If you were obliged to pick out a ration which would keep you alive, give you working power, and fit into the smallest possible bulk, you would take a protein, a sugar, and a fat in about equal amounts. Indeed, the German emergency field-ration, intended to keep soldiers in the field for three or four days without their baggage-wagons, or cook-trains, is made up of bacon, pea-meal, and chocolate. A small packet of these, which weighs only a little over two pounds, and which can be slipped into the knapsack, will, with plenty of water, keep a soldier in fighting trim for three days. Butter. The most useful and wholesome single fat is the one which is in greatest demand--butter. This, as we have seen, is the churned and concentrated fat of milk, to which a little salt has been added to keep the milk-acid (_lactic acid_) which cannot be entirely washed out of it, from "turning it sour" or rancid. The rancid, offensive taste of bad or "strong" butter is due to the formation of another acid call _butyric_ ("buttery") _acid_. Butter is the best and most wholesome of our common fats because it is most easily digested, most readily absorbed, and least likely to give rise to this butyric acid fermentation. We should be particularly careful, even more so almost than with other foods, to see that it is perfectly sweet and good, because when we swallow rancid butter, we are simply swallowing a ready-made attack of indigestion. Most people's stomachs are strong enough to deal with small amounts of rancid butter without discomfort; but it is a strain on them that ought to be avoided, especially when good butter is simply a matter of strict cleanliness and care in handling and churning the cream, and of keeping the butter cool after it has been made. Plenty of sweet butter is one of the most important and necessary elements in our diet, especially in childhood. And if children are allowed to eat pretty nearly as much as they want of it on their bread or potatoes, and plenty of its liquid form, cream, on their berries and puddings, it will save the necessity of many a dose of cod-liver oil, or bitter physic. Cream is far superior to either cod-liver or castor oil for keeping us in health.