They do, on account of water boiling there at a temperature below two hundred and twelve degrees. What is a _vacuum_? A _vacuum_ is a place that does not contain anything, not even air. When a vessel contains water and we pour the water out, does that produce a vacuum? No; because air passes in and fills the vessel. How then can a vacuum be produced? A vacuum in any place, can be produced only by drawing the air out of it. When a vacuum is produced, what does the surrounding air try to do? It tries to get in so as to fill the vacuum. With what force does the air try to fill a vacuum? With a force of fifteen pounds on every square inch of surface. When a vacuum is produced in contact with a liquid, what does the liquid do? The liquid tries to pass in to fill the vacuum. Can there be a vacuum in contact with a gas or a liquid? No; because the gas or the liquid will instantly fill the vacant space. Why does water make a gurgling noise when poured out of a bottle? Because air is passing into the bottle at the same time that the water is coming out, and the air makes the noise in passing through the water. Why does water make a gurgling noise when running out of a barrel? Because the air passes into the barrel at the opening where the water passes out, and thus the noise is made. How can the gurgling noise be prevented? By making an opening at the top of the barrel, so as to admit the air. Does water flow freely from a vessel that has but one opening? No; because part of this opening is needed to admit air into the vessel. Why does the water flow freely, when there is an opening at the top as well as at the bottom of the vessel? Because air is admitted at the top, and there is then nothing to obstruct the flow of water from the opening at the bottom. What causes fruit, vegetables, and meat to spoil? The air causes them to spoil. How do we know this? Because when kept from the air they remain good for a long time. What is this process of keeping fruit called? It is called canning. How is fruit canned? The fruit is put into a glass jar, or into a tin can, along with some water, and then heated until the jar or can is filled with steam and the air is all forced out; it is then sealed so as to be air-tight, and, in this condition, may be kept for many months. What becomes of the steam in the jar or can? As the fruit cools, the steam is condensed to water, and a vacuum is produced, so that the fruit is kept from the air. What is the plaything which boys call a sucker? A _sucker_ is a round piece of leather, with a string fastened to its centre. How is the sucker used? The leather is made very soft and wet, so that when pressed against a stone or brick, all the air under the leather is forced out; and when the leather is lifted up, the stone or brick is lifted with it, as is represented by Fig. 14. [Illustration: Fig. 14.] What keeps the leather against the stone?