Sonnet XLV

The other two, slight air and purging fire,
Are both with thee, wherever I abide;
The first my thought, the other my desire,
These present-absent with swift motion slide.
For when these quicker elements are gone
In tender embassy of love to thee,
My life, being made of four, with two alone
Sinks down to death, oppressed with melancholy;
Until life's composition be recured
By those swift messengers return'd from thee,
Who even but now come back again, assured
Of thy fair health, recounting it to me:
This told, I joy; but then no longer glad,
I send them back again and straight grow sad.

This relates to the previous sonnet. He finds that his thoughts and desires are not so much in himself, as with his beloved (hence present-absent.) The four elements of classical (Aristotelean) science were fire, air, earth and water. The Elizabethans had no idea of modern chemical or physical science, with its 100 plus elements. All substances were said to be made up of these four elements, hence when deprived of two of them, air and fire, (the other two) which correspond to thought and desire, the body sinks into melancholy and decay.

He finds himself in a state of continous fluctuation between joy and sorrow, but mostly sorrow, since his thoughts are continuously to-ing and fro-ing between himself and the one he loves.

The 1609 Quarto Version

THe other two,ſlight ayre,and purging fire,
Are both with thee,where euer I abide,
The firſt my thought,the other my deſire,
Theſe preſent abſent with ſwift motion ſlide.
For when theſe quicker Elements are gone
In tender Embaſſie of loue to thee,
My life being made of foure,with two alone,
Sinkes downe to death,oppreſt with melancholie.
Vntill liues compoſition be recured,
By thoſe ſwift meſſengers return'd from thee,
Who euen but now come back againe aſſured,
Of their faire health,recounting it to me.
This told,I ioy,but then no longer glad,
I ſend them back againe and ſtraight grow ſad.

Commentary

1. The other two, slight air and purging fire,: The other two elements of the four (fire, air, earth, water).
slight = flimsy, insubstantial.
purging = cleansing. Fire cleans by destroying, and also by burning up anything noxious.
2. Are both with thee, wherever I abide;: wherever I abide = wherever I am
3. The first my thought, the other my desire,: The first - the first element, air, corresponds to (or is) my thoughts.
the other - Fire, which is equivalent to my desire.
4. These present-absent with swift motion slide.: They (air and fire) are both present and absent because they reside with my beloved and yet are in me also.
5. For when these quicker elements are gone: quicker = livelier, more full of life, more swift moving. Earth and water were the heavier, hence slower, elements.
6. In tender embassy of love to thee,: Bringing tender messages of love to you. The image is that of an ambassador taking a message from one monarch to another. Ambassadors were often used on embassies of love, for example Eric the King of Sweden kept up a courtship with Elizabeth across the waters for many years, through a series of ambassadors, although he never managed to meet her. Other candidates did the same. Apparently in 1559 there were ten or twelve ambassadors at Court seeking Elizabeth's hand on behalf of foreign princes.
7. My life, being made of four, with two alone: with two alone - i.e the remaining two, earth and water.
8. Sinks down to death, oppressed with melancholy;: oppress'd with melancholy = overcome with despondency. melancholy was a medical term, meaning an excess of black bile, as a result of which the patient was sullen and gloomy. (OED.1.)
9. Until life's composition be recured: Until the material substances which make up my life regain health. recured = cured, brought back to health.
10. By those swift messengers return'd from thee,: By the return of thought and desire (air and fire). See note to line 6.
11. Who even but now come back again, assured: Who = which. Referring to the two elements, which have acted as messengers.
even but now = at this very moment. Compare from Othello:
Even now, now, very now, an old black ram
Is tupping your white ewe. OTH.I.1.89-90.
12. Of thy fair health, recounting it to me:: assured / Of thy fair health = confident that you are in good health; who have sure knowledge of your good health.
recounting it to me = giving me an account of it, telling me of it.
13. This told, I joy; but then no longer glad,: This told = the account having been given to me. I joy = I rejoice.
but then no longer glad = but immediately after when I am no longer gladdened by them; immediately after I become despondent again.
14. I send them back again and straight grow sad.: straight = straightaway, immediately.
grow sad - grow sad both as a result of your absence and the departure of the messengers once again, fire and air, the absence of which allows me to sink down to death. (Line 8).