The reason that the air we breathe out is not fit to breathe in again, is not that it contains carbonic acid, but because of the presence of foul organic matter, and because it contains too little oxygen.

Suppose an animal were put into carbonic acid gas, he is put into a gas of which he has already too much in his blood, and he will die if he takes much more. Now, if the action of respiration went on for the benefit of the animal under all circumstances, and were not a purely physico-chemical action, he would not take any more carbonic acid into his blood ; but if an animal is put into a bottle full of carbonic acid gas, and the mixture that remains afterwards be examined, it is found that he has taken carbonic acid gas out of the bottle, and put oxygen into it out of his lungs, so that respiration has gone on to his detriment; and nothing can show much clearer than this that respiration is a purely physico-chemical action, and will not necessarily go on for the benefit of an animal, but will, under unfavourable circumstances, go on to his detriment, and to his death if necessary.

A good deal of carbonic acid is given out from limekilns,-places where limestone or chalk is burnt to form lime. Limestone or chalk consists of lime combined with carbonic acid, and when the lime is heated carbonic acid is given off in very considerable quantities; and cases have occurred of persons going to sleep on the ground in the immediate vicinity of a lime-kiln, because it is warm, and then being suffocated in the night by the carbonic acid gas.

The great artificial source of carbonic acid in the air is due to the substances that we use for warming and lighting. These substances contain carbon and hydrogen, and some other things in smaller quantities, and when completely burnt in the air they produce carbonic acid, water, and small quantities of other substances. If incompletely burned, some of the carbon escapes into the air in the form of soot We all know that where gas is burned for some time in a room a considerable quantity of soot is deposited upon the ceiling and other parts of the room. Another result is the production, under certain circumstances, of a small quantity of gas which is called carbonic oxide, which consists of one atom of carbon combined with one of oxygen, and affords a remarkable illustration of the fact that when two things combine together you cannot predict the substance that will result.

Carbonic acid gas also consists of carbon and oxygen, and is a colourless gas, soluble in water, and making a precipitate in lime water.

Carbonic oxide gas is different from carbonic acid gas. When a match is placed in the latter it goes out, but when applied to a jet of carbonic oxide, that gas combines with the oxygen of the air and burns with a beautiful blue flame, and the result of the burning is carbonic acid gas. Carbonic oxide gas is not soluble in water, nor does it produce a precipitate in lime water.

Carbonic oxide is so poisonous a gas that one part in a thousand parts of air is enough to kill an animal. It has a stronger affinity for the corpuscles of the blood than oxygen gas, so that you will see the extreme importance of all contrivances by means of which the setting free of the smallest quantity of this gas can be avoided. Here, also, is another instance of the fact I have already stated-viz., that respiration is only favourable to the life of an animal under favourable circumstances ; for if an animal be placed in an atmosphere containing carbonic oxide he is killed by breathing air containing a gas which has a greater affinity for his blood than oxygen has.

We have to consider the materials used for lighting and warming, the apparatus employed, and the conditions necessary to be fulfilled in order that the substances may be completely burned, in the first place, and, in the second place, that the greatest amount of light or warmth may be got from their use, with as little detriment as possible; and we see that most detriment is caused when the substances are incompletely burned.

Now, the first among these substances that we will mention is a candle, and those candles which are the the most completely burned are always the hard candles. Soft tallow candles are never completely burned, and always throw a considerable quantity of soot into the air, and a considerable quantity of partially burned fats, which have a disagreeable smell The reason these soft tallow candles are not completely burned is because they melt at so low a temperature that the substance rises up into the wick faster than it can be burned. The substances of which the harder kinds of candles are made do not melt as quickly, and consequently do not rise up the wick so fast, therefore they burn slower and more completely. The result 8 that the injury to the air we breathe is much less. Of course these are only some of the evils, not all, as carbonic acid and water are given out into the air of a room, and you cannot have substances burned for lighting a room without this, except you have a special apparatus for carrying off the products of combustion.

With regard to oils used for lighting, the chief thing that has to be 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 combustion.

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 petroleum, found in oil-wells in the United States. It consists of various liquid oils, some of which are extremely volatile, having a white solid called paraffin,^md 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.