The principle of the syphon may also be adapted to our domestic filters. Charcoal, as we know, makes an excellent filter, and if we have a block of charcoal in one of those filters,—now so common,—we can fix a tube into it, and clear any water we may require. It sometimes (in the country) happens that drinking-water may become turgid, and in such a case the syphon filter will be found useful. [Illustration: Fig. 68.—Molecular attraction.] The old “deception” jugs have often puzzled people. We give an illustration of one, and also a sketch of the “deceptive” portion (figs. 70 and 71). This deception is very well managed, and will create much amusement if a jug can be procured; they were fashionable in the eighteenth century, and previously. A cursory inspection of these curious utensils will lead one to vote them utterly useless. They are, however, very quaint, and if not exactly useful are ornamental. They are so constructed, that if an inexperienced person wish to pour out the wine or water contained in them, the liquid will run out through the holes cut in the jug. To use them with safety it is necessary to put the spout A in one’s mouth, and close the opening B with the finger, and then by drawing in the breath, cause the water to mount to the lips by the tube which runs around the jug. The specimens herein delineated have been copied from some now existent in the museum of the Sèvres china manufactory. The _Buoyancy of Water_ is a very interesting subject, and a great deal may be written respecting it. The swimmer will tell us that it is easier to float in salt water than in fresh. He knows by experience _how difficult it is to sink_ in the sea; and yet hundreds of people are drowned in the water, which, if they permitted it to exercise its power of buoyancy, would help to save life. [Illustration: Fig. 69.—Vase of Tantalus.] The sea-water holds a considerable quantity of salt in solution, and this adds to its resistance, or floating power. It is heavier than fresh water, and the Dead Sea is so salt that a man cannot possibly sink in it. This means that the man’s body, bulk for bulk, is much lighter than the water of the Dead Sea. A man will sink in fresh, or ordinary salt water if the air in his lungs be exhausted, because without the air he is much heavier than water, bulk for bulk. So if anything is weighed in water, it apparently loses in weight exactly equal to its own bulk of water. Water is the means by which the _Specific Gravity_ of liquids or solids is found, and by it we can determine the relative densities of matter in proportion. Air is the standard for gases and vapours. Let us examine this, and see what is meant by SPECIFIC GRAVITY.