"Well, if it does change volume with temperature, that will be very convenient for measuring cold temperatures. . . . I wonder how cold it's possible for something to get."

"We can try to find out! Although it's kind of messy knowing how much heat you took from where if you do the river-freezing thing. Reckon you could do a weather barrier except backward? Push heat out instead of in?" 

"I don't see why not. Actually I think Savil theorized it was possible too. I'll give it a go." He fiddles with the magic, trying to get the same thing set up going the other way, from the inside of a spherical barrier to the outside.

It's a bit tricky to stabilize the weather-barrier spell in reverse, but with a couple of tries he can do it. 

And then he can make the room slightly warmer and the sphere of air a lot cooler!

Sandra stares intently at it, trying to figure out if anything weird is happening to the air. 

It gets a bit foggy or misty first, like the air above a pond on a cold day, and then - snows? - a very tiny bit. 

"Huh. I think I froze some water out of it."

"...I guess it makes sense, that you can have some water being a kind of air, and then it turns into rain, or snow if it's really cold." 

"Yeah . . . Like how puddles dry up after it rains, and presumably the water goes into the air? Water has to end up in the air somehow so it can rain again."

"Mmm." Sandra leans in. "I wonder what happens if you keep making it colder." 

"So do I; let's find out." He makes it colder, and then even colder.

There's...something like dew or condensation happening inside, maybe? Except it can't be dew because all the water is now ice. The weather-barrier isn't really a barrier exactly, it doesn't block solid objects from moving in and out of it, but it's substantial enough to block wind, and the tiny dewdrops are beading at the bottom of it. 

- some of them collect into a bigger drop, which is now heavy enough that the 'barrier' doesn't hold it, and it instantly vanishes in a puff of mist. 

"I wonder what that was. Is there something else in air that's like water, but goes into the air at a much lower temperature?"

"I don't know! I've never noticed that happen even in winter, but maybe it's colder in there than it ever gets naturally. ...I'm really tempted to stick my hand in but I probably shouldn't do that." 

"Yeah, I'm tempted too but probably don't, I let it get really cold in there. We really need a heat-measuring thing."

"We really do! ...I wonder if we could measure how much air shrinks when you make it cold? I don't know how, though." 

"Yeah. Hmm, when air heats up in a sealed container, it can't expand immediately, so it just presses on the inside of the container and expands out as soon as it's opened. But when it gets cold, does it actually shrink right then and leave part of the container with nothing in it? Or does it wait to shrink until the container opens?"

"Huh. It'd be really weird if it shrank and left nothing? I think maybe the difference between air and water is that air spreads out and water sticks to itself. But - hmm, I wonder if we had a stretchy container, instead of a rigid glass one - like a waterskin that's flat when it's empty and round when it's full - I wonder if it'd get flatter when the air was colder..." 

"Hmm. That's what I imagine happening when I imagine it, but I can't articulate why it would be that way. Seems easy enough to get a water skin and try it."

"Yes, let's! Do you have one? I think I lost mine." 

"Uhh, pretty sure it's in my room somewhere? I can go get it."

Sandra walks over with him to collect it, though she's mostly deep in thought and nearly walks into a bush. 

Bruce has done the exact same thing more than enough times and pretends he didn't see anything.

His waterskin is right where he thought it would be, namely "buried in the other stuff he hasn't used since the trip back from K'Treva".

Sandra frowns at it. "I guess you could - blow air into it and cork it? Like one of those children's balls they make out of pig bladders."