for determining its relative quantity. It has been ascertained by numerous experiments, that bodies in general are expanded by the same cause which excites in us the sensation of heat; and on this principle a number of instruments have been made, to measure, as is said, the quantity of heat, but which, in fact, are a measure of nothing else but their own expansion. But as the expansion of some substances, of mercury, for example, has been observed in conjunction with a number of other circumstances; as for example, when water boils, and would scald us, and when it freezes, and would produce a sensation of intense cold, the expansion of mercury becomes a measure of the expansion of other bodies, by the application of heat. By a reference, also, to the two intense sensations of burning and freezing, this expansion serves to give us some notion of the still more intense sensations we should probably have, were the body not before that period destroyed, when the heat is so small, that mercury itself freezes, and so great that iron melts. The degrees of heat mentioned in speaking of all thermometers, we here caution our readers, means, strictly speaking, only the expansion of the substance which indicates these degrees, and they are of no value and give no information beyond this expansion; but as the expansion of mercury in particular has been observed in conjunction with other circumstances, it is a sort of index to them. Thus a certain quantity of mercury contained in a glass tube of known and certain dimensions, expands a certain length when water boils under the ordinary pressure of the atmosphere; it does not expand so far when placed in contact with the human body, so as to mark what is called blood heat; it expands still less when wheat ripens and does not burn; and it is still less expanded when water becomes solid. Thermometers, therefore, do not measure, as is ordinarily said, degrees of

heat, but degrees of expansion; and they are particularly valuable, because we have learnt from experience, that when the mercury reaches various points in this scale of expansion, various other effects, which it is most important for us to know, always take place, and may therefore always be expected. Having made these few remarks, rather as a caution to our readers than as instructing them, for we shall hereafter treat of heat, we proceed to describe two thermometers for measuring small variations of temperature. The instrument, consisting of a single tube placed in a bottle, is an air thermometer; the other figure is a representation of Mr. Leslie's differential thermometer.

The air thermometer consists of a bottle, partly filled with any coloured liquid and partly with air. The lower end of a glass tube, having a ball blown at the top, is cemented or hermetically sealed in the bottle, so that it may nearly reach the bottom and penetrate below the surface of the coloured fluid. Any increase in the heat to which the bottle is exposed expands the air within it, and this expansion forces the coloured liquid up the tube, which has a scale affixed to it to measure the degree of the expansion. And because air expands by the application of heat above twenty times more than mercury, an air thermometer is, in that proportion, a more delicate instrument than a thermometer made of mercury. The degrees of its expan sion, however, are not so well known as those of mercury, nor have they, like the expansion of this metal, been observed in conjunction with other phenomena. The air thermometer is not, therefore, an instrument of equal utility with the mercurial thermometer, and its principal use is to discover slight variations of temperature. It is an instrument, however, which is easily made, and is advantageously employed in delicate experiments.

Mr. Leslie's differential thermometer is also an air thermometer;

ас

and, as its name 'signifies, is intended to measure small differences of temperature in points or spots when compared to the general temperature of a room, or of any other similar portion of space. Thus there are numerous and important experiments as to heat made by means of mirrors, in the focus of which a considerable degree of heat may exist while no alteration whatever has taken place in the temperature of the room. Now the differential thermometer is to measure the alterations of heat in the focus of the mirror; and we are indebted to it for nearly all the accurate knowledge which has been quired of the radiation of heat, and which has been already put to some very useful purposes. This thermometer consists of a small glass tube, bent so as to form three sides of a parallelogram, ás is seen in the plate, and each end of the tube terminates in a small hollow ball, both being of the same size, and both being full of air. The tube is nearly filled with sulphuric acid tinged red with carmine. To one of the legs of the tube a scale is affixed; and the upper surface of the sulphuric acid in the graduated leg rises to 0, and the ball of this leg is called the focal ball. Suppose the instrument brought into a warm room, the air in both balls will be equally expanded, and the liquor will remain station ary: but suppose the focal ball placed in the focus of a mirror exposed to beat, while the other is not, the air in the former will be more expanded, will press on the liquid, and will force it along the tube into the other ball. This serves to show the expansion of the air in the focal ball, and to measure the heat in the focus of the mirror. This thermometer is, therefore, peculiarly valuable for determining va riations of temperature at a particular point, while the surrounding atmosphere undergoes no altera tion.

1

COFFEE.

