Please explain the following lines from "A Refusal to Mourn the Death, by Fire, of a Child in London."
Deep with the first dead lies London's daughter,
Robed in the long friends,
The grains beyond age, the dark veins of her mother,
Secret by the unmourning water
Of the riding Thames.
After the first death, there is no other.
The central point of this poem is the author's radical reassessment of death given his belief that when we die we actually rejoin nature, which is an incredibly powerful force of regeneration and life in the universe. Thus, the speaker says, he deliberately eschews the forms of mourning that humans adopt because they fail to recognise the way that life metamorphosises and continues rather than ceases to be.
In the final stanza, which is the section of the poem that you are asking about, the dead girl that the poem's title refers to is said to join life in her death, becoming reunited with the "long friends" who have passed away before her and the "grains beyond age," which is a symbol of the sands of time and seeds, and finally the "dark veins of her mother," which refers to the earth and the way that the veins of the earth are its rivers. What is interesting is that nature itself is depicted as "unmourning" in the form of the river Thames, even though the girl is called "London's daughter." This is because, given the view of death and nature, the Thames is regarded as the river of regeneration, and is teeming with life as the cells of "London's daughter" form a part once again of the city. There is a pleasing symmetry in the way that rivers, journeying from their source, then flow into the sea, all being part of the same matter, just as humans, when they die, become part of the bigger life of nature and the universe, as it is all part of the same thing as well.
The final line of the poem cements the main argument of the speaker. Whilst individual life forms can die, we rejoin nature, which is a scientific force that is built to endlessly regenerate. Because of this, our first death will be our last, because when we rejoin nature, we can no longer "die" in the way that humans can.