of those compounds that we call hydro-carbons; and first and foremost both in giving size and heating power to the flame, as well as helping its luminosity, stands methane (CH)—a gas which at various periods in its career has been better known as marsh gas or light carburetted hydrogen. A natural product of the decompositions which led to the formation of coal from the monster vegetation of pre-historic times, its presence ready formed in the coal measures has proved one of the greatest dangers that the winners of our fuel have had to contend with; as mixed with the air in the workings of a mine it forms the dread firedamp that has cost us so many lives.

Taking the average composition of an ordinary 16 candle coal gas, free from enrichment, we may state it as follows:

these heat-yielding bodies must be fully represented in our future gas.

The only commercial and attainable sources of such hydrocarbons are found in the destructive distillation of coal and mineral oils; and although the hydrocarbon gases per se could probably be obtained most cheaply by the cracking of mineral oils, yet we should then be driven back for the large volume of diluting gas, which would be a necessity to lower the illuminating power and cost, to water gas, which under these circumstances would be the only available diluent. At once, however, three factors crop up which would immediately negative the use of a low grade carburetted water gas, and these are (1) that it would forthwith render useless the existing gas plant, (2) that cheap coke being an essential to the manufacture of water gas, the cutting off of the supply of this material would at once render the manufacture of water gas an impossibility at a commercial price, and (3) that blue water gas contains 40 per cent. of carbon monoxide, and that although in a 20 candlepower carburetted water gas this is reduced to 30 per cent., yet in a 12 candle-power mixture it would rise to 35 or 36 per cent., which under existing circumstances would never be permitted for a domestic supply.

In order to make our view of the subject as complete as possible, I shall deal later on with the economic side of a gas supply made on these lines, but I have thought it best to state at this point, in a few words, my reasons for considering that it never would be adopted. It is evident, therefore, that coal and not oil must be the main factor in producing those hydrocarbons which are to raise the calorific value of the gas to the required point, and we can now discuss the methods by which increased volume and lowered cost can be obtained on this basis. Amongst the suggestions made for cheapening the manufacture of gas at the expense of illuminating value, is that higher temperatures should be employed for retorting the coals, but I think the majority of gas managers will agree with me that the useful maximum temperature has been attained, and that a further pushing of the heats would accentuate troubles which have already shown themselves. If the present practice of using temperatures that yield about 10,000 cubic feet per ton of coal were departed from, the first thing that would make itself felt is the fact that only a small portion of the gas coal supply is fitted for use at higher temperatures than those em

ployed, and this limitation would soon tend to raise the price of such kinds of coal as could be used.

In a highly suggestive paper read by Mr. Harry E. Jones, this year, on the "Illuminating Power of the Gas of the Future," experiments seem to show that with a Durham coal it is possible to increase the yield of gas per ton of coal by something like 25 per cent., with a total increase in calorific value, by increasing the temperature of carbonisation and diminishing the pressure in the retort. Working in the same way, I have found results of the same character, but it must be borne in mind that this only means an increase in the volume of gas obtained per ton of coal from 1,000 to 1,250 feet.

The most complete work ever done on the distillation of coal was by Mr. Lewis T. Wright, and the paper which he read at the May meeting of the Incorporated Institute of Gas Engineers in 1895, will long remain one of the most valuable contributions to the subject of carbonisation. In it he showed that by pushing the temperature at which distillation was taking place, a rise in volume yielded per ton of a Derbyshire caking coal took place with increased heats, till a maximum of 12,190 cubic feet per ton was reached, and this. gradually fell again, the same thing being observed with other coals. It seems highly probable that the conclusion is correct that the more rapid distillation due to pushing the temperature in the retorts would not be attended by any very marked increase in the yield of gas per ton, as the more rapid evolution of gas in the early stages of the charge would hurry the gases out of the retort, and so prevent their being broken up by the increased temperature. Indeed, on purely theoretical grounds, it is impossible for increased temperatures to give more than a certain increase in volume, as the chief constituents once formed in the retort, they are so stable with regard to heat, owing to their character and to dilution, that but little extra gas is obtained from them, whilst in baking the already-formed gas, it is only the higher hydrocarbons that by their decomposition can give an increase; further, whilst some of these are being decomposed to carbon and hydrogen, others are being synthetically built up into such bodies as naphthalene.

In a recent experiment made under my direction a poor coal was retorted at the usual temperature, and after the candle power and calorific value had been determined, samples

If instead of taking a poor coal, as was done in this case, and using the ordinary retort temperature, which gives in the first case a large amount of hydrogen in the gas, a rich Newcastle coal is taken and is retorted first at a low temperature (dull red), and is then carbonised at the highest obtainable temperature (bright orange), this action is still more striking. As far back as 1884 Mr. Lewis T. Wright made an experiment of this character with the following results :

and it is here again evident that the breaking down of the hydrocarbons into hydrogen and carbon has given the increase in volume, and whilst there has been an increase in the candles per ton of coal carbonised, amounting to 10'3 per cent., the increase in calorific value per ton is only 5.6 per cent.

