of a Mixture of Air and Steam upon Cold Surfaces. (59 



allowed for the creation of a vacuum ; the vacuum which is attained 

 in the interval will therefore depend on the rapidity of condensation. 

 The interval will be very short ; and the better the vacuum the 

 shorter it will be ; so that unless the condensation is very sudden, 

 there will be but a slight reduction of pressure. 



If, however, the condensation is really instantaneous, a perfect 

 vacuum may exist for an instant. Hence, when there is water in the 

 flask, the rapidity of condensation is indicated by the height to which 

 the gauge rises, instead of the speed with which it rises ; and this is 

 much easier to estimate. 



7. The apparatus employed in making these experiments con- 

 sisted of a glass flask fitted with a mercurial vacuum-gauge and 

 pipes for admitting water and air, or allowing steam to escape. 



The flask and all the pipes were freed from air by boiling ; and 

 when all the air had been driven out the pipes were closed, the lamp 

 removed, and the flask allowed to cool until the gauge showed a 

 slight vacuum ; the water-pipe was then opened and a few drops 

 of water allowed to enter and fall through the flask ; as they did 

 so the mercury rushed up the gauge and, by its momentum, above 

 the point for a perfect vacuum, showing that the condensation was 

 instantaneous. Immediately afterwards the gauge fell nearly to 

 its starting-point. Next, the flask was allowed to cool and a little 

 air was let in (about equal to half an inch of mercury in the gauge, 

 or about a sixtieth of the volume of the flask). The lamp was 

 then replaced, and the operation was repeated as before ; 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 differ- 

 ence 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, two 

 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. 



LE. With the same apparatus as in method I. the rate of conden- 

 sation is measured by the quantity of water condensed in a given 

 time, obtained by counting the drops from the condenser, the pres- 

 sure 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 



