Melt a small quantity of the sulphate of potassa and copper in a spoon over a spirit lamp; it will be fused at a heat just below redness, and produce a liquid of a dark green color. Remove the spoon from the flame, when the liquid will become a solid of a brilliant emerald green color, and so remain till its heat sinks nearly to that of boiling water; when suddenly a commotion will take place throughout the mass, beginning from the surface, and each atom, as if animated, will start up and separate itself from the rest, till, in a few moments, the whole will become a heap of powder.
SUBLIMATION BY HEAT.
Provide two small pieces of glass; sprinkle a minute portion of sulphur upon one piece, lay thin slips of wood around it, and place upon it the other piece of glass. Move them slowly over the flame of a lamp or candle, and the sulphur will become sublimed, and form gray nebulous patches, which are very curious microscopic objects. Each cluster consists of thousands of transparent globules, imitating in miniature the nebulæ which we see figured in treatises on astronomy. By observing the largest particles, we shall find them to be flattened on one side. Being very transparent, each of them acts the part of a little lens, and forms in its focus the image of a distant light, which can be perceived even in the smaller globules, until it vanishes from minuteness. If they are examined again after a certain number of hours, the smaller globules will generally be found to have retained their transparency, while the larger ones will have become opaque, in consequence of the sulphur having undergone some internal spontaneous change. But the most remarkable circumstance attending this experiment is, that the globules are found adhering to the upper glass only; the reason of which is, that the upper glass is somewhat cooler than the lower one: by which means we see that the vapor of sulphur is very powerfully repelled by heated glass. The flattened form of the particles is owing to the force with which they endeavor to recede from the lower glass, and their consequent pressure against the surface of the upper one. This experiment is considered by its originator, Mr. H. F. Talbot, F.R.S., England, to be a satisfactory argument in favor of the repulsive power of heat.
HEAT PASSING THROUGH GLASS.
The following experiment is also by Mr. Talbot: Heat a poker bright red hot, and having opened a window, apply the poker quickly very near to the outside of a pane, and the hand to the inside; a strong heat will be felt at the instant, which will cease as soon as the poker is withdrawn, and may be again renewed, and made to cease as quickly as before. Now, it is well known that if a piece of glass be so much warmed as to convey the impression of heat to the hand, it will retain some part of that heat for a minute or more; but in this experiment the heat will vanish in a moment. It will not, therefore, be the heated pane of glass that we shall feel, but heat which has come through the glass, in a free or radiant state.
METALS UNEQUALLY INFLUENCED BY HEAT.
All metals do not conduct heat at the same rate, as may be proved by holding in the flame of a candle at the same time, a piece of silver wire and a piece of platina wire, when the silver wire will become too hot to hold, much sooner than the platina. Or, cut a cone of each wire, tip it with wax, and place it upon a heated plate, (as a fire-shovel,) when the wax will melt at different periods.
SPONTANEOUS COMBUSTION.
Mix a small quantity of chlorate of potassa with spirit of wine in a strong saucer; add a little sulphuric acid, and an orange vapor will arise and burst into flame with a loud crackling sound.
INEQUALITY OF HEAT IN FIRE-IRONS.