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 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 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, 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, cæteris 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 increases, 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 accomplished 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 I., 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 stream of water was kept constant, and in the other it was altered, so that the effluent water was kept at a constant temperature.

11. The results of these experiments are shown in Tables I., II., III.

The letters which head the columns have the following meanings:f stands for the volume of the flask in cubic centimetres.

a stands for the volume of the air at the pressure of the atmosphere.

h stands for the height of the barometer in millimetres of mercury at the time of the experiment.

h stands for the height of mercury in the gauge in millims. to stands for the temperature Centigrade of the effluent water. t stands for the temperature of the water in the flask, found from Regnault's tables of boiling-points.

P1=h-h, stands for the pressure within the flask in millims. of

mercury.

a

1

P2

t,+274

t+274

stands for the pressure of the air within the

flask corrected to the temperature T,.

P3-P1-P, stands for the pressure of the steam.

Pa stands for the ratio of the pressure of the air in the flask to

P3

that of the steam.

12. Table I. shows the result of an experiment after the first method, during which the flame and condensation remained constant, whilst the pressure within the flask increased with the quantity of air.

Table II. shows the result of an experiment after the second method, in which the pressure within the flask remained constant, whilst the flame and condensation were reduced as the air was admitted. In this experiment the rate at which the water passed through the condenser was constant from first to last, and consequently the temperature of the effluent water varied with the condensation.

Table III. shows the result of an experiment, also made according to the second method, but in which the quantity of water flowing through the condenser was so varied that the temperature of the effluent water remained constant.

Drops
P3

13. Each of these Tables shows the effect of air on the condensation in a very definite manner; but the results as given in the column P3 in Table I. cannot be compared with the in Tables II. and III. as they stand; for these show the effect of the air in a series of increasing figures. If, however, these figures show the power of the air to diminish condensation, then they will be inversely proportional to the quantity of water condensed, i. e. what would have been condensed if the pressure and other things had remained constant. Hence the numbers in the column should

be proportional to the numbers in the column and III.

Drops

P3

1

P3

in Tables II.

In order to compare the results of these experiments, the results in each Table have been multiplied by a common factor, so that they may be the same when the pressure of air is one tenth that of the steam. Thus the numbers in the column in Table I. have

1

P3

Drops
Pa

been multiplied by 2000, and numbers under in Table II. by 7. The result of the experiments thus reduced are shown in the curves 1, 2, 3.

The point of no air might have been chosen as the point in which the curves should coincide; but, as has been previously explained, the results under such circumstances are to be taken as indicating the power of the condenser to carry off the heat. Had it been possible to keep the condenser cool, then there is reason to believe that there would have been no limit to the condensation of pure steam, and that the true form of the curves is like that shown by the dots.

Although the curves do not coincide, yet they are all of the same form, and the difference between them is not greater than car be accounted for by the disturbing causes already mentioned. They all show that the effect of air begins to fall off rapidly when its pressure amounts to one tenth that of the steam, and that when it amounts to about one fourth that of the steam the admission of more air produces scarcely any effect.

Ratio of the pressure of air to that of steam.

14. Conclusions.-The conclusions to be drawn from these experiments are as follows:

:

1. That a small quantity of air in steam does very much retard its condensation upon a cold surface; that, in fact, there is no limit to the rate at which pure steam will condense but the power of the surface to carry off the heat.

2. That the rate of condensation diminishes rapidly and nearly uniformly as the pressure of air increases from two to ten per cent. that of the steam, and then less and less rapidly until thirty per cent. is reached, after which the rate of condensation remains nearly

constant.

3. That in consequence of this effect of air the necessary size of a surface-condenser for a steam-engine increases very rapidly with the quantity of air allowed to be present within it.

4. That by mixing air with the steam before it is used, the condensation at the surface of a cylinder may be greatly diminished, and consequently the efficiency of the engine increased.

5. That the maximum effect, or nearly so, will be obtained when the pressure of the air is one tenth that of the steam, or when about two cubic feet of air at the pressure of the atmosphere and the temperature 60° F. are mixed with each pound of steam.

15. Remarks. As this investigation was nearly completed my