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in certain definite proportions and form water; but the water can be again decomposed into its constituents.

Air, although I have called it a compound, is not, chemically speaking, strictly 80; it is a mixture of gases which preserve their individuality as sand and sugar would do if they were mixed in a vessel.

We are at present acquainted with more than sixty elements or simple bodies.

But to return to the atmosphere. It must not be imagined that we remove all the oxygen from the air we breathe; on the contrary, the removal of a small percentage renders air incapable of supporting life, and a still less diminution causes the difference between fresh air and vitiated.

I have said a good deal about the effect which the act of breathing has upon the atmosphere, because I want to make very clear the fact that the same air should not be respired more than once, and that it cannot be so, even in part, without danger to health. The other results of respiration, viz., carbonic acid and water, have been mentioned before, and it was shown that they contributed to foul the air.

I think we may, without serious error, divide noxious gases into two classes, placing those which are negatively poisonous in one class, and those which are positively, or actively, so in the other. To the former belong—as types—carbonic acid and nitrogen, both of which, though not in themselves injurious, are incapable of supporting life; so that an atmosphere composed wholly of these gases, or containing them in undue proportion, is fatal, from its negative qualities, to living beings. 'Carbonic acid gas, as before explained, is one of the products of combustion and respiration, while nitrogen forms 79 per cent. of the volume of the atmosphere. Its negative qualities are there, however, counterbalanced by the presence of oxygen.

With gases of the second class the case is very different. Some of them--such, for instance, as sulphuretted hydrogen and carbonic oxide--are fearfully active poisons. The former exists in putrid eggs, and some other animal and vegetable matters; also in certain waters, called hepathic, such as those of Harrowgate. It is usually formed

in the putrefaction of vegetable and animal matters; hence it is found in sewers, and especially in cesspools and similar places where accumulations of this filth take place. During the emptying of such receptacles, workmen sometimes suffer from asphyxia, or, as the French call it, “le plomb " (probably from the oppression on the chest which accompanies it). Parent Duchatelet states that the symptoms of poisoning by sulphuretted hydrogen are very alarming, for “the individual is either seized suddenly, and dies instantly, or, if the quantity of deleterious gas is too little to bring instantdeath, the asphyxiated person, suddenly losing consciousness, is taken with convulsive movements or other very grave nervous disturbances, and it is only after several days that he recovers perfect health.” The presence of sulphuretted hydrogen is readily detected by its odor, which resembles that of rotten eggs; but it is soothing in its nature if inhaled, which renders it especially dangerous when escaping into rooms where persons are sleeping.

It will be seen, then, of what extreme. importance is the prevention of the passage of sewer gases into our houses, and I have entered somewhat fully into the properties of these foul gases, because I wish to show clearly that they are not things to be trifled with.

Carbonic oxide is, like carbonic acid, a product of the combustion of carbon, or the union of carbon with oxygen, but the proportion of oxygen is less in the former than in the latter compound. Carbonic oxide, though a dangerous gas, need not be considered at length here, as it does not, I believe, exist in sewers, and is very seldom found in houses. It cannot, like sulphuretted hydrogen, be detected by its smell, as it is inodorous as well as colorless and tasteless. Carbonic oxide burns with a blue flame, which probably most people have noticed at some time or other, either playing over burning charcoal or dancing upon an ignited lime kiln.

I must now leave this part of the subject and say a few words useful, I hope, to those who are about to choose a house, whether as tenants or purchasers, and in

doing so I may, perhaps, travel somewhat away from ventilation.

The first consideration, after settling which town or part of the country it is desired to live in, should be the situation of the house, i.e., not only its aspect, but the condition of the soil on which it stands, and its position with regard to any ponds, streams, rivers, or other natural features.

Taking first the aspect. It is generally considered that a house is most favorably situated when its principal front is towards the south-east, for it then gets the morning sun, while the rooms are sheltered to a great extent from the midday heat. The south-west is the rainy quarter in the country, and should, therefore be avoided. A gravel soil is commonly to be preferred to any other, although, I believe, that in towns which are well sewered and drained, the nature of the soil is not of so much importance; indeed, one might readily imagine a case in which a gravel soil would be anything but a benefit—as, for instance, where there is a pond near the house, and on about the same level, or in towns where

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