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Mr. O. Reynolds on the Condensation of a [May 1, 



Second. To ascertain if (and according to what law) the effect of air on 

 the condensation increases as the proportion of air to steam increases. 



5. Of these two undertakings the first is much the most difficult. 

 The rate of condensation of pure steam is so great that it is practically 

 impossible to measure it; and to institute a comparison between this 

 and the condensation of a mixture of steam and air is like comparing the 

 infinite with the finite. It is practically impossible to keep any surface 

 cold when an unlimited supply of pure steam is condensed upon it, so 

 that under such circumstances the quantity of pure steam condensed is 

 limited by the power of the surface to carry off the heat. The best 

 method of obtaining a qualitative result seems to be by introducing 

 sufficient cold water into a flask of steam to condense it all, and ascertain 

 whether this condensation is effected suddenly or slowly. 



6. The presence of hot water in the flask with the steam very much 

 assists in ascertaining the rapidity of condensation. When there is no 

 hot water in the flask, the condensation by the injected water is only a 

 question of time ; the gauge will come to the same point whether the con- 

 densation is quick or slow, the only difference being in the speed at 

 which it will rise — a difference not easy to appreciate, especially when 

 the motion is quick. But if hot water is present, then as the steam in 

 the flask is condensed it is replaced by fresh steam from the water, 

 and the interval between the condensation and the consequent ebullition 

 is the only time 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 consisted 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 after- 

 wards 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 ; 



