1873.] Mixture of Air and Steam upon Cold Surfaces. 277 



this time, however, as the cold water entered the mercury did not rush 

 up the gauge, but rose slowly a small distance and there remained. 



8. This experiment shows, therefore, that there is a great difference in 

 the rates at which pure steam and steam with air condense on a cold 

 surface, so great in fact that the speed with pure steam must be regarded 

 as nearly infinite. 



9. To compare the various effects of different quantities of air, pwo 

 methods have been used, which may be described as follows : — 



I. A surface-condenser is formed within the boiler or flask, so that the 

 steam may be condensed as fast as it is generated. Then, when a flame 

 of a certain size acts on the boiler, the effect of the air is to cause the 

 pressure of steam in the flask to increase. This method is founded on 

 the assumption that the rate at which steam will condense at a cold 

 surface is, cceteris paribus, proportional to its pressure — an assumption 

 which is probably not far from the truth. 



II. With the same apparatus as in method I. the rate of condensation 

 is measured by the quantity of water condensed in a given time, obtained 

 by counting the drops from the condenser, the pressure within the flask 

 being kept constant. This method does not involve any assumption ; but 

 the conditions for its being accurate are such as cannot be obtained ; for 

 not only must the temperature of the condenser and the temperature of 

 the steam remain constant, but the pressure of the steam must also 

 remain constant, and if the two former conditions are fulfilled the latter 

 cannot be ; for the temperature of the steam will be the boiling-point of 

 the water in the flask ; and if this is to remain constant, the pressure of 

 air and steam must be constant, and therefore, as the pressure of the air in- 

 creases, the pressure of the steam must decrease. This variation of pressure 

 is not very great ; and its effect may be allowed for on the assumption that 

 the condensation is proportional to the pressure of steam. This is accom- 

 plished by dividing the drops by the pressure of the steam. 



These methods, neither of which, as it appears, is rigorous, seem 

 nevertheless to be the best ; and fortunately the law which the effect of 

 the additions of air follows is of such a decided character as to be easily 

 distinguished ; and the two methods give results which are sufficiently 

 concordant for practical purposes. 



10. The apparatus employed in these experiments consisted of a glass 

 flask, in which a surface-condenser was formed of a copper pipe passing 

 in and out through the cork. This pipe was kept cool by a stream of 

 water, and was so fixed that all the condensed water dropped from it, 

 and the drops could be counted. The flask was freed from air by 

 boiling; the volume of air passed into the flask could be accurately 

 measured ; and ample time was allowed for the air in the flask to produce 

 its effect before more was admitted. 



For the experiments according to method L, the flame under the flask 

 and the stream of water through the condenser were kept constant from 

 first to last. For those made according to method II., in one case the 



2 A 2 



