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the land slopes abruptly under water, these reefs are only a few yards in breadth, and they form a kind of stony ribbon or fringe round the shore. In places where the land slopes gently under the water, the reef always extends farther seaward, sometimes even as much as a mile from the land. From the circumstance that corals always grow more vigorously on the outside amid the breakers, and that the sediments washed within the reefs have a noxious effect upon the insects, the outer edge of the reef is always the highest part.

In accounting for the architecture of coral reefs, there are three things to be assumed as well-established facts: first, that no coral insects can live at a depth below 20 or 30 fathoms-that is, below 120 or 180 feet; secondly, that their

stony masonry must have a foundation to rest upon; and thirdly, that as soon as the corals build up their reef to such a height as to be left dry at low water, they cease to work. For the sake of understanding the formation of coral rocks, let us look again at the figure which represents the Island of Bolabola. We have supposed that the reef which surrounds it became dry land, through being upheaved by a sudden or gradual volcanic action. But now, let us imagine that that peak and that yonder reef are subsiding and sinking under the waters of the ocean. The island, with its present reef, represented by the unbroken lines in the next diagram subsides slowly.

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Fig. 100.-Growth of Coral on a subsiding Island.

so far from the shores which they form.

The same facts would come out, if, instead of an island, we had supposed the shore of a continent fringed with reefs to

In this illustration (fig. 100) ▲ ▲ represents the outer edges | AA'B'. You can now see why certain encircling reefs stand of the fringing reef at the present sea level. B B are the present shores of the island. As gradually as the island continues to sink, so progressively do the corals work upward; and a'a' represent the outer edges of the reef after its upward growth, during a period of subsidence, has been completed, and eventually converted into a barrier with islets on it. B' B' are the new shores of the now encircled island; and c c represents the lagoon between the fringing reef and the island, after a subsidence of several hundred feet, is given by the dotted lines

have subsided.

You must again imagine that the island of Bolabola has continued to subside for thousands of years, until there was formed around it a new barrier reef, represented by the broken lines in fig. 101.

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▲ ▲ represent the outer edges of the barrier reef at the level of the sea, with islets on it. B'B' the shores of the included island of Bolabola. cc, the lagoon channel between the reef and the land. On our supposition, as the barrier reef continues to sink down slowly, the coral insects go on working vigorously upwards. As the island sinks, the water gains inch by inch on the shore, and the two peaks, x y, form separate islands within one great encircling reef; and, finally, x the highest disappears. As soon as this takes place, a perfect atoll is formed, and ▲′′ A′′ represent the outer edges of the reef, now converted into an atoll, and c' is the lagoon in which a ship rides at anchor.

"We can now," says Mr. Darwin, "perceive how it comes that atolls, having sprung from encircling barrier reefs, resemble them in general size, form, in the manner in which they are grouped together, and in their arrangements in single or double lines; for they may be called rude outline charts of the sunken islands over which they stand. We can, farther see how it arises that atolls in the Pacific and Indian Oceans extend on lines parallel to the generally prevailing strike of the high lands and great coast lines of those oceans. I venture, therefore, to affirm that, on the theory of the upward growth of the corals during the sinking of the land, all the leading features in those wonderful structures, the lagoon island or

atolls, as well as in tne no less wonderful barrier reefs, whether encircling small islands or stretching for hundreds of miles along the shores of a continent, are simply explained.”. Darwin's Naturalist's Journal, p. 474.

The preceding figure represented to you the gradual formation of lagoon islands or atolls by subsidence. Fig. 102 will give you the appearance of an atoll when so formed. This engraving, after all, gives but a faint idea of the singular aspect of an atoll. Whitsunday Island is one of the smallest size, and has its narrow islets united together in a ring. "The immensity of the ocean, the fury of the breakers, contrasted with the lowness of the land and the smoothness of the bright green water within the lagoon, can hardly be imagined without having been seen.'

The rocks produced by coral insects are of immense extent. Coral reefs are scattered in the oceans, as if in certain lines of enormous length. On the eastern coast of Australia a coral reef stretches that is 350 miles long. In the Pacific there are two groups of islands, the one called the Disappointment Islands, and the other the Duff group. These two groups are 500 miles apart, but they are connected by coral reefs over which the natives can travel from one island to another. Also between New Guinea and Australia there is a line of coral reefs 700 miles long, in which there are no gaps wider than

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that in very remote periods in the earth's history, and in much higher latitudes than at present, these coral insects were among some of the most efficient architects employed by the Creator in the structure of the earth's crust, and that both the architects and the architecture in the ancient hills were the same as in the present day.

