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 Lord Salisbury said it was only a noun for the verb 'to undulate.'

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"Lord Salisbury"
Robert Arthur Talbot Gascoyne-Cecil, 3rd Marquess of Salisbury. He was Prime Minister of the United Kingdom three times in the latter half of the 19th Century. I don't know why he was weighing in on the possible existence of Æther except that people sometimes want to know what powerful politicians think. This guy seemed pretty smart if absolutely a model of conservative British politics. My favorite bit about him was his credo: "Whatever happens will be for the worse, and therefore it is in our interest that as little should happen as possible." Being the leader of the greatest imperial country of the 19th Century, this motto did not stop him from winding up with the most territory in the Scramble for Africa. I suppose that was more a case of the worst happening and less a case of as little should happen as possible. Because, you know, Britain could have sat out plundering Africa with the other imperialists, could they have not?

This guy looks like his credo.

"a noun for the verb 'to undulate.'"
I don't know if he really said this or this is just Pynchon fictionalizing some possible dialogues surrounding Æther at the time but it definitely sounds good! The main reason for Æther's existence was to give light waves something to travel, or undulate, through. The movement of light waves simply needed a medium, or noun, to make sense.

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 Some don't believe in it, some do, neither will convince the other, it's all faith at the moment.

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Think about science as theories based on observations and we can understand the kind of "faith" Heino means. If early "scientists" (just meaning curious people who actually consider the world around them rather than horny people just trying to get laid (which we need too! I'm not judging!)) were to first observe fish, and the only thing they knew was that they, and most animals they knew of, would drown underwater, they would probably come up with a story, or theory, as to how the fish survives. Religion is theory based on observation without evidence. That's where faith actually lies. Science is theory based on the current best understanding of what's being observed and which everybody knows is liable to change with new evidence. That's hardly faith! But back to the fish watchers! A theory one of the early scientists might come up with is that fish don't need to breathe. They're just different. Everybody might nod their heads and say, "Yeah, yeah! That sounds reasonable. They seem fine underwater yet no animals we know of can breathe underwater. Therefore fish don't breathe." Belief in that story would be Heino's version of faith. It makes sense through observation and an understanding of the world around them. But as soon as somebody takes a fish out of the water and sees the way it gasps and eventually dies, the theory would immediately change. Obviously the fish is trying to breathe! But it drowns on land the way people drown underwater! So they do breathe but they seemingly breathe water. Boom. That's the new story that some will believe, some won't, and nobody will convince the other because the evidence to support either assertion isn't in yet. It's all faith.

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 "You're quite right, of course, the Æther has always been a religious question.

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"a religious question"
I suppose all scientific theory must be born from faith or we would ultimately learn nothing. If we only believe what we see evidence of without somebody extrapolating further on their observations, then nobody would ever come up with theories to test. At what point does a scientific theory go from the religious to the scientific? It seems to me what's most religious in the arena of science is when the evidence has stacked up against a long-held theory and yet adherents continue to cling to it.
    Here Heino agrees with Merle that Æther has been a religious question because it has merely been theorized as something needed to uphold scientists' views on light and its properties. Kind of like how String Theory's multiple dimensions haven't been proved in any actual way but are only theorized because of the math. Or maybe dark matter is a closer comparison, pretty much being the modern day equivalent of Æther in that it seems to be needed for the current model of the universe and general relativity to work.

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 They strolled among the elm-shadows, eating sandwiches and apples out of paper bags, "a peripatetic picnic," as the Professor called it, slipping thereupon into his lecture-hall style.

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"elm-shadows"
Elm trees create plenty of shade and are one of the most popular trees to be used in public landscaping, often planted as boundary markers for paths, creating tunnel-like effects when planted to either side of a path. The elm is both a symbol of pastoral peace and death. Perhaps Heino and Merle are walking in the shadows of these border trees because they are, technologically speaking, crossing from a time of pastoral peace, where technology has begun to make impacts in our lives and seen as improving them, toward a time when technology will begin its decent from the peak of improvement, hurtling toward the ground as it becomes destructive and deadly, until it finally crashes beyond the zero into nuclear annihilation.
    Or maybe its just a bit of descriptive tat.

