ed by the approach or presence of some fearful epidemic, or of a calamity such as last year threatened this nation, that people seriously turn their attention to sanitary matters, and at times like these they accept the wildest schemes and act upon the crudest notions until, finding matters are no better, and perhaps rather worse than before, and the fright beginning to wear off, they relapse into carelessness, and vote sanitary science all nonsense. I think defective education is responsible for a great deal of this. I do not intend to assert that the generality of people, at any rate in the upper and middle classes, are what is commonly called ignorant; probably most of those who would be willing to turn their attention to sanitary matters, not being professionally engaged in them, have had at least the usual amount of education, as the term is commonly understood. The three R's are familiar to them as are also mathematics, Greek, and Latin, and they can probably converse in one or more of the modern languages, but what may be called the science of living, i.e., how life is sustain

ed, and the reasons why sanitary matters should be so carefully attended to in order that health and strength may be insured, are things of which comparatively few know anything at all.

In this paper I wish to impress upon the reader the very great importance of pure air to the human body, and to show how such air, or as pure as the district affords, may be insured in our houses.

It has been stated before, that one of the elements of the atmosphere unites in our bodies with certain of the elements composing the food we eat. This union of elements is a true combustion, though a slow one, as true and real as the combustion of coals in a fire grate.

Our food, having been decomposed in the body, is brought into a state proper for sustaining life and animal heat in us when, as venous blood, it is submitted to the purifying action of oxygen brought into the body by the lungs. Without the element, oxygen, which I have before stated forms part of the atmosphere, this purification could never take place, for the other

gases have no power of the kind. It will be obvious upon consideration that if a portion of oxygen in the air we have breathed has united with elements composing our food, that portion is removed from the atmosphere and must be replaced if breathing is to be continued.

I will here explain for the benefit of persons who may be wholly unacquainted with chemistry, what is meant by the term "element." The ancients held that there were four elements, or simple bodies, viz., earth, air, fire and water, but the progress of chemical knowledge soon revealed the fact that these were not elementary bodies at all, three of them being compound, while the fourth, fire, may be more properly described as a condition of matter. An element is a body which cannot, as far as we know, be divided or decomposed. It is unalterable and indestructible, and is therefore called a simple body or element.

Elements, however, combine with one another, and lose their individuality, so to speak, in the compound. Thus two simple bodies, gases, hydrogen and oxygen, unite

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 so; 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