annuals. The oak and ash are constantly the latest in pushing their leaves; the greatest number unfold them in spring; the mosses and firs in winter. These striking differences, with respect to so capital a circumstance in plants as that of unfolding their leaves, seem to indicate that each species of plant has a temperature proper or peculiar to itself, and requires a certain degree of heat to extricate the leaves from their buds, and produce the appearance in question. This temperature, however, is not so constant, as, to a superficial observer, it may appear to be. Among plants of the same species, there are some more early than others; whether that circumstanue depends, as it most commonly does, on the nature of the plants, or is owing to differences in heat, exposure, and soil. In general, it may be affirmed, that small and young trees are always earlier than larger or old ones. See GERMINATION, and Milne's Bot. Dict.

FROST, such a state of the atmosphere as causes the congelation or freezing of water or other fluids into ice. In the more northern parts of the world, even solid bodies are affected by frost, though this is only or chiefly in consequence of the moisture they contain, which being frozen into ice, and so expanding as water is known to do when frozen, it bursts, and rends any thing in which it is contained, as plants, trees, stones, and large rocks. Many fluids expand by frost, as water, which expands about one-tenth part, for which reason ice floats in water; but others again contract, as quicksilver, and thence frozen quicksilver sinks in the fluid metal.

Frost, being derived from the atmosphere, naturally proceeds from the upper parts of bodies downwards, as the water and the earth: so, the longer a frost is continued, the thicker the ice becomes upon the water in ponds, and the deeper into the earth the ground is frozen. In about 16 or 17 days frost, Mr. Boyle found it had penetrated 14 inches into the ground. At Moscow, in a hard season, the frost will penetrate two feet deep into the ground and Captain James found it penetrated 10 feet deep in Charlton İsland, and the water in the same island was frozen to the depth of six feet. Sheffer assures us, that in Sweden the frost pierces two cubits, or Swedish ells, into the earth, and turns what moisture is found there into a whitish substance, like ice; and standing water to three ells or more. The same author also mentions sudden cracks or rifts in the ice of the

lakes of Sweden, nine or ten feet deep, and many leagues long; the rupture being made with a noise not less loud than if many guns were discharged together. By such means, however, the fishes are furnished with air; so that they are rarely found dead.

The natural history of frosts furnish very extraordinary effects. The trees are often scorched and burnt up, as with the most excessive heat, in consequence of the separation of water from the air, which is therefore very drying In the great frost in 1683, the trunks of oak, ash, walnut, &c. were miserably split and cleft, so that they might be seen through, and the cracks often attended with dreadful noises like the explosion of fire-arms. Philos. Trans. Number 165.

The close of the year 1708, and the beginning of 1709, were remarkable, throughout the greatest part of Europe, for a severe frost. Dr. Derham says it was the greatest in degree, if not the most universal, in the memory of man; extending through most parts of Europe, though scarcely felt in Scotland or Ireland.

In very cold countries, meat may be preserved by the frost six or seven months, and prove tolerably good eating. See Captain Middleton's observations made in Hudson's Bay, in the Philos. Trans. Number 465, sect. 2.

In that climate the frost seems never out of the ground, it having been found hard frozen in the two summer months. Brandy and spirit, set out in the open air. freeze to solid ice in three or four hours.

Lakes and standing waters, not above 10 or 12 feet deep, are frozen to the ground in winter, and all their fish perish. But in rivers, where the current of the tide is strong, the ice does not reach so deep, and the fish are preserved. Id. ib.

Some remarkable instances of frost in Europe, and chiefly in England, are recorded as below; in the year

220 Frost in Britain that lasted five

months.

250 The Thames frozen nine weeks. 291 Most rivers in Britain frozen six

weeks.

359 Severe frost in Scotland for 14 weeks.

508 The rivers in Britain frozen for two months.

558 The Danube quite frozen over. 695 Thames frozen six weeks; booths

- built on it.

759 Frost from Oct. 1, till Feb. 26, 760.

827 Frost in England for nine weeks. 859 Carriages used on the Adriatic

Sea.

908 Most rivers in England frozen two months.

923 The Thames frozen 13 weeks. 987 Frost lasted 120 days: began December 22.

998 The Thames frozen five weeks. 1035 Severe frost on June 24: the corn

and fruits destroyed.

1063 The Thames frozen fourteen weeks.

1076 Frost in England from November till April.

1114 Several wooden bridges carried away by ice.

1205 Frost in England from January 14, till March 22.

1407 Frost that lasted 15 weeks. 1434 From Novem. 24, till Feb. 10, Thames frozen down to Gravesend.

1683 Frost for thirteen weeks. 1708-9 Severe frost for many weeks. 1715. The same for many weeks. 1739 One for nine weeks: began December 24.

1742 Severe frost for many weeks. 1747 Severe frost in Russia. 1751 Severe one in England. 1760 The same in Germany. 1776 The same in England. 1788 The Thames frozen below bridge; booths on it.

