To return to the vapour in the air-pump receiver, it must be observed that as long as the water continues to yield it we may continue to work the pump; and it will be continually abstracted by the barrels, and discharged in the form of water, because it collapses as soon as exposed to the external pressure. All this while the gauge will not indicate any more rarefaction, because the thing immediately indicated by the barometer-gauge is diminished elasticity, which does not happen here. When all the water which the temperature of the room can keep elastic has evaporated under a certain pressure, suppose half an inch of mercury, the gauge standing at 29.5, the vapour which now fills the receiver expands, and by its diminished elasticity the gauge rises, and now the water which had been attached to the pump by chemical or corpuscular attraction is detached, and a new supply continues to support the gauge at a greater height; and this goes on continually till almost all has been abstracted; but there will remain some which no art can take away; for as it passes through the barrels, and gets between the piston and the top, it successively collapses into water during the ascent of the piston, and again expands into vapour when we push the piston down again. Whenever this happens there is an end of the rarefaction.

While this operation is going on, the air comes out along with the vapour; but we cannot say in what proportion. If it were always uniformly mixed with the vapour, it would diminish rapidly; but this does not appear to be the case. There is a certain period of rarefaction in which a transient cloudiness is perceived in the receiver. This is watery vapour formed at that degree of rarefaction, mingled with but not dissolved in or united with the air, otherwise it would be transparent. A similar cloud will appear if damp air be admitted suddenly into an exhausted receiver. The vapour which formed a uniform transparent mass with the air is either suddenly expanded, and thus detached from the other ingredient, or is suddenly let go by the air, which expands more than it does. We cannot affirm with probability which of these is the case: different compositions of air, that is, air loaded with vapours from different substances, exhibit remarkable differences in this respect. But we see from this and other phenomena, which shall be mentioned in their proper places, that the air and vapour are not always intimately united; and therefore will not always be drawn out together by the air pump. But, let them be ever so confusedly blended, we see that the air must come out along with the vapour, and its quantity remaining in the receiver must be prodigiously diminished by this association, probably much more than could be, had the receiver only contained pure air.

Let us now consider what must happen in the pear-gauge. As the air and vapour are continually drawn off from the receiver, the air in the pear expands and goes off with it. We shall suppose that the generated vapour hinders the gauge from rising

pump, the air in the pear whose elasticity is 0.5 slowly mixes with the vapour at the mouth of the pear, and the mixture even advances into its cavity, so that if the pumping be long enough continued, what is in the pear is nearly of the same composition with what is in the receiver, consisting perhaps of twenty parts of vapour and one part of air, all of the elasticity of 0.5. When the pear is plunged into the mercury, and the external air allowed to get into the receiver, the mercury rises in the pear-gauge, and leaves not but of it filled with

60

1

or

1

60 × 20 1200 common air, the vapour having collapsed into an invisible atom of water. Thus the pear-gauge will indicate a rarefaction of 1200, while the barometergauge only showed sixty, that is, showed the elasticity of the included substance diminished sixty times. The conclusion to be drawn from these two measures (the one of the rarefaction of air and the other of the diminution of elasticity) is that the matter with which the receiver was filled, immediately before the re-admission of the air, consisted of one part of incondensible air, and twenty parts of watery vapour. Mr. Smeaton's pump, when in good order and perfectly free from all moisture, will in dry weather rarefy air about 600 times, raising the barometergauge to within one-twentieth of an inch of a fine barometer. This was a performance so much superior to that of all others, and by means of Mr. Nairne's experiments opened so new a field of observation, that the air-pump once more became an interesting instrument among experimental philosophers. The causes of its superiority were also so distinct that artists were immediately excited to a further improvement of the machine; so that this becomes a new epoch in its history.

There is one imperfection which Mr. Smeaton did not attempt to remove. The discharging valve is still opened against the pressure of the atmosphere. An author of the Swedish academy adds a subsidiary pump to this valve, which exhausts the air from above it, and thus puts it in the same situation as the pistonvalve. We do not find that this improvement has been adopted so as to become at all general. Indeed the quantity of air which remains in the passage to this valve is so exceedingly little that it does not seem to merit attention. Supposing the valve hole onetwentieth of an inch wide and as deep (and it need not be more), it will not occupy more than part of a barrel twelve inches long and two inches broad.

The last improvement which we shall mention is that by Mr. Cuthbertson. But we must be allowed to observe beforehand that the same construction was invented, and in part executed, before the end of 1779, by Dr. Daniel Rutherford, who was at that time engaged in experiments on the production of air during the combustion of bodies in contact with nitre, and who was desirous of procuring a more complete abstraction of pure aerial matter than could be effected by Mr. Smeaton's pump.

Fig. 1, plate I. PUMPS, is a perspective view of Cuthbertson's air-pump, with its two principal gauges screwed into their places. These need not be used together, except in cases where the utmost exactness is required. In common experiments one of them is removed, and a stop-screw put in its place. Wher