CONVECTION CURRENTS. 273 



arrangement is adopted; a cooling engine is provided (usually 

 involving the rapid evaporation of highly volatile liquids, such as 

 liquefied ammonia or sulphur dioxide gas) in place of a boiler ; the 

 coils of pipe intended to cool the air of the apartment are filled 

 with brine, or glycerine, or some other fluid that does not easily 

 freeze, and a continuous current of cold fluid is thus kept circulat- 

 ing, flowing outwards and downwards from the base of the cooling 

 vessel (which should now be placed at the highest elevation), and 

 returning at a somewhat higher temperature to the top of the 

 cooling vessel. When artificial ice is to be made, water is gradually 

 frozen into blocks by cooling the vessels containing it below zero 

 by means of pipes containing highly chilled brine. By placing a 

 horizontal network of pipes on the floor of a room and sprinkling 

 them with water, a crust of ice can be formed on the pipes ; which," 

 by cautiously repeating the addition of water, can be finally con- 

 verted into a solid flooring of smooth ice, thus producing an 

 artificial ice skating rink, such as is termed in various parts of the 

 country a glaciarium. ( In the manufacture of paraffin wax from 

 petroleum and shale distillates, &c., and in many analogous in- 

 dustries, chilling appliances of this description are largely employed 

 to cause the solidification of the more valuable portions, which are 

 then removed by pressing out the still fluid part by means of 

 powerful hydraulic presses (Expts. 26, 331). 



Anything that mechanically hinders the setting up of convection 

 currents in the air surrounding a given body will prevent its 

 rapidly altering in temperature if materially hotter or colder than 

 the air. The naked skin is soon chilled down, even in a room 

 with door and windows closed, so that there are no draughts or 

 horizontal currents of air ; partly because heat is lost by radiation, 

 but principally because convection currents are set up all over the 

 warm body, which continually bring fresh cool air in contact with 

 the skin, and thus rapidly abstract heat; but a loosely fitting 

 garment greatly retards this motion of the warmed air ; whilst a 

 series of garments one above the other, especially if wadded 

 with wool or quilted with eider down, &c., or lined with fur,, 

 effectually prevent the formation of convection currents and 

 consequent chilling action. Such clothing literally "keeps 

 one warm" by preventing escape of heat; but it in no way 

 produces heat, so that the phrase " warm clothing " is not exactly 

 correct. For exactly the same reason that materials of this 

 description will prevent a hot body from cooling quickly, they 

 will equally prevent a cold body from becoming warmed by 

 the air; 'thus to preserve a block of ice from melting rapidly in 

 a hot room, or in summer, it suffices to envelop it in several folds 