All the tertiary formation, especially the coralline crag, supplies numerous specimens of cargophylliæ, spongiæ, &c., while the eocene deposits contain astrea, meandrina, and mugenera, inhabitants of tropical seas.

In the chalk formation corals are abundant in certain localities, as in the sandy strata of Maestricht; but in the white chalk of England there is no appearance of coral reefs, though corals of a small and delicate species are found in it. It is evident that the white chalk was deposited in a profound ocean. As, therefore, the corals can only live at a moderate depth, coral reefs could not have been produced in the chalk sea, except in shallows or near the sea-shore. In some

LESSONS IN GREEK.-No. XII.
By JOHN R. BEARD, D.D.

A DEVIATION from the usual form of the Second Declension may here claim the student's attention.

THE SECOND DECLENSION CONTRACTED.

A small number of substantives in which an o or an e stands before the case-endings undergo contraction. By contraction is meant the blending of two vowels into a diphthong, or some other equivalent. The student must learn both the uncontracted and the contracted forms, frst horizontally, as πλοος, πλοῦς; πλου πλοῦ, &c.; and then perpendicularly, as πλοος, πλοου, πλοῳ, uncontracted ; and πλοῦς, πλοῦ, πλῷ, con tracted. Thus are declined ὁ πλοος, a sailing or voyage; περι πλοος, a sailing round or circumnavigation ; and το οστεον, a bone. EXAMPLES OF CONTRACTED NOUNS; SECOND DECLENSION. Uncont Contr Uncont. | Contrae. Uncon. | Contre. | πλοος πλοῦς περίπλους περίπλους οστεον οστοῦν πλοου πλοῦ περίπλοου περιπλου οστεου οστοῦ πλοῳ πλῳ περιπλου περιπλῳ οστεῳ οστῷ πλοον πλοῦν περίπλοον περιπλουν οστεον οστοῦν πλοε πλοῦ περίπλοε περιπλου οστεον οστοῦν πλοοι πλοῖ περίπλοοι περίπλοι αστεα οστά πλοων πλῶν περιπλοων περιπλων οστεων οστῶν πλοοις πλοῖς περίπλοοις περίπλοις οστεοις οστοῖς πλοους πλους περίπλοους περίπλους οστε πλοοιπλοῖ περίπλοοι περίπλοι οστεα D.N. & V. πλοω πλω περιπλου G. & D. πλοοιν [πλοῖν περιπλοοιν After this manner decline the

S.N.

G.

D.

Α.

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P.N.

G.

D.

Α.

V.

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an Atoll in the Pacific Ocean.

called the mountain limestone, which abounds in various forms of corals. The silurian system also teems with peculiar kinds of corals. From these facts we learn, as Dr. Mantell says, σε that an atom of living jelly floating in the ocean, at length becoming fixed to a rock, may be the first link in a chain of events, which, after the lapse of ages, may produce important modifications in the physical geography of our globe. When we bring the knowledge thus acquired to bear on the natural records of our planet, and examine the rocks and mountains around us, we find that, in periods so remote as to exceed our powers of calculation, similar effects were produced by beings of the same type of organization as those whose labours" are carried on at this day. "We are thus enabled to read the history of the past, and to trace the succession of events, each of such duration as to defy all attempts to determine, with any approach to probability, the period required for its development."-Wonders of Geology, p. 657.

t

ending in ooς (οὓς), οη (7), οον (ον), as ἁπλοῦ, ἁπλῆ, ἁπλοῦν, single or simple; also adjectives of two terminations in oos (ους) and oov (ον) formed from the substantive voor (νοῦς), the mind, as ὁ, ἡ εύνους, το ευνουν, well-minded, that is, well disposed ; and from the substantive πλοος (πλοῦς) ὁ, ἡ ευπλους, το ευπλουν, voyaging successfully. These differ from their substantives only in this, that in the neuter plural they suffer no contracDecline in the same tion, ending in -vou and -λοι. manner adjectives ending in oog, and denoting that of which a thing is made, as χρυσεος (χρυσοῦς), χρυσεα (χρυσῆ), χρυσεον (χρυσοῦν), golden. In the neuter plural ea is contracted into ᾶ. When the feminine termination sa is preceded by a vowel or p, the ca is contracted, not into, but into ã, as

ερε-εος (ερεούς), ερε-εα (ερεᾶ), ερεον (ερεοῦν), wooller. αργυρεος (αργυρούς), αργυρ-εά (αργυρᾶ), αργυρεον (αργορ ouv), of silver.

