heavy bullet, and previously placed in the water at the bottom of the glass, will not thaw, although the surface of the water at a few inches above, is in a state of full ebullition.

Then, if water were a conductor of heat, it should all fairly boil, when the heat is applied above, as we know that it does when applied below; the theory of its ebullition, in the latter case, is as follows.

The heat is conducted through the vessel, and thus expands the water that it first reaches; the portions thus expanded, are of greater temperature and of less specific gravity than others above, they accordingly ascend, the same as a lighter fluid through a heavier, they impart heat to these portions by contact; and thus losing temperature, they gain specific gravity; they fall, again acquire heat, and diminish in specific gravity; they again ascend, and thus by carrying heat, or by convection instead of conduction, ascending and descending or convective currents are established, until all portions of the water are equally heated to a degree at which it can no longer remain liquid; then vapour or steam simultaneously escapes from all portions, vaporization rapidly ensues, or in plainer terms, the water boils.

The whole of the water can never boil when the heat is applied at its surface, because convection, or ascending and descending currents cannot be established, the heated film, that in the other case is a convective film, is in this, a heated non-conducting film.

A very beautiful experiment regarding the ascent of

heated water and the descent of cold, can be made without much trouble, by attending to the following directions.

First, boil about two gallons of clean rain-water, in a clean tin saucepan for about an hour, then remove the vessel from the fire, cover it closely and allow the water to cool down to common temperatures.

Select a piece of glass-tube four feet long, about the twentieth of an inch thick, having a bore half an inch in diameter, and perfectly clean; if not so, it can be cleaned by passing a long string with a little cotton fastened to one end, several times through the tube.

Place a hand near each extremity of the tube, and thus hold it horizontally, that its centre may be at a distance of four or five inches above the heated air ascending from a large argand gas-burner, or above that of a bright charcoal fire in a small chafing dish or brazier; turn the tube round and round between the hands, that it may conduct the heat, or warm gradually at its centre, then continuing to turn, bring it lower and lower, or nearer the source of heat, and let it become fairly red-hot for about four or five inches; the hands will soon detect that it is soft and yielding, and by gently drawing them together in the same manner, as for bending a cane, the glass tube, or "cane" as it is technically called, will bend from its original straight form, and by still keeping this yielding portion in the heat, and still gently drawing the hands together, the tube will assume the form of a "syphon,' having its limbs parallel to each other, though about

four inches apart, and like a syphon, one of these limbs should be about four inches longer than the other: this is managed by heating the tube originally a little beyond its true centre; if, however, they should be of equal length, the matter can be remedied, of which hereafter.

Having wrought the tube into this shape, place its open ends in a box or jar that its bend may stand uppermost and thus slowly cool in the air; and when it is sufficiently cold to handle, it must be removed, and again heated in the following manner.

Hold the tube horizontally that about four inches of its longest end may become red-hot and soft, then by gently pressing against this with a sound piece of charcoal, or a stick of deeply charred wood, form it into a bend, exactly parallel with the first, though not quite so large; and this being done, place the first bend to stand in the box or jar, that the second may now cool.

When it is cold, remove the tube and place it flat upon the table, and very probably the straight limb of the tube will now appear the longest; if such be the case, take a sharp three-edged file, and gently make a deep cut across this, then hold the red-hot heater of an Italian iron on the cut for half a minute, and if the tube do not crack, remove the iron and touch the file mark with the wetted tip of the fore-finger, this sudden cold will cause it to fly across, and by a very gentle pull, a piece will be thus cut off, and the limbs of the syphon will bear the same proportion to each other as shewn in the next engraving.

Select a thin earthenware basin, large enough to admit the two ends of the syphon, when placed in it with its large bend uppermost; then remove it, and fill the basin with a portion of the boiled rain-water, and stain this deeply with the well-known dye called "Saxon blue;" this being done, select a very shallow pan that will hold the syphon when laid flat within it, pour the remaining uncoloured boiled rain-water into this, in order that the syphon may be perfectly filled, and every bubble of air excluded.

Fig. 65.

Then place a finger of each hand beneath the water, and close each end of the syphon; thus lift it, and when both ends are fairly immersed beneath the surface of the coloured water in the basin, remove the fingers, and the syphon will remain full on account of atmospheric pressure, as in the case of the "bird-fountain" and the "carafe" at page 212.

The basin thus containing the syphon must be now placed upon a ring supported on three feet,—a small tripod, and the syphon very gradually raised into a perpendicular direction, so that the orifice of its straight

end may be half an inch below the surface of the coloured water: it may be supported at this elevation by a string attached to its upper bend, and fastened to any convenient support above, as to the arm of a chandelier, suspended from the ceiling of a room.

Although the whole arrangement occupies much time in description, and even in its performance, it is worthy of both, on account of the instructive result that it will afford, the object being to prove that hot water is lighter, and therefore will ascend through cold water, which is heavier;-let the flame of a spirit lamp be applied to the bottom of the basin, in a line perpendicular with the straight end of the syphon; and in the course of a few minutes will be presented the phenomenon, of which we are in search, and which is represented in the above engraving.

The coloured water at the bottom of the basin be