Of architecture Gothic, all around
The well-known ballads flit, of Grubstreet fame!
The casement, broke, gives breath celestial
To the long dying-speech; or gently fans
The love-enflaming sonnet. All around
Small scraps of paper lie, torn vestiges
Of an unquiet fancy. Here a page
Of flights poetic; — there a dedication;—
A list of Dramatis Personae, bold,
Of heroes yet unborn, and lofty dames
Of perishable compound, light as fair,
But sentenc'd to oblivion!
On a shelf—
(Yclept a mantle-piece), a phial stands,
Half-fill'd with potent spirits! — spirits strong,
Which sometimes haunt the poet's restless brain,
And fill his mind with fancies whimsical.
Poor Poet! happy art thou, thus remov'd
From pride and folly! — for in thy domain
Thou canst command thy subjects; — fill thy lines—
The “phial… Half-fill’d with potent spirits” refers to opium, a drug that became quite common during the 19th century. Opium, the juice of the poppy plant, was generally used as a pain reliever and was often times mixed with differing quantities of alcohol, forming an anodyne  typically prescribed as laudanum. 
 Opium was highly addictive and is referenced by many poets for its enhancement of creative states. Samuel Taylor Coleridge, in particular, was known for his struggles with the drug. In his poem “ Kubla Khan ,” Coleridge writes of an opium-inspired dream. This poem was shown to Robinson in 1800, prior to its publication. The two poets were acquainted, and letters written both to Robinson directly and to those close to her show Coleridge’s concern for Robinson’s works, as well as for her well-being. It is highly likely that the two poets discussed the power of opium on the imagination. 
 
 Byrne, Paula.  Perdita: The Literary, Theatrical, Scandalous Life of Mary Robinson . Random House Publishing Group, 2007.
The Poets Garrett 1800