Sonnet XXII (22)

My glass shall not persuade me I am old,

So long as youth and thou are of one date;

But when in thee time's furrows I behold, 

Then look I death my days should expiate. 

For all that beauty that doth cover thee 

Is but the seemly raiment of my heart, 

Which in thy breast doth live, as thine in me:

How can I then be elder than thou art? 

O, therefore, love, be of thyself so wary 

As I, not for myself, but for thee will; 

Bearing thy heart, which I will keep so chary

As tender nurse her babe from faring ill. 

   Presume not on thy heart when mine is slain;

   Thou gavest me thine, not to give back again. 

4. Expiate. Bring to a close, finish. -- Schmidt's Lex. Malone compared King Richard III., Act iii. sc. 3, line 23, "Make haste; the hour of death is expiate." 

5. All the beauty, &c. The poet and his friend have exchanged hearts; and so the poet's heart is clothed with the beautiful form of his friend. 

9. Be of thyself so wary. Since thou hast my heart in thy breast. A gentle hint, possibly, in accordance with what has been said above. 

13. When mine is slain. Meaning, possibly, slain by thy present course of conduct.