cutting down the total amount of carbon dioxide in the body. The most conspicuous is a feeling of dizziness or light-headedness that comes on. If pushed to excess, there is a very definite feeling as though one were about to soar away into space, and this is followed by unconsciousness. Certain religious cults in India have interpreted this sensation resulting from deep breathing as an actual severance of soul from body, and maintain that during the time of unconsciousness the spirit really floats freely in space. Without venturing any statement as to the relation between the soul and the body during either consciousness or unconsciousness, we would point out that these bodily sensations are definitely due to the very simple fact that there is less carbon dioxide in the blood than is normal on account of the overventilation of the lungs, and just as soon as the metabolism that goes on all of the time in the tissues pours out enough carbon dioxide to bring the amount up to normal, consciousness will return and the ordinary condition of affairs will be resumed. Although this finishes what we have to say about the movements of gases into and out of the body, the general subject cannot be completed without a word concerning the conditions that should be maintained in the air immediately surrounding us. This makes up the topic of _ventilation_. We all know that some air is much more fit to breathe than other; until very recently, however, our ideas as to the conditions which make air fit or unfit to breathe have been hazy or entirely erroneous. Fortunately, of late years, the subject of ventilation has been actively investigated and we now have a satisfactory knowledge of its laws.