A large increase of temperature above that at present employed would, moreover, result in such trouble from stoppage of ascension pipes from pitch, and from naphthalene in the service pipes and mains, not to mention the increased quantity of sulphur compounds, and the reduction in ammonia and tar, that leaving out the question of wear and tear and fuel, such a method of increasing the yield could never be successful.

There is, however, another phase of the question, and that is by employing inferior coal bought at a cheaper rate, and retorting it at the highest temperatures found compatible with economical working, the low grade of gas of good calorific value might be obtained direct, but the economy to be attained would be very small,

The method, however, of obtaining a low grade gas which would be most welcome to the gas manager, and would comply with the necessities of the future, would be in no way to alter the working or arrangement of the present plant, to make the same gas as heretofore, and to dilute it down to the required point by a combustible diluent which must be cheap enough to offer sound economical advantages, and yet of sufficient calorific value to keep up the heating power of the mixture to the needed point.

The only gaseous mixtures which are available for this purpose are either water gas pure and simple, or mixtures containing water gas as their basis, and a glance at the calorific values of the so-called fuel gases makes this at once apparent.

Indeed with the exception of water gas the

with coal gas, do not lend themselves to distribution.

Blue water gas, as it is customary to call the non-luminous mixture of hydrogen and carbon monoxide generated by the action of steam or incandescent carbon, first made its appearance in this country on a manufacturing scale in 1887-8 as a fuel where high local temperatures were necessary, and shortly afterwards carburetted water gas, a mixture of water and oil gas, was introduced at Beckton as an enricher of coal gas in the place of cannel coal which had, at that time, risen to prohibitive prices. The ease and rapidity of manufacture at once secured it a complete success, and at the present time very few large gas works are without this valuable auxiliary. The attitude of the public towards this new introduction was at first one of apathy, but presently the gentlemen who delight in writing to the daily papers and seeing their names in print recognised a golden opportunity for self advertisement, and the public were informed that death and every form of horror were being let loose upon them. The exagger. ated and mostly false statements of the early agitators being to an extent supported by well-known and well-meaning medical practitioners, who had plenty of book-lore and little or no experience of the subject, a crusade against water gas was started, which culminated, in 1898, in the appointment of a Departmental Committee to inquire into the subject. Their report was issued in 1899, and it is probable that some legislation on the subject will shortly be brought forward.

Now the facts with regard to water gas are these. Made by passing steam over incandescent coke or anthracite, it has an average composition

and it is the 38 per cent. of carbon monoxide that has roused the storm of angry criticism.

Carbon monoxide, or carbonic oxide, the name by which it used to be more generally known, is a virulent poison, and when inhaled in even small quantity causes death, by combining with the blood and gradually cutting

thirds of their volume of inert nitrogen, and

being of high specific gravity as compared

by the blood.

0'4 per cent. of carbon monoxide in the air

will produce death if inhaled for over an hour, whilst o 05 in the air will only cause headache and giddiness however long the mixture may be inhaled. So that air containing 4 parts in a thousand is fatally poisonous, and air containing less than 5 parts in ten thousand is practically harmless.

Now this sounds alarming enough, and bears out the contentions of the opponents of water gas, but the points overlooked by them are that in an ordinary sized room it is very difficult even purposely to make a fatally poisonous mixture of air and gas, and, secondly, that blue-water gas with its 38 per cent. of carbon monoxide is not distributed as a domestic supply.

In order to give luminosity the otherwise non-luminous water gas is made to pass through chambers of heated chequer work, into which a thin stream or spray of petroleum is injected, and the oil being gasified by the hot brickwork into oil gas mingles with the hot water gas, and the mixture being fixed,-that is any oil remaining as vapour being converted into permanent gas by passage through another heating chamber,-the now highly illuminating mixture is known as "carburetted water gas.'

So that the dilution of water gas by oil gas reduces the percentage of carbon monoxide to 28.2, but with the exception of a short period in one of the divîsions of Liverpool, carburetted water gas alone has never been distributed in this country, although in America 296 of the largest companies send it out as a domestic supply.

In Great Britain it is almost entirely used to enrich and augment the supply of coal gas, and the amount added depends largely upon local circumstances, the following Table showing the maximum and average percentages put into the coal gas in some of the more important towns:

So that under ordinary conditions of working it is safe to say that 20 to 50 per cent. of carburetted water gas is added to the coal gas supply. The whole question of what quantity shall be permitted hinges on the percentage of carbon monoxide present in the mixture, and this should be kept down to a point that should prevent such small leakages as are possible in a dwelling room from being actively injurious or endangering life.