1794 Hard frost of many weeks. Ther. at London, mostly at 20 below 0 of Fahrenheit.

the cold.

FROTH spit, or Cuскow spit, a name given to a white froth, or spume, very common in the spring and first months of the summer, on the stems of certain plants; it includes and defends the larva of certain species of Cicada, and from the pores of which it is secreted at pleasure. See CICADA.

FRUCTESCENTIA, in botany, comprehends the precise time, in which, after the fall of the flowers, the fruits arrive at maturity, and disperse their seeds. In general, plants which flower in spring ripen their fruits in summer, as rye; those which flower in summer have their fruits ripe in autumn, as the vine; the fruit of autumnal flowers ripen in winter, or the following spring, if kept in a stove or otherwise defended from excessive frosts. The time in which plants ripen their fruit, combined with that in which they germinate and unfold their leaves, gives the entire space or duration of their life, which, in the same species, is proportionally short or long, according to the greater or less intensity of heat of the climate in which they are cultivated. In general, it appears, that, if the heat is equal and uninterrupted, the time betwixt the germinating or sprouting and flowering of annual plants is equal to the interval betwixt their flowering and the maturation of the fruits, or even the total destruction of the whole plant. In very hot climates, an annual plant generally lives as long before as after flowering. But in temperate climates, as France and England, plants which rise in spring, and flower before the month of June, live a little longer before than after flowering; such as flower in summer, as barley and oats, which flower in June, live as long before as after; while the later plants, which do not rise till autumn, live longer after flowering than before. These observations apply chiefly to herbaceous annuals. See Milne's Botanical Dictionary.

FRUSTUM, in mathematics, a part of some solid body separated from the

rest.

The frustum of a cone is the part that remains, when the top is cut off by a plane parallel to the base; and is other wise called a truncated cone. The frusafter the top is cut off by a plane parallel tum of a pyramid is also what remains,

to its base. To find the solid content of

the frustum of a cone, pyramid, &c. the base being of any figure whatever : add the areas of the two ends and the mean proportional between them together, then one-third of that sum will be the mean area, or the area of an equal prism, of the same altitude with the frustum; and consequently, that mean area multiplied by the height of the frustum will give the solid content for the product :

The cotton, or other materials, of which the wick is composed, is not scorched or consumed in the least, because the spirit with which it is constantly soaked is incapable of becoming hotter than 1740 Fahrenheit, which is considerably below the heat of boiling water. It is only the vapour that arises from it which is hotter, and this too only in its outer parts, that are most remote from the wick, and where only the combustion is going on, in consequence of communication and contact with the air. At the same time, as the alcohol is totally volatile, it does not leave any fixed matter, which, by being accumulated on the wick, might render it foul and fill up its pores. The wick, therefore, continues to imbibe the spirit as freely, after some time, as it did at the first. These are the qualities of alcohol as a fuel. But these qualities belong only to a spirit that is very pure. If, on the contrary, it be weak, and contain water, the water, being less volatile, does not evaporate so fast from the wick as the more spirituous part; and the wick becomes, after some time, so much soaked with water, that it does not imbibe the spirit properly. The flame becomes much weaker, or is altogether extinguished. When alcohol is used as a fuel, therefore, it ought to be made as strong, or free from water, as possible.

Oil, although fluid like spirit of wine, and capable of burning in a similar manner, is not so convenient in many respects. It is disposed to emit soot; and this applying itself to the bottom of the vessel exposed to it, and increasing in thickness, forms, by degrees, a foft and spongy medium, through which heat is not so freely and quickly transmitted. This was observed by Muschenbroeck, in his experiments upon the expansions of metalline rods heated by lainps. It is true we can prevent this entirely by using very small wicks, and increasing the number, if necessary, to produce the heat required. Or we may employ one of those lamps, in which a stream of air is allowed to rise through the middle of the flame, or to pass over its surface with such velocity as to produce a more complete inflammation than ordinary. But we shall be as much embarrassed in another way, for the oils commonly used, being capable of assuming a heat greatly above that of boiling water, scorch and burn the wick, and change its texture, so that it does not imbibe the oil so fast as before. Some have attempted a remedy, by making the wick of incom

bustible materials, as asbestos, or wire; but still, as the oil does not totally eva porate, but leaves a small quantity of gross fixed carbonaceous matter, this, constantly accumulating, clogs the wick to such a degree, that the oil cannot ascend, the flames become weaker, and, in some cases, are entirely extinguished. There is, however, a difference among the different oils in this respect; some being more totally volatile than others. But the best are troublesome in this way, and the only remedy is, to change the wicks often, though we can hardly do this and be sure of keeping always an equal flame.

