can it lose more Let us learn, by

Now what will happen if the substance be lighter, bulk for bulk, than water? How than its own weight? you may ask. means of experiment, what will take case as this.

place in such a

EXPERIMENT 22.-Here I have a piece of wood which is lighter, bulk for bulk, than water, and I force it beneath the surface of the water; but I find that the upward pressure caused by the buoyancy of the water is now greater than the weight of the substance, so that it is forced up to the top of the water and swims upon the surface.

Well, as the result of all these experiments, we may conclude, firstly, that any substance immersed in water appears to become lighter by the weight of its own bulk or volume of water. And secondly, that in consequence of this, if the substance be heavier, bulk for bulk, than water, it will sink; if of the same weight, bulk for bulk, as water, it will neither sink nor swim ; but if lighter, bulk for bulk, than water, then it will swim.

26. Comparative Density.-Now I wish to show you that we have here got a method by which we can tell how much any substance is heavier, bulk for bulk, than water.

* EXPERIMENT 23.-Let us imagine that we have a small piece of gold that weighs in air exactly 19 grains this is its weight. Let us next weigh it in water, and we find that it now weighs only 18 grains, showing a loss of weight equal to I grain. Now this loss is equal to the weight of its own bulk of water, which is therefore I grain. But the gold in itself weighs 19 grains, so that it weighs 19 times as much as its

own bulk of water. say that the specific gravity of gold is 19. Now we shall get the same result whatever be the size or shape of the piece of gold we use. But on the other hand, if a person put something into our hand that was not really gold, but only like it, we should no doubt find by weighing it in water that the substance was not so much as 19 times heavier than its own bulk of water. This method of finding out the specific gravity or relative density of bodies was discovered more than 2,000 years ago by a philosopher called Archimedes. Hiero, King of Syracuse, had a crown of gold, and he had reason to believe that the goldsmith had mixed a quantity of silver with the gold, but he could not think of any way of finding this out— so in his difficulty he applied to Archimedes. The true way of finding it out occurred to Archimedes one day when he had gone to take a bath, and the tradition is that he immediately ran out of the bath quite naked, shouting out "Eureka! Eureka!" which means "I have found it out! I have found it out!" He then went home and got a piece of gold which he knew was pure, and found that when weighed in water it lost one-nineteenth part of its whole weight, from which he argued as we have done, that pure gold is 19 times as heavy as water, bulk for bulk. He next took Hiero's crown, but he found that when weighed in water it lost more than one-nineteenth part of its whole weight, from which he argued that it was not made of pure gold, and doubtless the goldsmith was properly punished for his theft.

This is what we mean when we

27. Buoyancy of other Liquids. Other liquids besides water have buoyancy. Indeed, each

liquid has its own peculiar amount of buoyancy. A very light liquid, such as alcohol or ether, has comparatively little; while a very heavy liquid, such as mercury, has a great deal. To convince you of this, I have only to pour some of this mercury into a vessel, and put on its surface a bit of iron-the iron, as you see, floats; showing that it is lighter, bulk for bulk, than mercury. Gold, on the other hand, is heavier than mercury; in fact, mercury is 13 times as heavy as water, bulk for bulk; while gold, you have already seen, is about 19 times as heavy, bulk for bulk.

Salt water is somewhat heavier than fresh; and there is in Palestine an inland lake called the Dead Sea, so salt, and consequently so heavy, that a man immersed in it could not possibly sink.

28. Capillarity.-Before leaving liquids, let me just mention a well-known case in which water will rise above its own level.

EXPERIMENT 24.—If we hold a lump of sugar above the surface of water in a vessel, and allow its lower end to touch the surface, we shall soon find the whole lump wet. In like manner, if we dip a strip of blottingpaper or cotton-wick in water, we may convey it above its level by these means.

But if we hold the sugar or strip of blotting-paper with its lower end touching a surface of mercury, the mercury will not rise into the sugar or the blottingpaper; so that these two liquids, water and mercury, behave differently as regards the lump of sugar or the strip of blotting-paper. In the first place, we see the water rise into them, and not only rise into them, but remain there; the mercury, on the other hand, will not rise into them, and will not wet them; in fact,

mercury has not a sufficient attraction for sugar to rise into it, nevertheless mercury may be made to adhere to a surface of silver or of gold, because it has a great attraction for these metals.

PROPERTIES OF GASES.

29. Pressure of Air.-Gases have many points of likeness to liquids, but in other respects the two are very different. A liquid has a surface, so that you may fill a bottle half full with a liquid and shake the liquid against the sides of the bottle. But you cannot do this with a gas. Here, for instance, I have a bladder which contains gas, but the gas fills the whole bladder, and not a part of it. In fact, a gas has an intense desire to fill any vacant space that is not already filled, and will strongly exert itself to do so.

EXPERIMENT 25.-I can easily prove this by a very simple experiment. I have here an air-pump which I will afterwards describe to you; meanwhile let me tell you that by means of this air-pump, we can take out of this bell-jar the atmospheric air which it now contains. You see the india-rubber ball full of air which I will put under the bell-jar. Now I will exhaust the bell-jar, that is to say, take its air out, and what is the result? There is air in the india-rubber ball, but there is now none round about it, and in consequence the air in the ball tries to fill the empty space, but it can only do this by enlarging the ball, and you see the ball grow bigger and bigger as I continue the exhaustion. I shall now let the air in, and you see the ball once more resumes its former size.

EXPERIMENT 26.-We may vary the experiment in this way. I shall now place on the bed-plate of the air-pump a jar which is covered at its top by a piece of india-rubber tied tightly round the rim. I now exhaust the jar as before, and find that as I withdraw the air from the inside of the jar, the outside air trying to force itself into the void space presses down the india-rubber cover, and perhaps, before the experiment is

Fig. 13.

over, the pressure may be great enough to burst the india-rubber.

30 Weight of Air.-You thus see that air will force itself into any space that is empty, if it possibly cal, and indeed we have the greatest difficulty in emptying all the air out of any vessel. We can, however, take out the greater part of the air which fills a vessel. In fig. 14, for instance, is a vessel which we can attach to the air-pump, and by this means deprive it of air, and it will be found that the vessel full of air weighs heavier than the vessel empty, or, in other words, air has weight.

Fig. 14.

EXTERIMENT 27.-Let us now attach a it box bottom downwards to one of the arms of the balance, and ascertain its weight. This weight may be said to be that of the box filled with atmospheric air.

EXPERIMENT 28.-Let us next fill the box by dis