This, then, is another variety of the same carbonate of

lime.

It also occurs in the form of marble, of various qualities; and the chalk of the English Downs is entirely composed of it. In the deep sea, the coral builds with it his tiny palace; and the little shellfish forms it into pearls, worthy to adorn the diadem of kings.

These are some of the various forms assumed by this wonderful substance, but the list is by no means exhausted. Of its uses it is almost needless to speak. What should our builders do without marble and limestone? The former is employed in the construction of our finest edifices, and the latter, besides its use as a stone for plainer buildings, furnishes the cement by which all mason-work is kept together and consolidated. Lime for agricultural purposes is derived from the same source. Chalk is used in gilding

and several other arts; coral and pearls are esteemed as ornaments; and, finally, marble is the favourite material of the sculptor, in which have been enshrined many of the highest triumphs of artistic genius.

Lime unites with sulphuric acid to form another important salt, called sulphate of lime, or gypsum. Of this substance is composed the well-known plaster of Paris, so named from its being extracted in large quantities at Montmartre, near that capital. It is extensively employed for the purpose of making casts of statues and other objects, thus enabling us, at a trifling expense, to multiply faithful copies of the admirable works of ancient sculpture. It is also used for moulds in the porcelain and earthenware manufactures. When crystallized, it is perfectly transparent, and capable of being split into very thin plates, which sometimes served, in ancient times, the purposes of glass. In this state, it is called selenite. A finer quality of the same substance, remarkable for its purity and whiteness, has received the name of alabaster. It is chiefly found in Tuscany, where it is manufactured into graceful vases, and other articles of taste. Thus we have two kinds of alabaster, differing in appearance as well as in composition; the

calcareous variety formerly mentioned being rarely white, but generally veined and clouded with other colours. The value attached to these substances in ancient times is well illustrated by the interesting occurrence recorded in Matth. xxvi. 6-13.

ALUMINA.

SOME of the rarest and most beautiful productions of the mineral kingdom, as well as others less dazzling, but more useful, are wholly or partly composed of alumina. This substance is the oxide of a bluish white metal called aluminum, which can be wrought with the same ease as silver, and is extensively used by the jeweller and silversmith in the production of ornaments for dress. The name is derived from alum, a well-known salt, of which alumina forms an essential ingredient; but it is also found in other compounds of still greater importance. In a former lesson it was stated that a very large proportion of all our rocks and stones are compounds of silica; it has now to be added, that alumina is one of the substances with which that silica is most frequently combined.

On examining a piece of granite, it will be found that it is composed, as its name indeed implies, of grains or coarse particles of at least three different kinds of stony substance. One of these, the hardest of all, is quartz, which consists of silica alone. The others, mica and felspar, contain large quantities of alumina. The glittering particles in the granite are mica, and the great mass of the stone is usually felspar.

Felspar is, in fact, one of the most abundant substances in nature. Gneiss, porphyry, and some other rocks, are, like granite, chiefly composed of it. It is found of almost every colour, and is often beautifully crystallized. Some of its numerous varieties are even valued as ornamental stones. But, strange to say, it becomes most interesting when it is decomposed, and passes into the form of kaolin, or porcelain clay, a pure silicate of alumina. This change, the causes of

appearance? What are stalagmites? stalactites? For what purposes do we use limestone? chalk? marble? pearls ? plaster of Paris? What is selenite? Explain the composition of granite. What is felspar composed of? mica? quartz? From what is porcelain clay formed, and what is its composition? How do other clays differ from it? Name some of them. For what purposes are they useful? What is slate? shale? Which gems are composed almost entirely of alumina? Which of silica and alumina ?

ON THE THREATENED INVASION (1803).

[ROBERT HALL, a celebrated Baptist preacher and a distinguished theological writer, was born at Arnsby in Leicestershire, in 1764, and died at Bristol in 1831. Mr. Hall was gifted with a powerful and persuasive eloquence; but it was his "Sermon on Modern Infidelity," that established his fame as a divine. His works have been collected and published since his death, in six vols. 8vo.]

