If you could measure and weigh the water in your kettle to begin with, and then measure and weigh all the steam into which the heat of the fire changes it, you would find that the bulk of the steam was nearly 1,700 times as great as the bulk of the water, though the weight of the steam would be exactly the same as that of the water. If you had a small square cup like a die, the inside measure of which was exactly one inch each way, it would hold one cubic inch of water. If this cup full of water were heated till all the water was turned into steam, the steam would nearly occupy a cubic foot; since there are 1,728 cubic inches in a cubic foot. A cubic inch of water weighs 252½ grains, and the steam into which it is converted has just the same weight. Thus we may say that steam is water expanded by heat into a vapour which is of 1,700 times less specific gravity than water. On the other hand, a pint of steam allowed to cool, becomes converted into a quantity of water, which measures only 1/1700th of a pint, though it weighs just as much as the whole pint of steam did. The steam, therefore, is =condensed= to a 1/1700th of its volume of water. The power with which water expands when it is converted into steam is very great. If you were to stop up the nozzle of the tea-kettle, the steam, inside the kettle, in trying to expand, would burst open the lid; and if you were to fasten down the lid, it would pretty soon burst the kettle itself. You sometimes hear of the strong boilers of steam-engines being burst in this way. 35. =Gases or Elastic Fluids. Air.= Here is a glass flask with a long neck and an open mouth. If we pour water in at the mouth until it rises to the lip we say that the flask is full of water. If we now pour the water out we say that the flask is empty. But is it empty? Press the flask mouth downwards into a glass jar full of water. If the flask were empty there would be no reason why the water should not enter the neck of the flask and stand at the same height inside the neck as it does outside. If you take an “empty” glass tube open at each end and press it down into the water, the water inside and the water outside will stand at the same level. But if you put your finger on the upper end of the tube so as to convert it into a closed vessel, the water will enter the lower end only a little way. So with the flask, the water enters the neck only a little way. Hence there is something inside the “empty” tube and in the “empty” flask; something which is material, because it occupies space and offers resistance. In fact the flask is full of that form of matter which is termed =air=, a thick coat of which surrounds the earth as the =atmosphere=. Air has weight, as you will learn more fully by and by; and that air in motion can transfer that motion to other bodies you are taught by the effects of the winds, which are merely air in motion.