Well, that’s actually not correct.
Although metallic mercury toxicity is very overstated, it does have a significant vapor pressure at room temperature:
One common use for a bunch of mercury just in a container was old style glass tube vacuum gauges. You can stick a half dozen tubes in one container of mercury for hose lines going multiple places.
When I started work in the NHS, I was based at a hospital that had been built during WW2. After a year or so there we moved to a brand new building.
I went back to the old place a couple of weeks later and saw some guys fully kitted out on hazmat suits going into a now empty ward. I was told that, after 50 years of nurses breaking mercury thermometers and the contents rolling off into corners and crevices, it was considered to be a serious hazard. Bear in mind, that only a couple of weeks previously, there were eighteen sick people in that ward, plus all the staff in and out all day and night.
Was your dad into guns? I’ve known several old-timey shooters who used mercury to remove stubborn lead deposits from the bores of firearms. Basically the process is to plug the barrel, fill it up with mercury, and let it sit a while. The mercury removes the lead by forming an amalgam with it. Due to toxicity concerns, few still use mercury for this purpose today.
Thank you all for your replies. Unfortunately it looks like it will have to remain a mystery. The idea of making switches and all, or anything at that level, I suppose is possible. The trouble is, that those are component parts that could go into who knows how many things, so it doesn’t carry through for me to be able to think, ‘oh, yes, then clearly he wanted to build an X’ and feel the question is closed. 
The suggestion about cleaning gun barrels is nicely specific, but I’m very sure Dad had no interest in hunting or guns at all. He once said to me that the last time he’d touched a gun was during basic training for WWII. I suspect some exaggeration there (though he was an officer involved somehow in transportation coordination rather than direct fighting) but it gets the idea across.
I think the main reason none of the suggestions chime with me is the amount of the mercury: it just seems way, way too much if all he’d wanted to do was make a few switches or something like that. Unless that was the smallest size unit he could buy it in? But then, there’s no label at all on the jar, and surely it would have had one?
Possibly it was just something that fell into his hands through some accidental course of events at a job, and, like apparently a lot of you, he found it ‘interesting’ enough to keep around.
After I posted yesterday, I mentioned this in conversation with a neighbor and he had a possible idea – he said maybe my father was planning to make a special mirror, some non-standard shape. He said you used mercury to make mirrors out of regular glass way back, or at least, his father had once told him that.
Don’t know if that’s true at all. On the other hand, I do remember my father having at least a slight interest in astronomy. It would at least not be unbelievable that he would decide to build a ‘better than I can get from a store’ telescope, and some telescopes use mirrors, right?
Back in the 70’s when I was 12yo or so, I made a tilt switch. An old Testors paint bottle (model paint) 1/3 full of mercury, an unused extension cord and a lamp.
Raising the lid on my record player would tilt the paint bottle and turn on the light.
How I didn’t manage to burn down our house in my younger years I’ll never know.
Whatever the reason might be that your father had this jar of mercury, be sure to keep it away from anything you value that is made from aluminum.
Way back before Mercury was considered a hazmat thing, my father brought home from work a small jar that sounds just like was described in the OP. We would play with it and use it to put a brilliant shine on dimes and quarters–they were made of silver in those olden days. The shine wore off in a few days. I assume it either wore off or evaporated away.
When I worked in a biochem lab 60 years ago, there was some kind of machine that was studying metabolism in some kind of microorganism. I cannot recall the name. But there was a water bath, a rocker mechanism and a down or more individual cells and some measuring instruments, one in each cell, each with an open puddle of mercury in it. One of the techs claimed an unusual sensitivity to mercury and stayed out of the room, while the rest of us never noticed. While not claiming elemental mercury is benign, its danger has been highly exaggerated.
I took a blob of mercury I got from a lab spill. The company wanted to just toss it in the trash, but I bagged it and took it to our local hazmat collection event.
When they saw it, they radioed a guy in a nearby trailer. He was in full hazmat regalia, and came out of a trailer, gingerly took my sample, and scampered away. It was kind of freaky.
OP, your dad probably came across that mercury randomly and kept it. As you can see from this thread incidental exposure to mercury isn’t any great danger, but it little kids could ingest it, and you don’t want it open somewhere that you have extended contact with the fumes. Some years back around here National Grid was found to be keeping barrels of mercury outside in a storage yard after some teenagers broke in an stole some. They got heavily fined for storing the stuff, but then there was also a massive effort to find the kids, including private detectives staking out the house of one of the kids who ran off to avoid arrest. Among other things these idiots had tried putting mercury in a crack pipe and smoking it. As usual, the greatest danger of many things is putting them in the hands of morons.
Hg could have been used in a manometer for checking pressures/vacuums. Mercury used to be a common fluid used in these applications and the units associated w/ such are still in use today. I used to have one of these carburetor synchronizers that came with a small bottle of Hg for use in the chambers. I didn’t use it often and ended-up giving it to a friend who had a 4-cyl machine. I’ve only owned twins, so I’ve made my own manometer that doesn’t use Hg (but that’s another thread).
Like many others, as a kid I played with this stuff as often as I could get it. Rubbing it on silver coins to make them slick and shiny. Unless there are different types of mercury, I do not think it is particularly hazardous. Use common sense. Don’t eat it. Don’t get it into open cuts. Wash hands afterwards. You should keep it…or send it to me and I will properly dispose of it.
Maybe he was experimenting with a mercury telescope mirror. Newtonian telescope mirrors have a parabolic surface. It just so happens that if you rotate a shallow dish of mercury or any liquid it will theoretically form a parabolic surface. The faster you rotate the shorter the focal length.
This has been experimented with off and on for about 100yrs. Its a tough nut to crack and requires a very sophisticated mechanical setup. We are talking about millionths of an inch surface accuracy here. If I remember correctly though it does work after a fashion.
How much is mercury worth? Is it even legal to sell it?
I think it’s over $200 a pound, and it’s heavy so one pound is not a lot of mercury. It’s a hazardous material, but not illegal to sell that I know of.
There’s some old guy around here that was leaving 1940s war pennies (the steel ones) as part of a tip. I got a few. They are VERY shiny and clean. I thought at first he might be plating them, but now I am wondering…