Fewer children indeed die convulsed now, than formerly; this is because the rich learn, either from books, or conversation with physicians, how necessary fresh air is to life and health; hence they keep their houses well aired; but the poor, and servants, are not made to comprehend this matter properly; and therefore from neglecting to open their windows, and breathing a foul, tainted air, the greatest part of their time, many disorders are brought on, and others rendered worse than they naturally would be. [6] Having considered the purity of the air, let us next take a view of the changes in temperature which it undergoes, and the effects which these have upon the constitution. We find the air sometimes considerably below the freezing point; nay, even so much as 20 or 30 degrees; it is then intensely cold; and on the other hand, the thermometer sometimes indicates a great degree of heat. We then find ourselves much relaxed, and our constitutions exhausted. To understand how this happens, let us consider for a moment the nature of heat, and cold.--Heat is one of those stimuli which act upon the excitability, and support life: for if it was totally withdrawn, we should not be able to exist even a few minutes; and cold is only a diminution of heat. When heat is present, in a proper degree, or the atmosphere is about that degree of heat which we call temperate, it just gives such a stimulus, and keeps the excitability exhausted to such a degree, as to preserve the body in health; but if it continue for a considerable time to be much warmer than this temperature, the consequence must be, from the laws already laid down, an exhaustion of the excitability, and a consequent relaxation and debility; for, when the excitability has been exhausted by the violent application of heat, long continued, the common stimulant powers which support life, cannot produce a sufficient effect upon it, to give to the body that tone which is compatible with health. On the contrary, when the heat of the air falls below what we call temperate, or when cold is applied to the body, from the accustomed stimulus of heat being diminished, the excitability must accumulate, or become more liable to be affected by the action of the external powers.