EXPERIMENTS ON THE CONDENSATION OF STEAM. 423 



This gives a rate of 0-88 per degree-second. The lower half of the tube 

 was more thickly covered with water than, the upper half, the steam also 

 was full of flying spray, which may have assisted in conveying heat to the 

 metal, and in maintaining the same rate of condensation on the lower half 

 of the tube as on the upper half, in spite of the somewhat higher tempera- 

 ture of the circulating water in the lower half. 



3. With the same arrangement, but with the steam current reversed 

 and reduced until the escape was as gentle as possible consistently with 

 keeping the tube full of steam and entirely excluding air, a somewhat 

 larger rate of condensation was observed, namely, ti3-6 T.U.F. per square 

 foot per second. The pressure throughout the tube was very nearly 

 atmospheric, and the gentle upward current of steam tended to keep the 

 tube very thickly covered with drops and rivulets of water. The difference 

 of temperature was only 22°-0 F., giving a rate of condensation of 1-07 

 T.U.F. per degree-second. This is equivalent to 2-25 watts (joules per 

 second) per square cm. per 1° C, and was the largest value observed 

 throughout the work. It would appear probable that the surface exposed 

 by the di'ops is so much greater (in the present instance about twice as 

 great) than the surface of the metal, and that the drops themselves are in 

 such rapid motion, that the increase of surface by facilitating condensation 

 more than compensates for any resistance which the water-hlm may offer 

 to the passage of heat to the metal. 



4. To verify this view, the outer glass tube was replaced by a much 

 smaller tube, so as to leave very little space for the steam current. The 

 pressure of the steam was thus raised to nearly four inches of mercury 

 above the atmospheric at the entrance of the tube, and the surface of the 

 platinum was violently scoured by a spiral rusli of steam and spray. 

 Under these conditions, the condensation observed was reduced to 19-2 

 T.U.F. per square foot per second, instead of being increased as might 

 naturally have been expected with so strong a current of steam. The 

 effect of the energetic scouring of the metal surface was shown by a slight 

 rise of temperature of the metal as compared with the previous experi- 

 ments. The observed difference of temperature between the metal and 

 the steam in this case was 19°-8 F., giving a rate of condensation of 0-97 

 T.U.F. per degree-second. 



From these and similar observations, in which the conditions of the 

 experiments were varied to a certain extent in points of detail, it may be 

 concluded that the presence of water on a metal surface may tend to 

 increase rather than diminish the amount of condensation. The rate of 

 condensation of steam at 212° F., allowing for the fact that in these 

 experiments the surface was unduly increased by the presence and motion 

 of the water drops, would appear to be at least of the same order of 

 magnitude as the value deduced from experiments on the cyclical con- 

 densation in the cylinder of a working steam-engine in which the 

 temperature of condensation varied from 290 F. to 330° F., and the rate 

 deduced was 0-74 T.U.F. per square foot per degree-second. Since, how- 

 ever, it is possible that the latter value was diminished to an uncertain 

 extent by a slight film of grease on the hot and dry surface, and since the 

 value deduced from the surface-condenser method is perhaps a little too 

 large owing to the presence of the water-film, it would be unsafe to 

 conclude that the rate of condensation is the same at different tempera- 

 tures, although the evidence so far as it goes appears at present to point 

 in that direction. 



