"But what has the water to do with the fat burning?" persisted Margaret.
"I will try to explain, if you on your part will try to understand something which is difficult to understand. First of all, what is boiling water?"
"It is water which is so hot that it bubbles all over, and steam rises from it."
"Quite so. If we were using a thermometer, and were to put it into water which was bubbling all over, we should find that the silvery line, or mercury, in the thermometer rose until it came to 212°. We might put a hotter fire under the water, but under ordinary circumstances we should never get the mercury higher than 212°. Under extraordinary circumstances, I confess we could get it higher. For instance, if we were at the bottom of a mine, boiling-point would be two degrees higher, and if we were to put some salt in the water, boiling-point would be four degrees higher."
The little girls listened very attentively while Mrs. Herbert was speaking. When she paused, they looked very solemn, and said nothing.
"Fat, on the other hand, can be made very much hotter: more than three times as hot as boiling water. When heat is first applied to fat, it bubbles, but as it gets hotter it becomes still. As it gets hotter and hotter, it remains still, but it turns dark, and smokes, and smells burnt. This is what would happen to our fat in the tin if we were to let it come in contact with the heat of the oven shelf; but you can see that when water, which never rises beyond 212°, is under it, it cannot burn in this way."
"I see that perfectly," said Margaret, joyfully. "I like to be told difficult things when once I understand them. But, mother, will not the water boil away?"
"Yes; we must watch it, and as it does so, we must add fresh boiling water. It would never do to add cold water, because that would make the fat too cool, and would lessen the heat of the oven also."
"We should have to open the door, though, to see how the water was getting on," said Mary. "Would not that be a pity?"
"It would have to be done in any case to baste the meat," said Mrs. Herbert. "Remember, we can no more dispense with basting in baking meat than we can in roasting it before the fire. If we try to do so, our meat will be spoilt. We must baste every quarter of an hour, and to do this we must lift the meat right out of the oven, and shut the door as soon as possible. If we were to baste the meat while it was in the oven, the latter would become cool, and we wish to keep the heat up the whole time. We should be careful also to shut the oven door gently. If we slam it, we shall force some of the hot air out of it."