That’s a bit strong, but the elemental metal mercury ain’t all that bad. Playing with a little isn’t going to kill you. Problems come with the vapor (which is always present to a limited extent with the metal) and compounds, IIRC.
That mercury is toxic at all is fine evidence against the existence of a Just and Loving God. Cause seriously, what kind of bastard would you have to be to create something that frickin’ cool and then make it poisonous? 
The premise of the OP, regarding the madness of the syphilis vs the madness of mercury exposure, is kinda a red herring. Let me mention an alternative explanation:
If you have syphilitic sores, and you make them go away with mercury, you can then have sore-free genitals, which are in higher demand for sex partners than sore-ridden genitals. Your life expectancy, and mental qualities, are somewhat secondary concerns, on some level for many people. At least for some situations and applications.
Um, you ARE aware that the red stuff is just dyed alcohol? Perfectly safe. Don’t try to get drunk on it, however, 'cuz evaporation will take most of it off you as soon as you smash the glass, you’ll be smashing thermometers all night.
The silvery-grey stuff, on the other hand, is only dangerous in that it has a small vapour cloud around it, and that is what gets into your system. Quicksilver can be swallowed and touched and only a very low level of poisoning will result because it doesn’t get well absorbed by skin. “Animal data indicate less than 0.01% of ingested mercury is absorbed through the intact gastrointestinal tract”.
However, absorb enough of it, and it is viciously toxic.
It is highly reactive with selenium, and selenoenzymes “prevent and reverse oxidative damage” in the brain and hormone-producing organs. So, mercury interferes with the garbage collection system of your brain, the biggest oxygen user, which leads to oxidative damage to brain cells. Chronic exposure leads to tremors, impaired cognitive skills, and sleep disturbance. Get a higher dose, you’ve got tremors, emotional lability (irritability, excessive shyness, loss of confidence, nervousness), insomnia, memory loss, weakness, muscle atrophy, muscle twitching, headaches, cognitive function plummets. Get too much and you’re dead.
It inhibits the formation of myelin, which is why even low-level poisoning kills or severely disables fetuses.
Like most elements, quicksilver does not exist in nature, that it was produced at industrial levels to help refine silver. So, we kinda inflicted this poison on ourselves to satisfy our silverlust. (Oh, and to make our flouro-bulbs.)
Not totally true. Mercury ore, and in particular cinnibar can weep droplets of pure mercury. From many years ago I remember an article in National Geographic about mercury. One of the more stunning pictures was of a mine in Spain where the walls were red cinnibar that actually oozed liquid mercury. The occupational hazards of working there make the mind boggle. Mercury was also used for gold mining, and is still used by some illegal miners.
Actually it was lhumongus who revived the thread. So that lets you off the hook. 
Since at least two-thirds of syphilis patients never develop tertiary syphilis (which is the stage with potentially life-threatening manifestations), it’s a huge assumption that he was cured by the mercury treatment, rather than just being one of the fortunate people whose syphilis goes latent in the body and never returns.
I am glad this thread came out of latency, since it gives me a chance to post a link to a performance of one of my all-time favorite Xmas songs* (the visuals are pretty good too).
*my parents had a recording of this, which helped to warp me during my formative years.
Just like Giacomo Casanova’s experience with syphilis: There’s always The First Time!
“Jade, Cinnabar* and Salts of Mercury” One of the various formula for the alleged Elixir of Immortality of Ancient China.
And their fate is still unlearned?
Mercury was still considered a state of the art cure for health problems in the early nineteenth century. So the Lewis and Clark expedition took along a large supply. This has allowed modern archaeologists to precisely map out the expedition’s route. They just went to potential sites and tested the soil. Whenever they found mercury contamination they knew they had located a latrine.
And possibly beyond. I have read the following:
Civil War surgeons would ask soldiers in field hospitals if their “bowels were loose or slow” and give them morphine for the former and mercury for the latter; both were effective in dealing with the respective conditions.
Lincoln himself was taking a “medicine” called Blue Mass which contained lots of mercury
The dentistry profession had its own “civil war” over usage of mercury as a filling material – the mercury guys won
The treatment of syphilitic sores with mercury topically I am guessing worked; mercury is definitely something that kills micro-organisms-- as mentioned above, before penicillin, mercury was the only game in town (I don’t know if arsenic compounds were supposed to be used topically). Remember Mercurochrome? That was still around in the USA in the 1960s and maybe later.
Mercury was also used in the logging industry to prevent wood from rotting and thus tons of the element were deliberately dumped into rivers. It was also used as a fungicide in wallpaper.
When I was at school we thought that mercury was great fun to play with. You could roll it around in little balls and we used to use our fingers to rub it on to copper pennies to make them look silver.
Mind you - we also used to play with metallic sodium. That’s great fun if you drop it into some water. On one occasion we were doing just that in a sink in the lab, when a teacher came in. Some bright spark washed it down the plughole. WHOOSH!! He was lucky to get his face out of the way.
Mercurochrome was around until 1998. I definitely remember it long having a cherished place in my mother’s medicine cabinet throughout the 70s and 80s. I had no idea what iodine was, but definitely knew Mercurochrome!
I remember as a kid in the 50’s having a choice of iodine or mercurochrome to treat a cut. I always chose mercurochrome because it didn’t hurt. Iodine hurt like hell
There were only 2 choices back then. Thank God for whoever invented Neosporin.
I wonder if Washoe may have been referring to vermilion? There was no mention of thermometers in his/her post and none in the post that it was a response to.
from here: Mercury (element) - Wikipedia
Kind of ironic when you consider that it is commonly suspected that M. Lewis offed himself in the throes of syphilitic dementia. Must not have done him much good.
One important question asked was whether the dementia syphilis patients exhibited was in fact caused or exacerbated by the mercury they were taking or if indeed many of the symptoms of the disease were just mercury intoxication.
I wonder what the answer to the following question is: Is it better to take mercury than doing nothing in the case of syphilis?
My uneducated guess is that mercury might be a reasonable treatment topically although even in that case, it is probably only cosmetic. Or is it possible that the topical usage in the case of a sore might actually prevent the further spread of the illness, at least sometimes?
Untreated syphilis was one of mankind’s great curses, even after Dr. Ehrlich discovered salvarsan. The term “boogie” refers to the violent shaking observed in tertiary syphilis sufferers. Kurt Vonnegut wrote that such people were a common sight, in the pre-WWII American South-it was something that people knew all too well.
Recently, doctors have found evidence that Adolf Hitler was a tertiary syphilitic-his doctor either missed the symptoms, or feared reporting it might cause HIS demise.
No, I was talking about the stuff in outdoor thermometers. That’s not mercury? I’ve always thought it was because when I was a kid the old timers would look at the thermometer on the outside of the house and say something like “the mercury is rising.” Were they confusing thermometers with barometers or something?
Depending on your age, when you were a kid many (most?) thermometers likely did contain mercury, even ones for measuring a kid’s temperature. However, comments like “the mercury is rising” sound more like they were looking at a barometer (you guys still measue air pressure in mmHg, if I’m not mistaken). If the air pressure is rising it is an indication (just an indication) of good weather to come.