentive to the adoption of acetylene for the lighting of country railway stations, in place of the dim oil lamps now in use.

The improvements which have marked the last few years in connection with acetylene are largely due to the labours of the Home Office Committee on the Safety and Efficiency of Various Forms of Acetylene Generators, and also to the work of an excellent Acetylene Association, which has been formed to deal with all matters relating to calcium carbide and acetylene in the interests of the public and of the industry. The Association has already formulated regulations for the public guidance which have met with the approval of the Home Office authorities and also of scientific and practical men.

Like many new industries, carbide and acetylene have for a period suffered from overconfidence on the part of many of their earlier supporters, and on the Continent, where large water powers were available, factories were established one after another, as if it were only necessary to make unlimited quantities of carbide, in order to ensure a demand for it. The result of this was that, for a time there was considerable over-production, and capitalists and shareholders interested in carbide works suffered severely in pocket, as although the growth of the industry was wonderfully rapid, the supply was so great that stocks accumulated, and many factories had to close

whilst others worked to a small extent only of their output. The industry has now recovered, however, from this plethora of material, and this year there has been a danger of going to the other extreme, and at the present moment something closely approaching to a carbide famine exists in some countries.

The price of carbide, however, has remained fairly constant, and even during the period when large stocks were being held it never fell to any very low figure, whilst so far the scarcity has not forced the price up to any great extent.

At the present time the annual consumption of carbide on the continent for railway, domestic, and village lighting is probably not less than 40,000 to 50,000 tons, whilst in the United States and Canada the yearly consumption is not less than 20,000 tons, and fresh works are being erected to provide for the rapidly increasing demand. The colonies are being fed with carbide, both from the European factories and from America.

In Prussia, something like 6,000 tons are used annually in the railways, which almost entirely employ a mixture of 25 per cent. acetylene with 75 per cent. oil gas for lighting railway carriages, such a mixture giving a remarkably fine illumination; but in England this system has not yet received a fair trial, as although the Home Office has sanctioned the use of such a mixture, the railway authorities have not yet done more than experiment with it in a very haphazard and unsatisfactory

manner.

For the illumination of railway carriages, trams, and omnibuses, there is no doubt that acetylene has an enormous field before it. For some months the public in London were educated in the lighting power of acetylene as applied to omnibuses. Although owing to the faulty form of generators employed besides ample illumination, they also emitted an Occasional whiff of highly characteristic scent, at the same time it was realised that the ability to read the halfpenny press with ease and comfort gave them a distinct advantage over the oil lamp which had previously made the darkness visible.

Unfortunately there were difficulties in details which prevented this from proving economically successful. In the first place. acetylene generators in which the supply of water to carbide is so regulated as to give the volume of gas required to feed the burner in moving vehicles is under considerable disadvantages, owing to the jerking and jolting which upsets the arrangements for bringing

the water uniformly to the carbide, whilst the lamps being lighted at irregular hours and used for uncertain lengths of time caused another difficulty to be surmounted. A charge of carbide sufficient for the maximum possible supply must always be put in the apparatus, and if at the end of the day carbide still remains undecomposed, there is no help for it but to remove it with the sludge, and to replace it by a fresh charge. Moreover, a large amount of labour is entailed in cleaning and charging, whilst there are very grave difficulties in dealing with the large quantities of lime residue in a big city like London.

These troubles, however, are now in a fair way to be satisfactorily overcome by the introduction of acetylene dissolved under pressure in acetone, the solvent itself being absorbed by porous materials with which the cylinder is filled. The early researches upon compressed and liquefied acetylene show that it could be exploded under circumstances which made it inadvisable that it should be freely employed, whilst solutions of acetylene under pressure in acetone, although less liable to explosion, still could not be said to be sufficiently free from danger for general use. It has been discovered, however, by MM. Janet and Fouche that when the acetylene is dissolved in acetone absorbed by porous materials of the right kind under ten atmospheres pressure, it is impossible to produce explosion, even when electric sparks are passed through the containing cylinder, or platinum wires are fused in it. Exhaustive tests which I have seen carried out in France demonstrate the absolute safety cf acetylene compressed under these conditions, and these experiments have been corroborated both in Germany and England.

Under this new system acetylene is evolved from carbide in the ordinary way, and after proper purification is compressed under ten atmospheres into cylinders containing a porous medium of 80 per cent. porosity soaked with 43 per cent. of the capacity of the tube with acetone, and in this way one cubic foot of cylinder space is found to hold 100 cubic feet of acetylene, which is again to a large extent given off when the pressure is reduced by opening the stopcock of the cylinder.

Supplied to burners through proper regulating apparatus this gives a magnificent illumination, and such cylinders of small size, fitted to trams and omnibuses, are perfectly controlled by simply turning a cock, so that all waste of gas, trouble with residue and smell are obviated. As the system is now in the hands of the

Acetylene Illuminating Company in London, who have done such good work in all branches of the acetylene industry, and as the Home Office sanctioned its use in this form, it may be expected that public opinion and competition with the tubes will in the near future lead to the abolition of the old petroleum lamps, and the installation in its place of the more brilliant illuminant, especially as the difference in cost between the two amounts only to a few pence per week. For railway lighting the same method can be adopted with large tanks of the same character as those now used for compressed oil gas, but filled with the right form of briquette.

From the earliest introduction of acetylene attempts have been made to utilise it with incandescent mantles, but under the pressures which are usually obtained from the ordinary generating apparatus, this has been a prac tical impossibility. Although burners have been devised which for a short time gave a magnificent light with the incandescent mantle, it was very troublesome to prevent the flashing back in the burner tube with considerable explosive violence, an action which blew the mantle to pieces. With acetylene under pressure in acetone, however, any desired pressure at the burner can be obtained, and with a pressure of eight inches and upwards an acetylene burner can be made to work perfectly we'l with a mantle, the maximum illuminating power being given at a pressure of ten inches, when a light of 146 candles per cubic foot of acetylene consumed can be obtained. If the pressures rises above this point, although the total candle power is increased, the duty of the mantle falls. The results I have obtained are shown in the following Table :

ACETYLENE WITH INCANDESCENT MANTLES.