"eating sandwiches and apples"
Apples being the symbolic fruit of which Eve partook. Yeah, I know the fruit is never actually described but instead of being a pedantic nerd on the Internet, how about understanding the cultural symbols all around you. There's a reason the Apple logo is that of an apple with one bite taken out of it. And it's not because it's the mark of the beast and an indication of man's inability to resist temptation! It's about gaining knowledge! Maybe those are the same things though. See my comments on "elm-shadows."

"out of paper bags"
They didn't have plastic bags yet! If you want to learn more about the history of plastics, Pynchon covers that topic in Gravity's Rainbow.

"a peripatetic picnic"
This is a pun. The Professor is commenting on how the picnic is a traveling picnic but also it's a picnic where they discuss philosophical matters as they eat. So peripatetic as the adjective in that they're traveling but also as in The Peripatetic school founded by Aristotle, as we see Heino begins to lecture Merle as they walk.

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 "Guess I'd better go take a look. Probably that gear train again."

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The guy just came out with singed hair and smelling of burning flesh and cleaning products. I don't think it was the gear train. I think the Professor decided to try holding the Leyden Jar in his non-cranking hand. He may also have been re-attempting the "What happens if the jar is down my pants?" experiment. Maybe he was rubbing the glass spheres against his scrotum to generate the static electricity? Am I projecting too many of my own kinks onto the Professor? Although his name is Heino Vanderjuice. There's got to be a semen joke in their somewhere.

Maybe the more important part of Merle's response is for the reader to understand that Merle has the know-how to build and fix an electrostatic generator!

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 "Small confrontation with the Töpler Influence Machine, nothing to worry about."

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"Töpler Influence Machine"

An electrostatic generator which means a machine to generate static electricity! It was probably used for hilarious practical jokes and Jackass-style stunts. It's almost certain that August Töpler was the first guy in history to know what it felt like to shock your scrotum. Or, more probably, his lab assistant was the first and Töpler was the first person in history to piss himself laughing at seeing a guy get his balls electrocuted.

I am, of course, joking. Static generators were invented well over a century earlier and, as such, many, many balls had been shocked before Töpler was even born.

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 He discussed it one day with his friend at Yale, Professor Vanderjuice, who, having just emerged from another of the laboratory mishaps for which he was widely known, carried as always a smell of sal ammoniac and singed hair.

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Pynchon never fears going full trope at times. He uses the entirety of language at his disposal and a good portion of the language we share are pop culture tropes and characters. The literate dog aboard the Inconvenience is based on Scrappy Doo, for Christ's sake! And here we see Professor Vanderjuice as that bumbling, absent-minded professorial-type whose massive intelligence and passion for new discoveries can get him into dangerous situations, like always blowing up the lab he's in and coming out smoking with wild hair and big black holes burned into his lab coat. He's a cartoon (to me, he's Professor Pat Pending from The Wacky Races)! But being a cartoon, he's capable of more than real people and, as such, he's actually built a perpetual motion machine for the Chums of Chance (an invention his peers scoffed at, not because they didn't believe he built it but because the thing he built was impossible and thus deserved academic scorn).

One of the themes I feel Pynchon crafting in this book is that what we believe as human beings shapes the world in which we live. Like a cartoon coyote running on air off a cliff, we craft reality and reality only shifts or changes, causing us to plummet, when we finally observe some truth which causes us to give up our old beliefs. So the Frontier was more about what we believed it to be, and declaring its death once again changed the landscape of the West. Believing that light needed Æther to travel through meant the world was full of Æther and its strange lighting effects were seen by balloonists up until scientists could prove Æther didn't exist (which they haven't proven yet by 1893! But they have experiments which have failed to prove its existence where they should have, and thus it is on its way out). Later, we'll see the Chums of Chance travel through a hollow Earth, something we know isn't possible but lived on in theory and belief during their century.

Vanderjuice might be a ridiculous human being but that only means he's capable of accomplishing ridiculous scientific feats.