By a series of criminal enterprises, the liberties of Eúropel have been gradually extinguished; and wèl are the only people in the eastern hémisphere' who are in possession of equal láws and a free constitution. But freedom, driven from every spot on the continent, is pursued even hère, and threatened with destruction. The inundation of lawless power, after covering the whole earth, threatens to follow us hère; and we are most exactly, most critically, placed in the only aperture' where it can be successfully repélled— in the Thermopylae of the world. As far as the interests of freedom are concerned-the most important by far of sub'lunary interests-you, my countrymen, stand in the capacity of the federal representatives of the human race; for with you it is to determine (under God) in what condítion' the latest posterity shall be bòrn. If liberty, after being extinguished on the Continent, is suffered to expire hére, whènce is it ever to emérgel in the midst of that thick níght that will invèst it? It remains with yòu, then, to decíde, whether that freedom, at whōse voice the kingdoms of Europe awoke from the sleep of ages, to run a career of virtuous emulation' in everything great and good -the freedom' which dispelled the mists of superstition, and invited the nations to behold their God, whose magic tórch kindled the rays of gènius, the enthusiasm of poetry, and the flame of éloquence-the freedom which poured into our lap opulence and arts, and embellished lifel with

innumerable institutions and improvements, till it became a theatre of wonders-it is for yoù to decide, whether this freedom shall yet survive, or be covered with a funeralpáll, and wrapped in eternal glòom. It is not necessary' to await your determination. In the solicitude you feel to approve yourselves worthy of such a trúst, every thought of what is afflicting in wàrfare, every apprehension of dánger must vànish, and you are impàtient to mingle in the battle of the civilized world. Gò then, ye defenders of your country, accompanied with every auspicious ómen; advánce with alàcrity into the field, where God himself musters the host to war. Religion is toò much interested in your succéss not to lend you her àid; she will shed over this enterprise' her selèctest influence. While yoù are engaged in the field, mány will repair to the clòset, mány! to the sanctuary; the faithful of every náme will employ that prayer' which has power with Gòd; the feèble hands, which are unequal to any óther weapon, will grasp the sword of the Spirit; and from myriads of humble contrite hearts' the voice of intercèssion, supplicàtion, and weeping, will mingle in its ascent to heaven, with the shouts of battlel and the shock of arms. And it is next to impossible for víctory not to crown your exertions; for the extent of your resources, under God, is equal to the jústice of your cause. But should Providence' determine otherwise should you fáll in the struggle, should the nation fall-you will have the satisfaction (the purest allotted to man!) of having performed your part; your names will be enrolled with the most illustrious dead; while postèrity to the end of tíme, as often as they revòlve the events of this period (and they will incessantly revolve them) will turn to you a reverential eye, while they mourn over the freedom! which is entombed in your sèpulchre. I cannot but imagine that the virtuous heroes, legislators, and pátriots of every age and country, are bending from their elevated seats to witness this contest, as if they were incapable, till it be brought to a favourable issue, of enjoying their eternal repòse. Enjoy that repose' illustrious immortals! Your mantle fell' when you

ascended; and thousands inflámed with your spirit, and impatient to tread in your steps, are ready to swear by Hím! that sitteth upon the throne, and liveth for ever and ever, they will protect freedom in her last asylum, and never desert that cause, which you sustained by your lábours, and cemented with your blood.

ROBERT HALL.

THE ARCTIC EXPEDITION.

[FOR more than three centuries it had been the ambition of brave and skilful navigators to immortalize their names by the discovery of a North-west passage through the Arctic Seas to the golden realms of the East. Previous to 1845, more than one hundred and twenty expeditions had successively sailed to the Northern seas in order to discover that mysterious passage, but had all failed in their object.,

In the spring of 1845, the Erebus and Terror with a gallant crew under the command of Sir John Franklin sailed to the far north. Sir John Franklin resolved once more to renew his efforts to solve the great problem of a Northwest passage. For three years nothing was heard of this gallant expedition and its leader. The Government sent out expedition after expedition in search of the gallant hero and his crews without any very satisfactory result. Then Lady Franklin, finding the Government unwilling to renew the search, with a devotion that no disappointment could damp, fitted out the Fox and despatched the expedition which, under the command of Captain M'Clintock, proved so successful. Franklin had discovered the North-west passage in 1847, and died on the 11th of June that same year, on board his ship.]

By the pale beams of the aurora darting across the dark sky, a little vessel is seen wedged in the ice. Her decks being roofed over and covered with snow, she is scarcely to be recognized in the icy waste around. Arrested by the conglomerated floes, her brave crew have bidden farewell to the sun, but not farewell to hope, and not farewell to the chivalrous enterprise on which they have gone forth. Here they wait till Spring, with smiling face, dancing over the dreary region, shall break up their prison by her magic tread, and bid them go free; then, undeterred by past dangers, from that vantage-ground they will pursue their search. But no! while apparently stationary, every surrounding object retaining its relative position, the whole icy plain is drifting southwards. Old Winter, indignant at their intrusion so near his ancient throne, hurls them back