Typically the heroic couplet, as used by Dryden, Pope, Johnson, and others, is a kind of self-contained entity in which a single idea is encapsulated, often holding within itself an ironic juxtaposition or a tension between two parts of the thought expressed. In Eloisa to Abelard, chiefly because the subject is quite different from Pope's mock-heroic pieces and satires, his use of the couplet does not so much follow this pattern as it does enhance the emotional climate of the poem in a manner that other poetic forms (such as blank verse, for instance) would not have done, in keeping with the neo-classical ideals of Pope's time.
It can be argued that iambic pentameter in English has a special gravity that no other metrical form approaches. Yet rhyme added to it has a different effect. Given that poets as great as Shakespeare, Milton, and Wordsworth wrote in blank verse, it's obvious that, for emotional effect, especially in dramatic monologue, rhyme is not necessary. It is also true that, unlike in other languages, such as Italian and German, rhyme in English has sometimes been found by readers to have a kind of sing-song effect, distracting from the emotional directness and seriousness of the poetry.
Rhymed iambic pentameter couplets, in the hands of a less skilled poet than Pope, could fall into this trap of sometimes trivializing the expression. It is the miracle of Pope's skill that this does not happen. Pope's diction in Eloisa to Abelard has two features that maintain and enhance the seriousness and dignity of the verse. First, as one would expect, the tone is stark and dignified throughout. The manner in which Eloisa describes her prison is a case in point:
Relentless walls! whose darksome round contains
Repentant sighs, and voluntary pains:
Ye rugged rocks! which holy knees have worn;
Ye grots and caverns shagg'd with horrid thorn!
Shrines! where their vigils pale-ey'd virgins keep,
And pitying saints, whose statues learn to weep!
Though cold like you, unmov'd and silent grown,
I have not yet forgot myself to stone.
Her personification of the prison and both the likening of herself to it, and her distancing herself from it, are given in language that is elevated and extraordinarily creative, and yet, natural and almost conversational, as is typical of Pope. The relentlessness of the couplet form has a kind of cumulative effect. It never changes, and unlike a poem written in quatrains, one does not get a sense of closure at the end of stanzas, but rather a continuum. Though each couplet, as in Pope's satiric works, can be said to express a single idea or thought, the stream of couplets gives the impression of an unending, unvaried experience, the tragic drawing out of Eloisa's isolation and suffering. It is almost a hypnotic effect that is created, a trance-like expression of the horrible loneliness to which she is subjected:
The dying gales that pant upon the trees,
The lakes that quiver to the curling breeze;
No more these scenes my meditation aid,
Or lull to rest the visionary maid.
But o'er the twilight groves and dusky caves,
Long-sounding aisles, and intermingled graves,
Black Melancholy sits, and round her throws
A death-like silence, and a dread repose.
The muted, resigned tone is perfectly expressed by the almost mechanical quality (not in any negative sense) of the couplet form. Moreover, in her yearning for Abelard and, simultaneously, the unending guilt she feels, Eloisa is trapped, as if condemned to address him in a controlled, dignified poetic form in which her emotion is fully conveyed, but in which her expression is under a kind of lock and key like that of the prison that encloses her.
The poignancy of the whole story is enhanced by the final lines in which Pope obliquely refers to himself and his passion, at the time, for Lady Mary Monatgu:
And sure, if fate some future bard shall join
In sad similitude of griefs to mine,
Condemn'd whole years in absence to deplore,
And image charms he must behold no more...
Let him our sad, our tender story tell....
It is as if Pope is linked to Eloisa not simply by being separated from his own love, but by the fact that he, as the poet, is under the control of his chosen poetic form which he has imparted to Eloisa, as the true expression of her grief and pain.