Mystery Theme VI: Topic Occluded by KlionskyG

What lawyer successfully defended British soldiers against murder charges arising from the Boston Massacre?

Until English engineer Edward Nairne developed a prototype of this common item, people used wax, pumice, or sandstone -- or crustless bread -- to do the job. Joseph Priestley may have given this item some oxygen within the first few months when he wrote, "I have seen a substance excellently adapted to the purpose of [redacted] ... It is sold by Mr. Nairne, Mathematical Instrument-Maker".

What item did Nairne invent?

Who became Chancellor of the Exchequer beginning the day before his 57th birthday and Prime Minister of the UK the day after his 57th birthday? Ironically (as his name suggests longevity) he served as PM for the shortest period of anyone to hold the office: 119 days in the spring and summer of 1827.

Who led the first European expedition known to encounter the Eastern coastline of Australia?

What is the unsurprising name of this life-size Thomas Gainsborough painting? When it changed hands for $728,000 in 1921, it was the most expensive painting ever sold.

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Who was awarded his PhD from the University of Königsberg for his Dissertation on the Form and Principles of the Sensible and the Intelligible World? I suppose I shouldn't make you read it unless I thought everyone ought to read it.

Such is life: At least 133 people died when a fireworks display at a post-wedding celebration set mannequins and other décor aflame, leading to a panic in which many were trampled and others drowned in the adjacent river. Name both the bride and groom at this affair. (Dessert was presumably served four days before the tragedy, when the teenagers' wedding itself was held.)

When Lexell's Comet passed by the earth, it was -- and remains -- the closest approach by a comet in recorded history.

Call the average distance between Earth and the Moon (about 240,000 miles) 1 LD. Within how many LDs did Lexell's Comet pass? Answers within 15% in either direction accepted.

Joseph Louis Lagrange proved Bachet's conjecture, which states that every natural number can be represented as the sum of four integer squares. But Bachet didn't come up with the problem; he merely translated the much earlier Arithmetica into Latin in 1621.

Name the Greek mathematician who wrote this earlier work (which also led to a better-known conjecture about which I've written a truly marvelous question that this text box is too narrow to contain).

With her poetic elegy to Rev. George Whitefield, who became the first African-American woman to have her work published?

What composer, in whose honor many concert series were planned throughout 2020, and whose birthday is coming up shortly, once premiered two symphonies and a piano concerto, among other works, in a single concert of 4+ hours?

What is the theme of this quiz?

Like in the Google quiz, here's a question that's not really part of the quiz. Don't money this question.

You can feel free to leave this blank if you like, no one's getting points for it. But if you want to describe your approach to getting the theme, go ahead, type it in.

(It's hard to judge tone online. This really really is not a hint or a clue or anything, I just like the idea of this open-window 13th question.)