THE extensive use of this berry, to form a very refreshing drink

throughout Europe, makes us sup pose a short notice of it will not be unacceptable to our readers. It is one of the luxuries which have been discovered in modern times. The ancients, however renowned they may have been for slinging stones and hurling javelins, certainly knew nothing of the noble science of good living, and were as much inferior to us in the delicacy and number of their viands and drinks as in the management of ships or in the weaving of cloth. Coffee was not used by either the Greeks or Romans, and appears to have been introduced into Arabia by-let him be for ever illus-trious MEGALEDDIN, Mufti of Ada; some persons call him STYK SADLY, and it is said the Arabs never meet to drink coffee without wishing him bliss in Paradise for the benefits he conferred on them.. From Ada the use of coffee was gradually extended to other parts of Arabia, and in a short time coffeehouses were generally established both in that country and in Persia. At that period, however, it was supposed that coffee excited intoxication and improper feelings; and the enlightened governments of Egypt and Turkey, as the governments of Europe have done with regard to spirits and wine, took it into their heads to forbid the use of coffee. Further, Ma-" homet, who seems to have been one of the most cunning impostors the world ever saw, had forbidden, with great certainty that his com- |mands would be obeyed, his devoted Musselmen to use charcoal as a food. His successors in power equalled him in ambition butnot in cleverness, and willing, like the ministers of every religion, to extend their dominion on every possible occasion, pretended that in this prohibition of the prophet the use of charcoal for the roasting of coffee was included, and they pro cured the drinking of coffee to be prohibited. They made it out, also," to the satisfaction of those who at that period, thinking their own conduct of no importance, took the morals of nations under their spe

cial protection, that coffeehouses were the haunts of the vicious and disaffected, and they caused them to be shut up: the system of paying for a permission to be vicious, and of subjecting houses to magisterial inspection not having been then discovered, and being in fact one of the improvements of our own age and country. Though the Muftis and Cadis succeeded very well in keeping up among the people the conviction that Mahomet was the true prophet of God, and that they were his legal vicegerents, and the legitimate successors of his power, they were not so successful in persuading them that coffee was an abominable drink, which they ought to leave to Muftis and Cadis, or pay largely for the privilege of drinking it; and after a few commotions and some throat-cuttings, all of course for the benefit of public tranquillity, the people maintained their right to drink coffee, and it has ever since, in the East, at least, been drunk with impunity. It is certainly curious to observe that the governments of modern Europe rather encourage the drinking of coffee, as a means of checking the consumption of ardent spirits. In our time, coffeehouses are places of gentility and elegance, and the reprobation they met with in Turkey and Arabia is reserved in Britain for publichouses and dram-shops. If any drink more maddening than alcohol should be discovered, which seems not unlikely, dram-shops may be expected to become places of elegant resort, and " HODGES' BEST CORDIAL" will rise to that eminence of reputation which its maker seems to think it deserves. After the Arabs had indulged in the use of coffee somewhat more than a century, it seems to have found its way into Europe. Some of it was imported into Marseilles in France, in 1664; towards 1670 it became known in Paris, and in 1672 the first coffeehouse is said to have been opened in that city, by an Armenian, who afterwards went to London. Before that, however, the use of this berry had been

brought into this metropolis. In 1652, Mr. Edwards, a Turkish merchant, brought a Greek servant with him to England, who understood the art of preparing coffee, and he was the first person who publicly sold it in England. Coffeehouses soon became numerous in the metropolis; and the care of our morals, for which the government of this country,-whether republican or monarchical-whether under the influence of bishops or presbyters,-has been at all times distinguished, soon induced it to endeavour to check the licentiousness of coffee drinking, by laying, in 1660, a tax of 4d. on every gallon, and by compelling, in 1663, all coffeehouse keepers to take out a licence at the quarter-sessions. We believe that this measure had the effect of promoting the use of coffee, for we know that coffeehouses were rapidly increased in number. It is somewhat difficult to explain the cause of their subsequent decrease, coffee having almost gone out of use in England till within a few years, when it has been again revived; and not only the genuine berry, but numerous imitations of it are now consumed in every part of the country. It has been computed that the quantity of coffee now annually consumed in Europe amounts to 120,000,000lbs. avoirdupois; and though this may not be a correct calculation, it will at least serve to show the vast increase which has taken place in the use of coffee since the year 1660.

Up to the beginning of the 18th century all the coffee which was drunk in Europe was brought from the East; now it comes principally from the West. The Mocha coffee, however, still preserves its superiority, and has the finest flavour, and fetches the highest price. According to some accounts, the Dutch first brought coffee-plants from Mocha, and planted them in Batavia in 1690; according to other accounts, this did not take place till 1722. The former is the more likely, because it is said that in 1714 a plant raised at that settle