PORTLAND VASE

When Erasmus Darwin (grandfather of Charles Darwin) brought his book to Joseph Johnson to be published he was in need of engravers to supply the illustrations. Following the suggestion of Fuseli, Blake was engaged to engrave several pictures over a period of time. Darwin's book consisted of Part I of The botanic garden: a poem, in two parts. Part I. Containing the economy of vegetation. Part II. The loves of the plants. With philosophical notes. The larger part of the book is Darwin's copious notes.
 
We learn how Blake became involved in this project from the article, THE PORTLAND VASE: SIR WILLIAM HAMILTON, JOSIAH WEDGWOOD AND THE DARWINS by Milo Keynes.
"With the poem, there were 120 pages of Additional Notes; Note XXII (pp. 53-59) was on the Portland Vase, and was illustrated by four engravings: (1) the Portland Vase (figure 10); (2) and (3) the two compartments with the figures; and (4) the handles and bottom of the Vase (figure 5). From a letter written to him by Wedgwood on 17 November 1789,49 Darwin had thought of using the Bartolozzi prints, but was worried that their use might infringe Sir William Hamilton's copyright.
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On 9 July 1791, Darwin wrote to Wedgwood that the engraver suggested by Joseph Johnson (1738-1809), his publisher, wished to see the Bartolozzi prints, and that 'Johnson said He is capable of doing anything well'. Johnson wrote to Darwin on 23 July that: It is not the expense of purchasing Bartolozzi's plates that is any object; they cannot be copied without Hamilton's consent, being protected by act of pari1.
Blake is certainly capable of making an exact copy of the vase, I believe more so than Mr. B[artolozzi], if the vase be lent him for that purpose.. .
It was William Blake (1757-1827), thus recommended by Johnson, who engraved the four plates for 'The Botanic Garden', but it is not known whether he worked from the Portland Vase itself, not yet on loan to the British Museum, had access to a Wedgwood copy, or adapted Cipriani's drawings and Bartolozzi's engravings. His work was finished by 1 December 1791 —The Economy of Vegetation', though dated 1791, probably was not published until June 1792.52 Later, Blake was to provide engravings of Wedgwood ware for Josiah II in 1815 and 1816."

Blake contributed seven engravings to Darwin's book:
Fertilization of Egypt
Tornado
Amaryllis and
Four images of the Portland Vase to illustrate the philosophical note. 

Blake had become interested in studies of Greek and Roman literature through his friend Thomas Taylor. The Portland Vase, which dated from first century Rome, was decorated with scenes from classic mythology. The exquisite craftsmanship interested many in London's intellectual community, but the puzzling figures created the main fascination. Josiah Wedgwood became absorbed by both. Darwin's interpretation of the decorations as delineating the Eleusinian Mysteries would have made Blake's task of providing illustrations all the more interesting to him. 

The best place to view Blake's four engravings is this website. 

From Note XXII of Part I - Containing the economy of vegetation is Darwin's commentary on the Portland Vase:

"This central figure then appears to me to be an hieroglyphic or Eleusinian emblem of MORTAL LIFE, that is, the lethum, or death, mentioned by Virgil amongst the terrible things exhibited at the beginning of the mysteries. The inverted torch shews the figure to be emblematic,
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The man and woman on each side of the dying figure must be considered as emblems, both from their similarity of situation and dress to the middle figure, and their being grouped along with it. These I think are hieroglyphic or Eleusinian emblems of HUMANKIND, with their backs toward the dying figure of MORTAL LIFE, unwilling to associate with her, yet turning back their serious and attentive countenances, curious indeed to behold, yet sorry to contemplate their latter end. 

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mediterranees.net/art

Engraving by William Blake

2. On the other compartment of this celebrated vase is exhibited an emblem of immortality, the representation of which was well known to constitute a very principal part of the shews at the Eleusinian mysteries, as Dr. Warburton has proved by variety of authority. The habitation of spirits or ghosts after death was supposed by the antients to be placed beneath the earth, where Pluto reigned, and dispensed rewards or punishments. Hence the first figure in this group is of the MANES or GHOST, who having passed through an open portal is descending into a dusky region, pointing his toe with timid and unsteady step, feeling as it were his way in the gloom. This portal Aeneas enters, which is described by Virgil,—patet atri janua ditis, Aen. VI. l. 126; as well as the easy descent,—facilis descensus Averni. Ib. ... The MANES or GHOST appears lingering and fearful, and wishes to drag after him a part of his mortal garment, which however adheres to the side of the portal through which he has passed. The beauty of this allegory would have been expressed by Mr. Pope, by "We feel the ruling passion strong in death."
A little lower down in the group the manes or ghost is received by a beautiful female, a symbol of IMMORTAL LIFE. This is evinced by her fondling between her knees a large and playful serpent, which from its annually renewing its external skin has from great antiquity, even as early as the fable of Prometheus, been esteemed an emblem of renovated youth. The story of the serpent acquiring immortal life from the ass of Prometheus, who carried it on his back, is told in Bacon's Works, Vol. V. p. 462. Quarto edit. Lond. 1778. For a similar purpose a serpent was wrapped round the large hieroglyphic egg in the temple of Dioscuri, as an emblem of the renewal of life from a state of death. Bryant's Mythology, Vol II. p. 359. sec. edit. On this account also the serpent was an attendant on Aesculapius, which seems to have been the name of the hieroglyphic figure of medicine. This serpent shews this figure to be an emblem, as the torch shewed the central figure of the other compartment to be an emblem, hence they agreeably correspond, and explain each other, one representing MORTAL LIFE, and the other IMMORTAL LIFE.

mediterranees.net/art

Engraving by William Blake

This emblematic figure of immortal life sits down with her feet towards the figure of Pluto, but, turning back her face towards the timid ghost, she stretches forth her hand, and taking hold of his elbow, supports his tottering steps, as well as encourages him to advance, both which circumstances are thus with wonderful ingenuity brought to the eye. At the same time the spirit loosely lays his hand upon her arm, as one walking in the dark would naturally do for the greater certainty of following his conductress, while the general part of the symbol of IMMORTAL LIFE, being turned toward the figure of Pluto, shews that she is leading the phantom to his realms.

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The figure of PLUTO can not be mistaken, as is agreed by most of the writers who have mentioned this vase; his grisley beard, and his having one foot buried in the earth, denotes the infernal monarch. He is placed at the lowest part of the group, and resting his chin on his hand, and his arm upon his knee, receives the stranger-spirit with inquisitive attention; it was before observed that when people think attentively they naturally rest their bodies in some easy attitude, that more animal power may be employed on the thinking faculty. In this group of figures there is great art shewn in giving an idea of a descending plain, viz. from earth to Elysium, and yet all the figures are in reality on an horizontal one. This wonderful deception is produced first by the descending step of the manes or ghost; secondly, by the arm of the sitting figure of immortal life being raised up to receive him as he descends; and lastly, by Pluto having one foot sunk into the earth.
There is yet another figure which is concerned in conducting the manes or ghost to the realms of Pluto, and this is LOVE. He precedes the descending spirit on expanded wings, lights him with his torch, and turning back his beautiful countenance beckons him to advance. The antient God of love was of much higher dignity than the modern Cupid. He was the first that came out of the great egg of night, (Hesiod. Theog. V. CXX. Bryant's Mythol. Vol. II. p. 348.) and is said to possess the keys of the sky, sea, and earth. As he therefore led the way into this life, he seems to constitute a proper emblem for leading the way to a suture life."