EXAMPLES OF CONTRACTED ADJECTIVES; SECOND DECLENSION. Μ. Ν. M. F IN. ἁπλοῦς ἁπλῆ ἁπλοῦν ἁπλοῦ ἁπλῆς ἁπλοῦ ἁπλῷ ἁπλῷ ἁπλῳ ἁπλοῦν ἁπλῆν ἁπλοῦν ἁπλοῦς ἁπλῆ ἁπλοῦν ἁπλοῖ ἁπλαῖ ἁπλᾶ ἁπλῶν ἁπλῶν ἁπλῶν ἁπλοῖς ἁπλαῖς ἁπλοῖς ἁπλοῦς ἁπλᾶς ἁπλᾶ ἁπλοῖ ἁπλαῖ ἁπλᾶ ἁπλῶ ἁπλᾶ ἁπλῶ ἁπλοῖν ἁπλαῖν ἁαλοῖν

Γ.

S.N.

G. D.

A.

V.

P.N.

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χρυσούς χρυσή χρυσοῦν χρυσου χρυσῆς χρυσοῦ χρυσῷ χρυσῇ χρυσῷ χρυσοῦν χρυσῆν χρυσοῦν χρυσοῦς χρυσῆ χρυσοῦν χρυσοῖ χρυσαῖ χρυσᾶ χρυσῶν χρυσῶν χρυσῶν χρυσοῖς χρυσαῖς χρυσοῖς χρυσοῦς χρυσᾶς χρυσᾶ χρυσοῖ χρυσαῖ χρυσᾶ D.N. & V. χρυσῶ χρυσᾶ χρυσῶ χρυσοῖν χρυσαῖν, χρυσοῖν

περίπλω οστεω οστω περίπλοιν οστεοιν οστοῖν multiplicative adjective,

G.D.

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VJS υντος

comes from a nominative in wr

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comes from a nominative in wv, ous
comes from a nominative in w, ws, nûs
comes from a nominative in wp, og
comes from a nominative in ms, os, ws
comes from a nominative in e
comes from a nominative in TNP

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The understanding is a teacher to men. The well-disposed friend is honoured (epaπEUW). Well-disposed friends are honoured. To the well-disposed ar many friends (that is, the well-disposed have many friends). Abstain from the senseless. Strive after benevolent friends. Bring the bread in a basket. Avoid senseless youths. Senseless youths are avoided. The goblet is golden. Silver goblets are beautiful. Pass life (Blov ayew) with understanding. Contend ye not with the senseless.

Remark that as a general rule the subject (or what is commonly called the nominative) has the article, the predicate being without it. Thus, if, as in the last Greek sentence, you meet with a sentence having two nouns connected by the verb eval, take first, that is, take as the subject, that which has the article before it, as

Subject.

ὁ θάνατος
Death

λέγεται
is called

χαλκοῖς
a brazen

Predicate.
ύπνος
sleep.

THE THREE DECLENSIONS (review).

