Stanza 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Lines 10-11

A mere nothing
A spark they strike only to let it go out again sometime later

We switch gears pretty suddenly here. We follow the idea that the speaker's eyes and love are going down the wrong road with the sentiment "A mere nothing." We then hear that this nothing is a spark, struck by a mysterious "they," only to burn out: poof.
These two lines echo the structure of the first two lines, where a small first line is expanded upon by a longer second line (more on form in "Form and Meter"). This reinforces the ideas, and takes our minds as readers from the small to the big.
In the first of these lines, we get zip, "a mere nothing." This could refer to the wrong road that the speaker's romance is on, or the force that's keeping him alive, or life and the world in general. We're not quite sure, but the sentiment lets us know that things aren't all they seem in this speaker's world.
The second line brings us back to the metaphor of heat that runs throughout the poem. Whatever the speaker is saying is "nothing" is also just a spark, struck only to burn out. Amid the alliteration, or repetition of that beginning S sound of "spark" and "strike," we get a mysterious "they." This "they" could be anyone, but we're guessing that it's purposefully ambiguous.
Who knows who strikes the spark of life, and decides when it will go out? We certainly don't claim to, and neither does our speaker.
These lines take us from the despair and confusion that we felt in the previous stanza and drive it deeper. We've got a short-circuited system, a broken engine, a mysterious force keeping us on our feet though we feel we shouldn't be running, and a misled love life. On top of it, add a little doubt that the world and life is more than nothing, more than a short-lived blip of fire.
Anyone ready to party yet? Reverdy's given us a recipe for depression, driving home the despair felt when, perhaps, you're in love but can't be with the one you love for one reason or another.

Lines 12-13

I've had enough of the wind
I've had enough of the sky

These lines seem to illustrate more of the speaker's emotions relating to his (and we're just assuming it's a he since Reverdy was a man) breakdown. The world, and his life, seems like nothing, and he's had enough of the wind and sky, two things which, in general, are pretty important, right? It's not as if, in real life, you can up and decide that you've had enough of the wind and sky. If you've ever been caught outside in a nasty storm, you know that the wind and sky do what they want, regardless of what we as humans have to say about it.
Potentially, these two lines mean that the speaker is sick of the outside world, and that he is just going to take himself inside a building and stay in there. Maybe he'll start collecting old newspapers and cats and try to get on the new season of Hoarders.
Or perhaps they mean that he's sick of the world, the world that is beyond his control. He's going to avoid it in general, wrapping himself up in a protective layer of disbelief in reality. (Take that, reality.)
Note the repeated structure of these two lines. Imagine them read aloud, and how the repetition helps to build a sense of frustration, a sense of breakdown. (More on this sort of stuff in "Form and Meter.")

Line 14

Essentially everything visible is artificial

This line wraps up a lot of the frustration and doubt that has been building throughout the poem. Everything visible, like the sky, the speaker claims, is artificial. Wha?
This is a quite philosophical line. This poem was published in 1916, right smack in the middle of World War I, which was thought to be the war to end all wars and which resulted in massive casualties. In the face of such tragedy, it's no wonder that the nature of reality would become questionable.
This line is sweeping in its accusations. In its definition of what is artificial, it's not just including the wind and sky, or perhaps the woman that the speaker loves. It's including… everything, and we mean everything, that anyone can see. Your favorite t-shirt? Your pet labradoodle? The city of Rome? Everything that our eyes show us, this line claims, is fake.
You can almost think about this in terms of the movie The Matrix, except minus Keanu Reeves' stellar acting chops.
According to this line, the world we live and breathe in is not what it seems. It's a fake, and our speaker has had enough of it.
It's also possible to read this line as a rejection of materialism, which is the view that stuff money can buy is important, but also the view that science and the visible provide the only explanation of the world around us. Our speaker seems to think that everything we see is artificial. That doesn't necessarily mean that they aren't real, but could mean the speaker places more value on what's underneath the visible, whatever "electromagnet" that may be.

Lines 15-16

Even your mouth
Despite the fact that I warm up wherever your hand touches me

After the speaker's resolve that he's had enough, and the world is artificial, it seems that he's actually having doubts.
First, he says that even "your," or, we can guess, his lover's, mouth is artificial. Hmm. We've heard of fake eyelashes, but that's pushing it.
From the fact that he feels a special need to make this point, though, we can deduce that his lover's mouth is one of the things that feels most real, most authentic. He's saying, perhaps without really believing it, that everything he sees is artificial, despite the intensity of the mouth of the woman he loves.
Second, he mentions the heat he feels when his lover touches him. That means that her mouth and the touch of her hand are the things that most threaten his view of the visible-as-artificial. These lines suggest that, in addition to everything we see, everything we sense, too, is artificial.
Or, these exceptions are included to give us as hints that this is just a moment of breakdown, and that, most of the time, the speaker is so wrapped up in the passionate reality of his lover's mouth and touch that the world doesn't seem very artificial at all. It's as if the speaker is saying, "Laters for this reality stuff—it's all a bunch of mumbo jumbo. Still, come to think of it, it does feel mighty real when we touch…"
Line 16 also ties back to the theme of central heating in the poem. We return to the idea of heating up, and that the touch of another human, and perhaps, especially, a lover, can warm our bodies up.

Line 17

The door is wide open but I refuse to enter

So we've got this line on a literal level, first: there's a door that's not just open, but wide open. Like, somebody ran out in a hurry and forgot to shut it. And yet, our speaker is being stubborn, refusing to enter.
This line seems like a pretty sudden jump from the line before, about his lover's hand warming him with its touch. So that makes us think, on a metaphorical or figurative level, this door that's open is truly embracing his love for this woman. He knows that she's there, waiting for him, but he just can't make himself walk through the door to her.
This could also be read on another level, too. The door to the world being real, to embracing the world instead of having had enough of it, is open. But the speaker, again, refuses to enter. He gives reality the cold shoulder. He'd rather have his broken-down heart and artificial world, and wallow in his confusion and doubt, because, well, it seems more real to him than whatever joy or misery might be on the other side of that door.