The second kind of fuel mentioned, peat, is so spongy, that, compared with the more solid fuels, it is unfit to be employed for producing very strong heats. It is too bulky for this; we cannot put into a furnace, at a time, a quantity that corresponds with the quick consumption that must necessarily go on when the heat is violent. There is, no doubt, a great difference in this respect among different kinds of this fuel, but this is the general character of it. However, when we desire to produce and keep up, by means of cheap fuel, an extremely mild gentle heat, we can hardly use any thing better than peat. But it is best to have it previously charred, that is scorched, or burnt to black coal. The advantages gained by charring have been already explained. When prepared for use in that manner, it is capable of being made to burn more slowly and gently, or will bear, without being extinguished altogether, a greater diminution of the quantity of air with which it is supplied, than any other of the solid fuels. Dr. Boerhaave found it extremely convenient and manageable in his Furnus Studiosorum.

The next fuel in order is the charcoal of wood. This is prepared by piling up billets of wood into a pyramidal heap, with several spiracles, or flues, formed through the pile. Chips and brushwood are put into those below, and the whole is so constructed, that, when kindled, it kindles almost over the whole pile in a very short time. It would burst out into a blaze, and be quickly consumed to ashes, were it not covered all over with earth or clay, beaten close, leaving openings at all the spiracles. These are carefully watched; and whenever the white watery smoke is observed to be succeeded by thin blue and transparent smoke, the hole is immediately stopped; this

being the indication of all the watery vapour being gone, and the burning of the true coaly matter commencing. Thus is a pretty strong red heat raised through the whole mass, and all the volatile matters are dissipated by it, and nothing now remains but the charcoal. The holes be ing all stopped in succession, as this change of the smoke is observed, the fire goes out for want of air. The pile is now allowed to cool. This requires many days; for, charcoal being a very bad conductor of heat, the pile long remains red hot in the centre, and, if opened in this state, would instantly burn with fury.

Small quantities may be procured at any time, by burning wood in close vessels. Little pieces may be very finely prepared, at any time, by plunging the wood in lead melted and red hot.

This is the chief fuel used by the chemists abroad, and has many good properties. It kindles quickly, emits few watery or other vapours while burning, and when consumed leaves few ashes, and those very light. They are, therefore, easily blown away, so that the fire continues open, or pervious to the current of air which must pass through it to keep it burning. This sort of fuel, too, is capable of producing as intense a heat as can be obtained by any; but in those violent heats it is quickly consumed, and needs to be frequently supplied.

Fossil coals charred, called cinders, or coaks, have, in many respects, the same properties as charcoal of wood; as kindling more readily in furnaces than when they are not charred, and not emitting watery, or other gross smoke, while they burn. This sort of charcoal is even greatly superior to the other in some properties.

It is a much stronger fuel, or contains the combustible matter in greater quantity, or in a more condensed state. It is, therefore, consumed much more slowly on all occasions, and particularly when employed for producing intense melting heats. The only inconveniences that attend it are, that, as it consumes, it leaves much more ashes than the other, and these much heavier too, which are, therefore, liable to collect in such quantity as to obstruct the free passage of air through the fire; and further, that when the heat is very intense, these ashes are disposed to melt or vitrify into a tenacious drossy substance, which clogs the grate, the sides of the furnace, and the vessels. This last inconvenience is only troublesome,

however, when the heat required is very intense. In ordinary heat the ashes do not melt, and though they are more copious and heavy than those of charcoal of wood, they seldom choke up the fire considerably, unless the bars of the grate be too close together.

This fuel, therefore, is preferable, in most cases, to the charcoal of wood, on account of its burning much longer, or giving much more heat before it is consumed. The heat produced by equal quantities, by weight, of pit-coal, woodcharcoal, and wood itself, are nearly in proportion of 5, 4, and 3. The reason why both these kinds of charcoal are preferred, on most occasions, in experimental chemistry, to the crude wood, or fossil coal, from which they are produced, is, that the crude fuels are deprived, by charring, of a considerable quantity of water, and some other volatile principles, which are evaporated during the process of charring, in the form of sooty smoke or flame. These volatile parts, while they remain in the fuel, make it unfit (or less fit) for many purposes in chemistry. For, besides obstructing the vents with sooty matter, they require much heat to evaporate them; and therefore, the heat of the furnace, in which they are burnt, is much diminished and wasted by every addition of fresh fuel, until the fresh fuel is completely inflamed, and restores the heat to its former strength.

But these great and sudden variations of the heat of a furnace are quite inconvenient in most chemical processes. In the greater number of chemical operations, therefore, it is much more convenient to use charred fuel, than the same fuel in its natural state.

There are, at the same time, some kinds of fossil coal, which are exceptions to what has now been delivered in general. We meet with some of them that leave a smaller proportion of ashes than others, and the ashes of some are not so liable to melt in violent heats. There is one species too, such as the Kilkenny coal of Ireland, and which occurs likewise in some parts of this country, that does not contain any sensible quantity of water, or other such volatile principles. But this may be called a sort of native charcoal. It has the appearance of or dinary coal, but, when thrown into the fire, does not emit smoke or soot. It merely becomes red, gives a subtile blue flame, and consumes like charcoal; only it lasts surprisingly long, or continues to gives heat for a very long time before it