certain point before the oxidation can begin, and this point can not be reached until the moisture is evaporated, which uses up a good deal of heat.

This process of decay is continually going on in our bodies; but during life the matter which is burned up is being constantly renewed from the food we eat. The body is not only decaying, as dead animal matter decays, but it is also wearing out. With every motion a part of the muscles is actually consumed, and must be replaced by fresh material. The heat of the body is likewise due to combustion, and must be kept up by proper fuel, like the fires in our stoves and furnaces. The products of all this burning are carbonic acid and water, which pass out of the body through the lungs.

The rusting of metals is a slow combustion, and scientific men have proved that, like decay, it develops heat. Iron can be easily burned in pure oxygen, with the production of intense light and heat. Zinc and some other metals can be burned in the air if heated very hot, and most metals are rapidly consumed in the flame of the oxyhydrogen blow-pipe. Indeed, every form of matter known to us can be burned, unless it has already been burned. All substances belong to one of these two classes—those that will burn, or unite with oxygen; and those that have been burned, or are products of oxidation. Water belongs to the latter class, and so do nearly all the rocks and solid matter of the earth.

Slow burning sometimes becomes rapid, and then we have what is called spontaneous combustion. When cotton or tow which has become soaked with oil is laid aside in heaps, the oxygen of the air begins to unite with it; but the heat developed causes the oxidation to go on faster and faster, until in some cases the mass bursts into a flame. The same thing sometimes takes place in moist hay, the moisture starting the process, as explained above, and the confined heat increasing until it is sufficient to set the heap on fire.

[By special arrangement with the author, the cards contributed to this useful series, by W. J. Rolfe, A.M., formerly Head-Master of the Cambridge High School, will, for the present, first appear in Harper's Young People.]

GETTING WEIGHED.

DAVE'S GREAT LUNCH.

BY J. B. MARSHALL.

It was the great day at the State Fair, and the sidewalks were nearly deserted as Dave Burt went down Main Street toward the post-office. As Dave approached the Town Hall, or the City Hall, as the good people of Rawley were pleased to call that fine building, he glanced up at it, and saw Mr. William Henry Barrington, the great lawyer, standing at one of the large windows of his office. Mr. Barrington was frowning, and looked up the street and down it as if impatiently waiting for some one.

"I'll bet he's mad 'cause he can't go to the fair," thought Dave.

A few days before, Billy Barrington, a nephew, had been telling the boys of that fine office, with its brass-studded revolving chairs, great bookcases of books, and a private room where the great lawyer ate his dinner, which was sent up to him on a dumb-waiter from the restaurant in the basement of the City Hall the moment he touched an electric bell.

Dave was recalling all the delightful possibilities of such a room, when click! went something on the pavement before him.

"A penknife," said he, picking up the article, and then, looking in vain among the branches of the tree for its owner. Examining the knife, he noticed a slip of paper shut in under the largest blade, and on which was written: