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Henry's Law


Consider this story the next time you’re installing an air separator, or wondering why the air keeps coming out of that compression tank.


Episode Transcript

The first time I saw a Spirovent air separator in action was at a plumbing supply house in Brooklyn, New York. It was set up with see-through, plastic piping and I got to use a bicycle pump to inject air into the moving water. I stood there for a while, pumping away. The Spirovent caught the air and spit it out. It was fun, but then, I’m easily amused.

The salesman smiled at me and told me about microbubbles, which was a new term for me, and how they would collide with and adhere to the inner workings of the Spirovent, and then leave by way of the vent. He also mentioned Henry’s Law, which was something I had never heard of at the time, but I nodded with great respect nevertheless. Nobody wants to look dumb.

When I got back home in those pre-Google years, I looked up Henry’s Law in the dictionary and learned that it had to do with the way a gas will dissolve in a liquid, depending on pressure. It turns out the more pressure you put on a liquid, the more gas it will hold in solution. Vice versa, of course. Oh, and the hotter the water gets, the less air it will hold in solution (and vice versa). I sort of knew all of that by watching water boil and from shaking bottles of club soda, but I had never put a name to it. Now I knew. Henry’s Law.

And that got me thinking a lot about club soda. I had written a book called Pumping Away and I needed a simple visual that would show what happens to dissolved gas when you lower the pressure on the water. Club soda fit the bill. I started taking a bottle of that fizzy stuff with me whenever I left home to do a seminar. Shaking a bottle of club soda and popping the top is a marvelous way of demonstrating Henry’s Law to a group of heating professionals, and one they will long remember. Well, at least the ones in the front row will remember.

I did this for years, but I never gave much thought to who Henry of Henry’s Law was, so one day I decided to look into that. Here’s what I learned:

Bill Henry’s daddy, Tom, was a rich doctor who also owned an industrial chemical business in Manchester, England in the late 1700s. Try putting those two professions together nowadays. You could probably make your own patients. Tom Henry was the first guy to suggest that you could bleach clothing with chlorine. How about that? Isn’t it good to know that Clorox and the proper location of hydronic air separators have their roots within the same family?

Anyway, Little Billy showed up just before Christmas in 1774 and all went well until he reached the age of 10. That’s when a beam fell from the ceiling, landed right on top of him, and left him with chronic pain for the rest of his life. Because of this, he wasn’t able to play with the other kids so stayed inside and hit the books. And he hit them hard.

At 16, he began to study medicine, and at 21 he entered the University of Edinburgh in Scotland, but only stayed a year. He left the university to help his father with his medical practice and to work in the family business. He spent the next 10 years doing original research in chemistry, which plays a big part in this story. And then, because he didn’t like to leave things undone, he returned to medical school at 31 and got his medical degree two years later. Oh, and he did his dissertation on, of all things, uric acid (a.k.a. pee), which I think is splendid because it ties Bill Henry even closer to the business of plumbing and heating.

You know we are all influenced by those who came before us, and Bill Henry was no different. A generation earlier there had been a fellow in France by the name of Antoine Lavoisier. Ever hear of him? He was the guy who first said that matter can neither be created nor destroyed, and he also gave names to two things that are pretty important – those being oxygen and hydrogen. Lavoisier also came up with the first extensive list of elements, and helped create the metric system.

So there. What have you done lately?

And you would think that the French would have appreciated all of this, but at the height of the French Revolution, someone accused Antoine Lavoisier of selling watered-down tobacco, so they chopped off his head.

Smoke ‘em if you got ‘em.

Bill Henry was fascinated by Lavoisier’s work, and in 1801, while still working with his dad, he put together a book about it, and he did a fine job of explaining all of it. He called the book, Elements of Experimental Chemistry, and this went through 11 editions over the next 30 years. He kept adding to it, and it was this work that introduced generations of chemists to the Frenchman’s careful use of experimental measurement. Bill Henry was just 27 years old when he wrote that book, and two years later he published the paper that established what we now call Henry’s Law. Years and years after that, I’m in some plumbing supply house in Brooklyn and an air-separator salesman is bringing it to my attention.

Ain’t life grand?

And how about this? Henry’s Law came about because Bill Henry was sitting around wondering why our atmosphere, which is composed of all these different gases, each with its own density, doesn’t separate into layers like oil and water. I’ve never wondered about that. Have you? I mean I’ve gone through 71 years of life without once considering that. Gosh.

It was Bill Henry’s initial thinking about our mixed-up atmosphere that led to the theory of mixed gases, which we now today credit mostly to Bill’s pal, John Dalton.

I have to tell you about him. You’ve probably never heard of the guy. I sure hadn’t. He’s the fella who figured out that atoms make up everything. These two guys were hanging out together. One’s figuring out atoms and the other’s wondering why all the atmosphere doesn’t look like a seven-layer cake. And they’re just hanging out!

Dalton was brilliant but he was also clumsy and careless around the lab. Oh, and he had very little money for experimenting (he was a teacher). His buddy, Bill, though, had lots of money, and even more patience, so the two men worked together like salt and pepper.

They did most of their experiments with gases because gases are chemically simpler than other forms of matter, and when you’re looking for atoms, this helps. Out of all that experimenting came Henry’s Law.

Think about that the next time you’re installing an air separator, or wondering why the air keeps coming out of that compression tank.

So John Dalton goes on to become famous for the atomic theory, which is very cool because suddenly, the world could identify and order elements. From this comes the Periodic Table of the Elements, something you probably had to study in school at some point. Ugh.

John Dalton and Bill Henry are the guys who gave us (are up ready?) H2O. Think about that the next time you’re purging those pipes.

And how’s this for being wonderfully human? Although Bill Henry’s experimenting helped John Dalton come up with atomic theory, Bill didn’t want to back it. As he got older, he became more reluctant to accept change. He didn’t like it when his experiments pointed to something other than what he expected. He held onto his old beliefs, such as insisting that heat has mass. It doesn’t.

In 1824, a series of unsuccessful surgeries on his hands took away his ability to manipulate instruments. He quit chemistry and turned his full attention to medicine, specifically to the spread of contagious diseases.

In 1831, a cholera epidemic hit the United Kingdom and it was horrible. Nowadays, we know that the way to prevent the spread of cholera is to wash our dirty clothing in real hot water and chlorine bleach, which Bill’s father, Tom, had promoted years earlier. But they didn’t try chlorine then. Rather, Bill came up with an inexpensive and simple device that used heat to disinfect clothing. It worked and it probably would have saved countless lives, but for some reason, Bill Henry decided that he didn’t like the idea of the device, so he abandoned it. Thirty years later, Louis Pasteur came up with the germ theory of disease and we all began to pasteurize things with heat. Thirty years later.

In 1836, chronically depressed and filled with the pain that had been with him since that long-ago childhood accident, Bill Henry took his own life.

And I hope you’ll never look at those air separators and compression tanks in quite the same way ever again. I sure don’t.

And I hope you enjoyed this story. And if you did, please share it with your friends. And please subscribe to this podcast if you haven’t already. I have many more Dead Men Tales to share with you. I so appreciate you taking the time to stop by and listen because, let’s face it, without you, I’m just talking to myself. Thanks for always being there for me.

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