14 SCIENTIFIC AMUSEMENTS. 



Still more marked effects are produced when gases are strongly 

 compressed by a powerful pump or by generating them by chemical 

 action in a confined space; the gas becomes converted into a 

 liquid, just as steam condenses to water on cooling ; but in the 

 case of many compressed gases becoming liquefied, cooling is not 

 indispensable as it is with steam. Some gases require to be cooled 

 as well as compressed before they will liquefy ; with a few a very 

 enormous amount of cooling and most intense pressure are both 

 necessary to produce the effect. 



Change of State produced by Causes involving Chemical 

 Action. 



The term "change of state" in the narrowest sense is only 

 applicable to such alterations of condition as take place with 

 water when moderately heated so as to produce steam or aqueous 

 vapour, or chilled so as to freeze to ice ; or, conversely, when ice 

 melts or steam condenses again to liquid water ; in each case one 

 substance only is acted upon, producing only one product when 

 the action is complete.* But in a wider sense "change of state" 

 includes a number of actions where, by the application of heat 

 or electricity, solids give rise to liquid or gaseous products, liquids 

 to gases, or, conversely, gases to liquids or solids ; the most marked 

 feature in each case being that the number of products formed is 

 not the same as the number of kinds of materials employed. 

 Such actions are spoken of as chemical changes, and are divided 

 into two classes, viz., those where decomposition or breaking 

 up occurs, in which cases the number of products is greater than 

 the number of different materials used ; and those where combi- 

 nation or synthesis occurs, in which cases the number of products 

 is less than the number of materials. 



Thus when water is heated to a much higher temperature than 

 that required to convert it into steam, or when it is subjected to 

 the action of electricity under certain conditions, the water (or 

 steam) disappears, and in its stead there are obtained two entirely 

 new substances, both of which are gases analogous to the atmo- 

 sphere, but quite different from each other and from steam ; one 

 of these gases is termed hydrogen and the other oxygen ; both of 

 them will be frequently met with in our subsequent experiments, 

 being capable of formation in other ways besides this "decom- 

 position " of water into its two constituents. On the other hand, 



* Under certain conditions solid ice, liquid water, and a small amount of 

 aqueous vapour may be simultaneously present. 