Line 18

I see your face but lack all faith in it

We see, in this line, what may be a reason for why the speaker is refusing to enter the door. He sees his lover's face (is it in the doorway?), but just can't have faith in it.
We need to break this down. On one level, we know that the speaker is claiming that everything that can be seen is artificial.
That means that he may not have faith that the woman's face is even real, a real touchable thing on this world, and not just some façade, or mask, over the nothingness beneath it.
It could also mean that he sees the woman's facial expression opening up the door for him to embrace her and her love, but cannot have faith in her love and intentions.
It's handkerchief time, y'all. This is a deeply sad line—here's a woman, whom the speaker seems to have strong feelings for, and he can't bring himself to believe in the physical reality of her face, which invalidates any emotions he may feel for that face and the person behind it. Are you weeping openly yet?

Line 19

You're so pale

Here, we get a detail to describe the face. The woman is pale, meaning she hasn't seen very much sun lately. Or maybe she's feeling ill.
If our lover was reading this poem to us, we would probably feel quite ill, or shocked. After all, he's thinking about how this woman is not even real. That's not something you'd want to hear on a regular basis.
This little detail gives quite a ghostly, wan quality to the poem, yet in one sense it is loving at the same time. The speaker can't help but notice her appearance, even though he is debating the reality of that appearance.

Line 20

One night when we were unhappy we sat down together on a trunk

This line seems to leave the doubts about the reality of the world behind to move to the memory of an action in the real and tangible world. It's simple enough: the speaker is remembering a night when he and his lover were sad, and they sat down together. He even remembers that they were sitting on a trunk (like a steamer trunk, not an elephant trunk), and not a couch or a bed or a comfortable chair.
We can imagine this trunk was probably not very comfortable to sit on for any length of time. Yet the trunk was a place for sad lovers to sit on at night together, as unhappy as they may be, as their rear ends got numb.
This memory seems to be fighting against the view of the world as artificial, but also gives us a clue about the state of the speaker's relationship. The two weren't joyous and gleeful as they sat on the trunk. Something is wrong in their relationship, but we don't know why they were unhappy. They could have been unhappy with each other, unhappy because of a tragedy or dissatisfaction in their life. Remember, the poem was published in the middle of a devastating war, so although the source of unhappiness is a mystery, it's not hard to imagine what could be making this speaker and his lover full of doubt about their world.

Lines 21-22

Men were laughing somewhere off in the distance
Nearly naked children walked by now and then

These lines give us some details about the world that surrounded the speaker and his lover on the unhappy night. There is some gathering of men going on, because they can be heard laughing in the distance.
Somewhere in this artificial world, where our speaker is unhappy, men are laughing. Joy is present in the world though our speaker is not currently part of this joy, and neither are we as readers. There is the possibility that this joy gives the speaker hope, but most likely, knowing that it exists but not being included in it just deepens his depression.
In addition, children walk by occasionally. This makes us think that this trunk must be in a public place, perhaps out on the street. The children are nearly naked—perhaps they are poor and in rags, or perhaps it's a hot summer day, and their parents can't keep the kids dressed no matter how hard they try.
The children may be nearly naked, but it seems that the image of the small children adds to the joy of the men laughing. The speaker and his lover may be sad, sitting on a trunk, but the other people of the world keep moving around them. Men laugh, children clad in summer freedom walk on by, and none of them have a care for the sad couple sitting on the trunk, watching the world around them while their rear ends get numb.

Lines 23-24

Water flowed by in perfect purity
Copper wire conducts the light

Scene change alert. Now we switch from a description of the people around the couple to the description of the physical world and objects around them. We start with water. Reverdy lived in Paris at the time this poem was published, so perhaps they are sitting by the shore of the Seine, the river that runs through Paris. It's doubtful that this water, in the middle of a city, would be perfectly pure (or really, even remotely pure), so we could also think of a stream elsewhere—maybe water in a fountain, or freshly fallen rain water.
We could also think of the water that could be used in a central heating system, which, heated up, would be pure.
Either way, the speaker is thinking of water, and its perfect purity. Water, indeed, is often used as a metaphor for cleansing and purity. So it could be that this water is pure on more of a figurative level than a literal one.
After we hear about the water, we hear about copper wire. Copper wire is a great conductor, taking electricity to things like light bulbs.
So there's both water and light flowing by in these two lines. It seems as if, even though he's not sure the world around him is real, the speaker is including details about it to give his unhappy night a bit of context.

Line 25

The sun and your heart are compacted of the same substance

This last line is the kicker, almost the punch line of the whole poem, bringing the theme of love together with the imagery of central heating.
Here we've got the sun and the speaker's lover's heart. Then we've got the idea that the sun and the heart, are compacted, or made, of the same material.
This means that the lover's heart is a source of heat—perhaps, for the speaker, the source of his central heating, the electromagnet that is keeping him running.
These lines are undeniably romantic. In the face of the speaker's inner breakdown, and his doubts about the world and its reality, he gives as much significance to his lover's heart as he gives to the sun. Romantic, right?
But he remains ultimately, tragically unconvinced. He could believe that he is powered by the energy, the heat, of this woman, whose hand warms him up when it touches him, though he despairs about the potential artificiality of the wind and sky. It seems like he's just not sure. (By the way, if you're curious what the sun is actually made of, check this out.)