A. The moment the water in the larger pot is boiling hot (or 212°), steam is formed, and carries off some of its heat; therefore, 212 degs. of heat can never pass through it, to raise the inner vessel to the same heat.
Q. Why do sugar, salt, &c. retard the process of boiling?
A. Because they have a tendency to fix water by chemical attraction; and therefore retard its conversion into steam.
Q. If you want water to boil, without coming in contact with the saucepan, what plan must you adopt?
A. Immerse the pot (containing the water you want to boil) in a saucepan containing strong brine, or sugar.
Q. Why would the inner vessel boil, if the outer vessel contained strong brine?
A. Though water boils at 212 degs. of heat, yet brine will not boil till raised to 218 or 220 degs. Therefore, 212 degs. of heat may easily pass through brine to raise the vessel immersed in it to boiling heat, before any of it is carried off by steam.
Q. Why will brine impart to another vessel more than 212°, and water not so much?
A. Because both liquids will impart heat till they boil, and then they can impart heat no longer.