It must be remembered that coal gas itself contains a certain proportion of carbon monoxide, the quantity being dependent upon the composition of the coal and upon the conditions of temperature and exhaust in the retorts. It has been found that the presence of combined oxygen in coal gives rise during carbonisation to the formation of oxides of carbon, and M. Ste Claire-Deville, from a long series of experiments on 59 different kinds of coal, established the following relations :

The presence of the oxygen also in large percentage in the coal generally means a high yield of volatile hydrocarbcns, hence a gas of high illuminating power, but such coals are rather avoided by the gas manager, as they are high in price and ruin his coke. High retort temperatures also seem to have an effect in increasing the percentage of carbonmonoxide. In the analysis of gas by Mr. Lewis T. Wright, already quoted, it will be seen that whereas a Derbyshire caking coal yielded 8.72 per cent. of carbon-monoxide when distilled at a dull red heat, the same coal yielded 13.96 per cent. of this gas at a bright orange heat.

A heavy exhaust would also tend in the same direction, as any trace of air leaking into the retort would form the monoxide. With the coals most used, however, from the Newcastle and Durham districts, the average percentage of carbon monoxide rarely exceeds 4 to 5; and it is now possible to gain an idea of the quantity that might with safety be allowed in the mixture sent out as illuminating gas.

There is no danger of poisoning by such mixtures during the day time, as the smell of the gas attracts attention to the danger long before the gas reaches a serious proportion; nor is it any good considering the cases of large leaks, as from broken mains or torn down fittings, for there would always be the risk of fatal results, whether the gas was coalgas only or a mixture of coal and water gas.

The real risk is during sleep from such leakages as might be produced by leaky joints or partly re-turning a loose tap when putting out the gas. Such a leak would practically never be a serious one, as, if it were, it would be detected before the occupant of the room could get to sleep.

My own view is that practical safety is assured as long as the percentage of carbon monoxide in the gas supply does not exceed 16 to 17. In the departmental committee's report they recommend "that the proportion of carbon monoxide in the public gas supply at night should be regulated to 12 per cent., or such greater amount as the department may consider desirable; " and in the body of their report they say "In some cases 12 per cent. of carbon monoxide in the gas supplied might be proper, in others 16 or perhaps 20. . . . With the present condition of gas supply, 20 per cent. is the highest proportion of carbon monoxide that should be allowed, and this percentage should be used only under special circumstances."

The views of the authorities in London are that the amount of carbon monoxide in the gas distributed should not exceed 16 per cent., and the Legislature is always so chary of doing anything to hamper unnecessarily a great industry, that the limit is not likely to be fixed below this point; indeed, as coal gas per se may contain up to 12 per cent., fixing anything lower than 16 would be practically prohibiting the use of a valable adjunct to gas manufacture. Taking, for the sake of calculation, the allowable limit as 16 per cent., this would mean that an ordinary coal gas containing 5 per cent. of carbon monoxide, might have its bulk increased by carburetted water gas until the mixture contained 52 of coal gas and 48 of carburetted water gas, whilst if blue water gas were used, the limit would be reached when the mixture contained 66 of coal gas to 34 of blue-water gas in other words, to 100 volumes of coal gas you might add 92 of carburetted water gas, or 51.5 of blue-water gas, before reaching the limit of 16 per cent. of carbon monoxide in the mixture. bearing of these figures will become apparent when we consider the factors that must govern the production of low-grade gas.

Correspondence.

The

SWEET POTATOES AND YAMS. During the period of stress which our sugar; growing colonies in the West Indies are passing through, pending the abolition of the foreign sugar bounties, the attention of the planters has naturally been given to other produce. In Barbado's great

success has been achieved in the cultivation of sweet potatoes and yams of the very best quality, and an endeavour is now being made to introduce these into this country. The sweet potato is a cheap and palatable vegetable, but a good yam is a positive luxury.

During a long residence in London I imported several barrels every year for my own use, and out of the numerous guests who tasted them at our table there was not one who did not highly appreciate them. I may add that here the flavour is even more delicious than in the West Indies, as the butter, which is a vital ingredient in a well-cooked yam, is so much better.

I am returning to Barbados almost immediately, but any information on this subject will be given by Messrs. W. Pink and Sons, of Portsmouth, who are importing regular supplies. Receipts for various ways of cooking both sweet potatoes and yams re sent out with every parce'.

FORSTER M. ALLEYNE,

Legislative Council of Barba los.