How does a vacuum keep in both heat and cold?
A vacuum flask is a handy gadget to carry in your lunch box any day of the year. In winter, it can keep a helping of hot soup hot all day. In summer, it can stop a cold drink from becoming lukewarm. The magic vacuum, however, cannot work unless the flask is sealed securely. A vacuum is no more than an empty space. There may, of course, be an empty space .on a shelf and another in the back of the bus. But these empty spaces are not vacuums because they are not really empty. True, there are no solid cane on the shelf and no solid body in the vacant bus seat. But these spaces, nevertheless, are filled up to the brim with air. A vacuum is an empty space with the air, or most of the air, removed. Air is a mixture of gaseous molecules too small for our eyes to see. We cannot see clean, pure air or taste or smell it. Air seems to be nothing at all but you would be surprised at how many things it can do. It carries sounds. When you speak, your words are shoved along by busy molecules of air. A