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I wish you, with the aid of this table, to review the groundk over which we have gone. With it you should possess the utmost familiarity before you pass on to the next topic. In order to assist you, and at once to ground you in what you have learnt, and to enlarge your acquirements, I subjoin With the nouns of the first and second declension, the stu-exercises bearing on the three declensions. These exercises dent, if he has thoroughly mastered the foregoing lessons, are taken from the best Greek authors, and from the Sacred will find no difficulty in any attempt he may make to construe Scriptures. When you have mastered them, you will feel classical Greek. It is somewhat different with nouns of the that already you have made some progress. third declension, the discovery of the nominative of which is I premise a few syntactical remarks. In Greek, as in necessary in order to consult a Greek lexicon with ease and Latin, adjectives, adjective pronouns, and participles, agree effect. I therefore subjoin the following, which will enable with their nouns in gender, number, and case. That is, if the him from the genitive case to find the nominative; in which noun is in the accusative singular, in the accusative singular form substantives and adjectives appear in dictionaries. I must the adjective, &c., be. If the noun is in the genitive give the genitive, because the genitive is, as it were, the key plural, the adjective must be in the genitive plural. If the to the remaining oblique cases. Thus, if you meet with avopa, noun is of the neuter gender, put the adjective in the neuter you know the genitive must have two of these letters, namely, gender; and so in all other cases, the adjective, the adjective op; if you meet with Xeuwves, you know the genitive will pronoun, and the participle, when they agree in sense, must have the letters xeuwv; if you meet with μehaves, you know agree also in form, both being in the same gender, number, the genitive will have the letters μehav. Now, from the and case. μελαν. Thus, we say ayabos avno, a good man; but if we genitive you may get to the nominative, and you may do so use yuvŋ instead of avnp, we must change ayados into ɑyabn. by the aid of what has already been said. But for this you Also we write ανδρα αγαθον θαυμαζω, I admire a good man must bear in mind that the in μeλav, though belonging but yuvaika ayaðŋv Davμa(w, I admire a good woman-where

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αγαθος becomes αγαθον to agree with ανδρα, and αγαθην το agree with γυναικα. Compare the declensions of adjectives and nouns combined in the fourth and sixth lesson.

As a general rule, a transitive verb, or a verb which has an object after it, has that object in the accusative case, as in the sentence just given—ανδρα αγαθον θαυμαζω. Many verbs, however, put their object in some other case ; some require the genitive, and some the dative. Examples have already appeared. When two nouns come together in a state of dependence, the dependent noun is put in the genitive case: e. g., Ο Αλεξανδρος του Φιλίππου ην υἱος, Alexander was the son of Philip ; where Φιλιππου is in the genitive case because it is in sense dependent on vioç.

When two verbs come together in a state of dependence, the dependent verb is put in the infinitive mood: e. g. βουλομαι ύδωρ πινειν, I wish to drink water; where πινειν is governed in the infinitive mood by βουλομαι, the former being in sense dependent on the latter.

RECAPITULATORY EXERCISES FROM THE GREEK CLASSICS.

3. Πελοπι from Πελοψ, Πέλοπος, a proper name, governed in the dative case by ησαν; to Pelops there were, that is, Pelops had ; Ατρευς (5. εως), Atreus; Θυέστης (g. ου), Thyestes. Observe that the English y represents the Greek v.

4. παρ' for παρα, against, παρ' ἐλπιδα, contrary to their expec tations, ελπιδα, acc. sing., from ή ελπις (g. έλπιδος), hope; why has the plural adjective πολλα the verb in the singular ?

5. τροπος, ου, ο, a turning, disposition; χρυσία, neut., pl., from χρυσιον, & diminutive of χρυσος, gold, and so denoting golden ornaments, jewels.

pleasing in sound, nom. pl., from εύφωνος (ευ and φώνη, α 6. τεττιγες, grasshoppers, from ὁ τεττιξ (Β. τεττιγος) ; ευφωνοι, the third person plural, passive voice, present tense, from λεγω, voice), an adjective of two terminations ; λεγονται, are said, I say; it governs ειναι, to be, in the infinitive mood.

7. μυρμήκων, gen. pl. governed by βιος, from ὁ μυρμηξ, μυρμηκος, an ant; μελισσων, gen. pl. governed by βιος, trom μελισσα, ης, ή, a bee; πολυπονος, ον (from πολυς and πονος), laborious.

8. γιγνωσκει (from γιγνωσκω, I know), indicative mood, active voice, third person singular agreeing with its subject, or nominative φωρ; φωρ, φωρος, ό, a thief, λυκος, ου, o, a wolf. 9. χρησις, εως, ή, use ; οργανον, ου, τo, a means, our organ. 10. ανευ, without ; τυφλον, from τυφλος, η, ov, blind; the adjective is in the neuter gender, denoting disparagement, a blind thing; διχα, separate from, ελλιπες, from ελλιπης, ες, defective (from λειπω, I leave).

11. προστιθει, adds, from προστιθημι, I add, επιστημη, ης, ή, understanding.

12. βουκερω, Παving the horns of an or, from βουκερως, ω, and that from Bous and κέρα; Τους, Io, from Ιω, οὓς, πλαναι, uanβους derings, from πλανη, ης, ἡ.

14. αληθως, truly; ως αληθώς, very truly.

