With regard to oils used for lighting, the chief thing that has to bo considered is the contrivance in which they are burned, so as to produce the greatest amount of light with the most perfect combustion; and the kind of lamps in which this is done are those in which the wicks are round, like the Argand burners, or in which wicks are placed parallel to one another, and in which there is a chamber of air communicating with the flame so as to ensure complete eombustion.

The oils I have been speaking about are especially vegetable oils, but a large amount of mineral oil is now used for the purposes of lighting. The most important of these mineral oils is paraffin oil, made from petro leum, found in oil-wells in the United Slates. It consists of various liquid oils, some of which are extremely volatile, having a white solid called paraffin, and a gas which is one of the constituents of coal-gas, dissolved in them. It is, then, a mixture of different substances, of different densities, and different evaporating points. The danger of using these substances is considerable, because when heated beyond a certain degree some of the more volatile substances contained in them rise up into the air, and form with it an explosive mixture, and a good many accidents have taken place, both in petroleum works and also where petroleum is used in lamps. And so it is important, when these lamps are used, to burn only the best kind of oil, or at any rate mineral oils that have been distilled, so that some of the more volatile substances have been driven off.

Another thing that should be carefully remembered is that some of these mineral oils have a penetrating power, and will pass through a china lamp and appear on the outside, and evaporate into the air, causing a disagreeable smell in the room.

Many of you burn paraffin in lamps, and perhaps you have noticed that invariably when you have taken hold of them you have found oil on your fingers, and then thought that the lamp had not been properly cleaned. This is not so; the oil has come through the pores of the substance of which the lamp is made.

We will now consider Coal Gas. Gas may be prepared by the dry distillation of various substances, but is now exclusively made from coal The coal is placed in large vessels called retorts, and heated; various substances are given off, and something remains; this is the valuable substance we call coke, used chiefly in engines, when it is necessary not to have smoke, as coke burns without smoke.

The substances given off consist, in the first place, of a large quantity of tar, which is used for an immense variety of purposes in commerce, very largely for the preparation of carbolic acid, the well-known disinfectant; the aniline colours, mauve, magenta, etc., are also prepared from coal tar. Further, a considerable quantity of ammonia goes off. This is collected by washing the gas with water, which absorbs the ammonia, and this ammoniacal liquor is used for the preparation of manures, and is now the great source from which ammoniacal manures are obtained. Besides these, a mixture of gases is given off which requires to be still further purified before it can be used for burning. It has sulphuretted hydrogen in it, which has a smell like that of rotten eggs, and has to be separated from the gas, and this is done either by passing it over lime, or over oxide of iron, other compounds of sulphur being partly separated at the same time. The gas is more efficiently purified by the lime process, but the mixture of waste lime and sulphur compounds is liable to cause a serious nuisance to the neighbourhood.

The remaining mixture is the gas we burn, and it consists chiefly of a gas called light carburetted hydrogen, or marsh gas, which forms carbonic acid and water when burnt in air. It contains, also, pure hydrogen gas, which has the property of burning in air, and producing water; and heavy carburetted hydrogen gas, which burns in air to form carbonic acid and water, giving an intense light. This last is the substance to which the lighting properties of coal gas are mainly due. Coal gas always contains carbonic oxide, sometimes as much as 11 or 12 per cent. It always contains compounds of sulphur, which,when burned in the air, produce sul- phurous acid with a little sulphuric acid.

The gas we use for burning is exceedingly poisonous. It contains a sufficient percentage of carbonic oxide gas to kill any animal, and it is therefore extremely important that the gas should not be allowed to get into rooms, even in the smallest quantity. It contains one of the most poisonous substances with which we are acquainted. It is very fortunate that the gas has a very strong and unpleasant smell, as otherwise a great many people would be poisoned by it. A very foolish thing was once done. A man took out a patent for depriving the gas we burn of its smell, but the gas companies fortunately did not adopt it.

When coal gas escapes into the air, it forms a mixture which explodes very readily when a spark is applied.

What are the dangers from breathing air into which the products of the combustion of the substances we use for lighting have been allowed to escape?

It produces in persons who breathe it for a lengthened period what is called an‘mia, or bloodlessness; and especially when the combustion is incomplete, and soot escapes into the air, it produces cough, lung complaints, and is particularly fatal to persons who are liable to consumption. It is, therefore, extremely important that the products of combustion should not be allowed to escape into the air of rooms. They should be conducted away, or, at any rate, plenty of fresh air should be admitted.