[12] Town and village paving plans will benefit by knowledge of the recent satisfactory experience of New York City authorities in paving with wood blocks soaked in a preparation of creosote and resin. As compared with the other two general classes of paving, granite blocks, and asphalt, these wood blocks are now considered superior. The granite blocks are now nearly discarded in New York because of their permeability, expense, and noise, being now used for heavy traffic only. Asphalt is noiseless and impermeable (thereby serving the "double sanitary purpose" mentioned by Dr. Price). But the wood possesses these qualities, and has in addition the advantage of inexpensiveness, since it is more durable, not cracking at winter cold and melting under summer heat like the asphalt; and there is but slight cost for repairs, which are easily made by taking out the separate blocks. These "creo-resinate" wood blocks, recently used on lower Broadway, Park Place, and the congested side streets, are giving admirable results.--EDITOR. CHAPTER II =Ventilation= =Definition.=--The air within an uninhabited room does not differ from that without. If the room is occupied by one or more individuals, however, then the air in the room soon deteriorates, until the impurities therein reach a certain degree incompatible with health. This is due to the fact that with each breath a certain quantity of CO2, organic impurities, and aqueous vapor is exhaled; and these products of respiration soon surcharge the air until it is rendered impure and unfit for breathing. In order to render the air pure in such a room, and make life possible, it is necessary to change the air by withdrawing the impure, and substituting pure air from the outside. This is _ventilation_. _Ventilation_, therefore, is the maintenance of the air in a confined space in a condition conducive to health; in other words, "ventilation is the replacing of the impure air in a confined space by pure air from the outside." =Quantity of Air Required.=--What do we regard as impure air? What is the index of impurity? How much air is required to render pure an air in a given space, in a given time, for a given number of people? How often can the change be safely made, and how? These are the problems of ventilation. An increase in the quantity of CO2 [carbon dioxide gas], and a proportionate increase of organic impurities, are the results of respiratory vitiation of the air; and it has been agreed to regard the relative quantity of CO2 as the standard of impurity, its increase serving as an index of the condition of the air. The normal quantity of CO2 in the air is 0.04 per cent, or 4 volumes in 10,000; and it has been determined that whenever the CO2 reaches 0.06 per cent, or 6 parts per 10,000, the maximum of air vitiation is reached--a point beyond which the breathing of the air becomes dangerous to health.