[Sidenote: 37. How, in the case of the lower animals? Tendency of certain occupations?] 37. The lower animals confined in the impure atmosphere of menageries, contract the same diseases as man. Those brought from a tropical climate, and requiring artificial warmth, generally die of consumption. In the Zoological gardens of Paris, this disease affected nearly all monkeys, until care was taken to introduce fresh air by ventilation; and then it almost wholly disappeared. The tendency of certain occupations to shorten life is well known; disease being occasioned by the fumes and dust which arise from the material employed, in addition to the unhealthful condition of the workshop or factory where many hours are passed daily. [Sidenote: 38. Give the fact as set forth in the table.] 38. The following table shows the comparative amount of carbonic acid in the air under different conditions and the effects sometimes produced:-- PROPORTION OF CARBONIC ACID. In 1000 parts of Air. Air of country. .4 " " city. .5 In hospital, well ventilated. .6 In school, church, etc., fairly ventilated. 1.2 to 2.5 In court-house, factory, etc., without ventilation. 4. to 40. In bedroom, before being aired. 4.5 In bedroom, after being aired. 1.5 Constantly breathed, causing ill health. 2. Occasionally breathed, causing discomfort. 3. Occasionally breathed, causing distress. 10. Expired air. 40. Air no longer yielding oxygen 100. {141} [Sidenote: 39. What can you state of the diffusive power of gases? The added influence of the winds?] 39. NATURE'S PROVISION FOR PURIFYING THE AIR.--We have seen that carbonic acid is heavier than air, and is poisonous. Why, then, does it not sink upon and overwhelm mankind with a silent, invisible wave of death? Among the gases there is a more potent force than gravity, which forever precludes such a tragedy. It is known as the diffusive power of gases. It acts according to a definite law, and with a resistless energy compelling these gases, when in contact, to mingle until they are thoroughly diffused. The added influence of the winds is useful, by insuring more rapid changes in the air; air in motion being perfectly wholesome. The rains also wash the air. [Sidenote: 40. How is the constant purity of the air secured? Explain the process?]