1. Μια χελιδων εαρ ου ποιει. 2. Παντα ο χρονος προς φως αγεί. 3. Πελοπι υἱοι ησαν Ατρευς και θυεστης. 4. Πολλα ανθρωποις παρ' ελπίδα γιγνεται. 5. Γυναιξι κοσμος ὁ τροπος (sc. εστιν) ου τα χρυσία. 6. Οἱ τεττιγες ευφωνοι λεγονται ειναι. 7. Μυρμηκων και μελισσων βιος πολυπονος εστι. 8. Γιγνώσκει φωρ τον φωρα και λυκος λυκον. 9. Ου κτησις αλλ' ή χρησις των βιβλιων οργανον της παιδειας εστιν. 10. Η μεν φυσις ανευ μαθησεως τυφλον, ή δε μαθησις διχα φύσεως ελλιπες. 11. Ο χρόνος τω γηρᾳ προστιθει την επιστημην. 12. Πολλαι ησαν αἱ της βουκερω Ιοῦς πλαναι. 13. Ανηρ ανδρα και πολις πολιν σώζει. 14. Επαμινώνδας ως αληθώς εν ανδρασιν ανηρ ην. 15. Γερων γεροντι γλωσσαν ἡδιστην έχει, παις παιδι, και γυναικι προσφορον γυνη. 16. Παντες οἱ των αρίστων Περσων 15. ήδίστην, sweetest, the superlative degree of ἡους, sweet; παιδες επι ταις βασιλεως ταις βασιλεως θυραις παιδευονται. 17. Ξίφος προσφορον, pleasant, from προσφορος, ον, conducive to (προς and τιτρώσκει σωμα, τον δε νουν λογος. 18. Ἡ φρόνησις μεγιστον φέρω). 16. αριστων, the best, that is, noble, from αριστος, a superla εστιν αγαθον. 19. Πολεως ψυχη οἱ νομοι. 20. Ἡ τυραννις tire of ἀγαθος. | αδικιας μητηρ εστιν. 21. Ὁ δειλος της πατρίδος προδοτης 17. Ξίφος, ους, το, a sword; τιτρώσκει, wounds, from τιτρωσκω, εστιν. 22. Οἱ αγαθοι ανδρες θεων εικονες εισιν. 23. OiIwound. Νομάδες των Λιβυων ου ταις ημέραις, αλλα ταις νυξιν αριθμουσιν. 24. Χαλεπον εστι λεγειν προς γαστερα, στα εχουσαν. 25. Ηφαιστος τω ποδε χωλος ην. 26. Ἡ Μηδεια γραφεται τω παιδε δεινον ὑποβλέπουσα. 27. Εθους βασανος εστιν ανθρωποις χρονος. 28. Οἱ ὄφεις τον ιον εν τοις οδουσιν εχουσιν. 29. Ο Παρνασσος μεγα και συσκιον όρος εστιν. 23. Νομάδες, the nomads, or wandering tribes, from νομας, Εν βοιωτια δυο εστιν επισημα όρη, το μεν Ελικων καλουμενον, αδος, and that from νεμω already explained ; αριθμούσιν, they ετερον δε Κιθαιρων. 31. Ὁ Νειλος εχει παντοια γενη ιχθυων. number, from αριθμεω, I number, our arithmetic. 32. Τιμα τους γονεῖς. 33. Ανάχαρσις την αμπελον είπε τρεις φερειν βοτρυς" τον πρωτον, ηδονης τον δεύτερον, μεθης τον τριτον, αηδιας. 34. Πονος ευκλειας πατηρ (sc. εστιν). 35. Ωκεανου και Τηθυος παις ην Ιναχος. 36. Οἱ τεττιγες σιτούνται την δροσον. 27. Κλεανθης εφη τους απαιδευτους μονῇ τῇ μορφή των θηριων διαφέρειν. 35. Αναχαρσις ονειδιζομενος οτι Σκύθης ην, είπε, τῳ γενει αλλ' ου τῷ τρόπῳ. 39. Κολάζονται εν άδου παντες οἱ κακοι, βασιλεις, δουλοι, σατράπαι, πενητες, πλουσιοι, πτωχοι. 40. 40. Αἱ Φορκου θυγατέρες γραίαι ήσαν εκ γενετης. 41. Ζηνων εφη, δειν τας πολεις κοσμειν ουκ αναθη μασιν, αλλα ταις των οικούντων αρεταις.

