39 E.A. Schaffer, "Artificial Respiration in its Physiologic Aspects," _The Journal of the American Medical Association_, September, 1908. 40 Testing the prone-posture method by suitable apparatus, Professor Schaffer has found it capable of introducing more air per minute into the lungs than any of the other methods of artificial respiration, and more even than is introduced by ordinary breathing. 41 Osier, _The Principles and Practice of Medicine_. 42 Huber, _Consumption and Civilization_. 43 To prepare limewater some small lumps of _fresh_ lime (either slacked or unslacked) are added to a large bottle of water and thoroughly shaken. This is put aside until the lime all settles to the bottom and the water above is perfectly clear. This is now ready for use and may be poured off as needed. When the supply is exhausted add more water and shake again. 44 An _element_ is a single kind of matter. Those substances are classed as elements which cannot be separated into different kinds of matter. Two or more elements combined in definite proportions by weight form a _compound_. The elements are few in number, only about eighty being known. Compounds, on the other hand, are exceedingly numerous. 45 The term _energy_, as used here, has the same general meaning as the word _power_. See Chapter XII. 46 The oxygen pressure of the atmosphere is that portion of the total atmospheric pressure which is due to the weight of the oxygen. Since oxygen comprises about one fifth of the atmosphere, the pressure which it exerts is about one fifth of the total atmospheric pressure, or, at the sea level, about three pounds to the square inch (15 x 1/5 = 3). This is the oxygen pressure of the atmosphere. The low oxygen pressure in the tissues is due to its scarcity, and this scarcity is due to its entering into combination at the cells. 47 See footnote on oxygen pressure, page 109. 48 The impression prevails to some extent that carbon dioxide, on account of its weight, settles out of the atmosphere, collecting in old wells and at the floor in crowded rooms. Any such settling of the carbon dioxide is prevented by the rapid motion of its molecules. This motion not only prevents a separation of carbon dioxide and air after they are mixed, but causes them to mix rapidly when they are separated, if they still have surface contact. The carbon dioxide found in old wells is formed there by decaying vegetable or animal matter. In rooms it is no more abundant at the floor than in other parts.