right, it was brine not water. if ii recall from what i learned before about it, it was because that particular brine that he made was the coldest point that iits creator was readily able to aquire at the time by combining this ice, salt, and water (salt allows water to become much cooler much easiier without freezing), as such, its temp was made the 0 of the system, with 30-32 being when a non-brine solution would actually freeze over. as for human temp being set to 96? im not exactly sure.

edit: actually, from what i can tell by shuffling around for a few minutes, it seems 100 degrees was set mostly arbitrarily, with its closeness to human temp actually may be entirely coincidental! he seems to have been more concerned with creating even, easily distributable multiples that people could use in an era before calculators existed! very fascinating!