334 8GIENCE IN SHORT CHAPTERS. 



various kinds would be engendered; whereas, if the air were 

 cooler than the walls, the latter must be absolutely dry." 



Upon these assumptions, Dr. Siemens condemns steam- 

 pipes and stoves, hot-air pipes, and all other methods of, 

 directly heating the air of apartments, and thereby making 

 it warmer than were the walls, the ceiling, and furniture 

 when the process of warming commenced. It is quite true 

 that stoves, stove-pipes, hot-air pipes, steam -pipes, etc., do 

 this; they raise the temperature of the air directly by con- 

 vection, i.e., by warming the film of air in contact with 

 their surfaces, which film, thus heated and expanded, rises 

 towards the ceiling, and, on its way, warms the air around 

 it, and then is followed by other similarly-heated ascending 

 films. When we make a hole in the wall, and burn our 

 coals within such cavity, this convection proceeds up the 

 chimney in company with the smoke. 



But is Dr. Siemens right in saying that the air of a room, 

 raised by convection above its original temperature, and 

 above that of the walls, deposits any of its moisture on these 

 walls? I have no hesitation in saying very positively that 

 he is clearly and demonstrably wrong ; that no such con- 

 densation can possibly take place under the circumstances. 



Suppose, for illustration sake, that we start with a room 

 of which the air and walls are at the freezing point, 32 F., 

 before artificial heating (any other temperature will do), 

 and, to give Dr. Siemens every advantage, we will further 

 suppose that the air is fully saturated with aqueous vapor, 

 i.e., just in the condition at which some of its water might 

 be condensed. Such condensation, however, can only take 

 place by cooling the air below 32, and unless the walls or 

 ceiling or furniture are capable of doing this they cannot 

 receive any moisture due to such condensation, or, in other 

 words, they must fall below 32 in order to obtain it by 

 cooling the film in contact with them. Of course Dr. Sie- 

 mens will not assert that the stoves or steam-pipes (enclos- 

 ing the steam, of course), or the hot-air or hot-water pipes, 

 will lower the absolute temperature of the walls by heating 

 the air in the room. 



But if the air is heated more rapidly than are the walls, 

 etc., the relative temperature of these will be lower. Will 