ουκ

30.

In giving the vocabulary of these recapitulatory exercises, I shal take each sentence in the order in which it stands, because the learner will here need more aid than he has hitherto received.

VOCABULARY TO THE EXERCISES FROM THE CLASSICS.

1. Μια, one, from the numeral εἷς, μια, έν, one, χελιδων, nom. sing., fem., agreeing with μια ; χελιδων, χελιδονος, α swallow.

2. See note.*

This sentence contains nothing that the student ought not to know. I therefore leave him to the knowledge he has, or may have, already attained, and so in future shall I do without giving notice thereof.

18. μεγιστον, the greatest, superlative from μεγας, great. 20. τυραννις, ίδος, ή, usurped power, tyranny; αδικιας, of injustice (α privative, and δικη, right, justice). 21. δειλος, η, ov, cowardly, ὁ δειλος, the coward; προδοτης, ου, ό, a betrayer, traitcr.

22. εικονες, images; εικων, ονος, o, an image.

21. εχουσαν, καving, present participle from εχω, I have; it agrees with γαστέρα.

25. Ηφαιστος, Vulcan; χωλος, η, ον, lame.

26. Μήδεια, ας, ή, Medea ; ὑποβλέπουσα, scowling at, from ύπο, under, and βλεπω, 1 look.

27. ήθους, of character, from το ηθος ; βασανος, ου, ή, a touchis stone, test.

a

28. οφις, οφεως, δ, a serpent; ιος, ου, ο dart, sting.

29. Παρνασσος, Parnassus, a mountain of Phocis, on which was Delphi; συσκιος, ον, ονεrhung with clouds. from συν, with, and σκια, a shade.

от

30. επίσημος, ov, distinguished, remarkable, from επι, (here an intensive), and σημα, a sign, whence our semaphore, that is, a telegraph; Ελίκων, Helicon; Κιθαιρών, Cithaeron; καλουμενον, called, named, participle agreeing with το, that is, ορος; έτερος, α, ον, other, the other.

33. Ανάχαρσις, Anacharsis; ειπε, said; ἡδονης depends on βότρυς ; μεθη, ης, ἡ, intoxication; αηδια (from a, not, and ἡδυς, sweet), disgust.

34. εύκλεια, ας, ή, glory, distinction.

35. Ωκεάνος, ου, o, Oceanus, Ocean considered as a divinity ; Τηθύς, ος, η, Tethys, a sea-goddess.

36. σιτεομαι, I feed on; δροσος, ου, ή, dew.

37. Κλεάνθης, Cleanthes, εφη, said, απαίδευτος, ον, untaught, uneducated; μορφη, ης, ή, form; διαφέρω, I difer.

38. ονειδίζω, Ι reproach, Anacharsis being reproached ; Σκύθης, a Scythian.

WorM

39. Koλalw, I punish; ev gidov, douw is understood, in the abode of Hades, in hell; σarρaπns, ov, ò, a satrap or governor of a province; Tεvns, Nτos, poor; πTwxos, n, ov, begging; oi πтwxоi, beggars. γραια,

πενης, ητος, πτωχος, η,

40. ypaιa, i, old, an old woman, grey-haired.

41. dev, that it was necessary, proper; avalημα, 70s, 70, an offering, public monument, from ava, up, and rienu, I place; Twv oikovvтwy of their inhabitants, from ouкew, I inhabit, compare oikos and oikia.

των οικούντων οικεω,

(To be continusd).

ON PHYSICS OR NATURAL PHILOSOPHY. No. VII.

MOLECULAR FORCES.

Nature of Molecular Forces-The phenomena which bodies constantly exhibit lead to the conclusion that their particles are always under the action of two opposite forces, one of which tends to make them attract, and the other to repel, one another. The first, which is called molecular attraction, varies in the same body only with the distance of the particles; the second, which is produced by heat, varies with the intensity of the agent and with the distance of the particles. From the mutual relation of these forces, and from the disposition and arrangement which they give to the particles, arise the different states of bodies, namely, solid, liquid, and gaseous. Molecular attraction only acts at distances incalculably small. Its effect is nothing at any sensible distance, a property which distinguishes it from gravity and universal gravitation, which act at all distances. We are ignorant of the precise laws according to which molecular attraction operates. According to the manner in which it is viewed, it receives the different names of cohesion, affinity, and adhesion.

