In Act III, Scene 5 of "Romeo and Juliet", the final words the lovers speak together are filled with apprehesion. In what way?

Juliet and Romeo share a night together after Romeo has been banished for killing Tybalt, Juliet's cousin, and must go far away from his home and from Juliet. Morning inevitable arrives, and Romeo must soon leave. Nurse warns the two lovers that Juliet's mother is coming soon. Romeo and Juliet must now part.

JULIET

    Then, window, let day in, and let life out.

ROMEO

    Farewell, farewell! one kiss, and I'll descend. 

JULIET

    Art thou gone so? love, lord, ay, husband, friend!
    I must hear from thee every day in the hour,
    For in a minute there are many days:
    O, by this count I shall be much in years
    Ere I again behold my Romeo!

ROMEO

    Farewell!
    I will omit no opportunity
    That may convey my greetings, love, to thee.

JULIET

    O think'st thou we shall ever meet again?

ROMEO

    I doubt it not; and all these woes shall serve
    For sweet discourses in our time to come.

JULIET

    O God, I have an ill-divining soul!
    Methinks I see thee, now thou art below,
    As one dead in the bottom of a tomb:
    Either my eyesight fails, or thou look'st pale.

ROMEO

    And trust me, love, in my eye so do you:
    Dry sorrow drinks our blood. Adieu, adieu! 

The two lovers' parting is full of sorrow. Romeo and Juliet declare their undying love for one another and swear to meet again. Juliet, however, inexplicably begins to feel a dark apprehension. She has a vision of Romeo "dead in the bottom of a tomb." Her apprehension at this moment is hard to explain. We cannot say what is causing Juliet to have this premonition. Shakespeare is using the literary device of foreshadowing in this scene. Romeo will indeed die in the bottom of a tomb, and these two lovers will find that their apprehension was not unfounded.