Cohesion is the force which unites similar particles of matter to each other, that is, matter of the same kind, as for instance two particles of water, or two particles of iron. This force is almost nothing in gases, sensible in liquids, and very great in solids. Its intensity is diminished when the temperature of a body is raised, while the repulsive force arising from heat is increased. Hence, when solid bodies are heated, they ultimately become liquid, and even pass from this state into the aeriform or gaseous state.

Cohesion varies not only with the nature of the bodies, but also with the arrangement of their particles. To the modifications which cohesion undergoes in different circumstances are to be attributed the different qualities of tenacity, ductility,

and hardness.

In liquids, taken in large quantity, gravity overcomes cohesion. Hence liquids, constantly yielding to the action of gravity, and assuming no particular form of their own, take always that of the vessels in which they are contained. In small drops of liquids, however, cohesion overcomes gravity, and they assume the spherical or spheroidal form. This may be seen in the drops of dew suspended on the leaves of plants; and the same phenomenon is observed when a liquid is poured on a plane horizontal surface and does not wet it, as mercury upon wood. The same experiment can be made with water, if the surface be previously rubbed or sprinkled with a light powder, such as lamp-black, &c.

Affinity is the attraction which takes place between heterogeneous substances; in water, for instance, which is composed of two atoms of hydrogen to one of oxygen, it is affinity which unites these two bodies; but it is cohesion which unites two particles of water. Hence, it is evident that in compound bodies cohesion and affinity act together, while in simple bodies it is only cohesion that unites the particles. Affinity is the form of attraction to which we refer all the combinations and decompositions of chemistry.

Every cause which tends to weaken cohesion increases affinity. The latter is, in fact, increased by the state of division in a body; it is also increased by the liquid or the gaseous state of a body. This force is particularly developed by a body when it is disengaged from combination with another body and isolated or left free to yield itself to the action of other bodies for which it may have an affinity. This

force also exhibits very variable effects, according to the elevation of the temperature of bodies. In certain cases, by separating the particles and diminishing cohesion, heat produces combinations. For example, between sulphur and oxygen the affinity is without effect at the ordinary temperature, while at a high temperature these bodies combine and produce a fixed compound called sulphurous acid. In other cases, on the contrary, heat destroys combinations, by communicating to their elements unequal expansibility. Hence many metallic oxides are decomposed by the action of heat.

Adhesion is the molecular attraction exhibited in bodies which stick together by contact. Two plates of glass, for example, when placed in contact with a weight upon them, adhere so strongly that they cannot be separated without breaking, after the weight is removed. The force of adhesion acts between solids and liquids, and between solids and gases. Adhesion between solids is not merely the effect of atmospheric pressure, for its action is exhibited in a vacuum. This force increases in proportion to the degree of the smoothness of the surfaces in contact, and to the length of the duration of contact; for the resistance to their separation is greater in proportion to the time that their contact has continued. Moreover, adhesion between solid bodies is independent of their thickness a fact which indicates that the molecular attraction acts at indefinitely small distances.

When solid bodies are immersed in water, alcohol, and most other liquids, they are found covered with a coat of the liquid when taken out of it; and this is simply the effect of adhesion. Adhesion is produced between solids and gases, similar to that between solids and liquids. Thus, if we immerse a plate of glass or of metal in water, we perceive air-bubbles floating. on the surface. Now, in this case the water does not penetrate the pores of the plate, but the air-bubbles arise only from the expulsion of the air which surrounded the plate like the coating of a liquid. A series of phenomena proceeding. from molecular attraction, under the names capillary attraction, endosmose, absorption, and imbibition, shall be brought under our notice in the sequel.

PARTICULAR PROPERTIES OF SOLIDS.

Having explained to the student the principal properties of matter common to solids, liquids, and gases, we shall in this lesson treat of some particular properties of solids; such as the elasticity of traction, the elasticity of torsion, the elasticity of flexure, tenacity, ductility, and hardness.

the nature of elasticity in general, and referred chiefly to that Elasticity of Traction. In our second lesson we explained developed by pressure. In solids, however, elasticity is developed also by traction or extension, by twisting or torsion, and by flexure or bending.

In ascertaining the laws of the elasticity of traction, M. Savart employed an apparatus represented in fig. 18. This

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