SOLUTIONS 133 



times its volume of carbon dioxide and 90 times its volume 

 of ammonia. It has such an affinity for bromine gas that if 

 a piece of it is dropped into a jar of this gas, it will absorb it so 

 strongly as to produce a vacuum in the jar. Under proper 

 conditions, charcoal may be made to absorb the air from a 

 vessel, making a more perfect vacuum than can be obtained 

 by the best air pumps. The rare metal palladium will take 

 up 800 times its volume of hydrogen, and spongy platinum 

 absorbs th s gas so rapidly that the striking of the molecules 

 against it, soon generates enough heat to raise the gas to the 

 kindling temperature. Self-lighting gas mantles are con- 

 structed on this principle. Solids are said to hold gases by 

 occlusion. Water may also be dissolved in solids and is then 

 known as water of crystallization. When gypsum and certain 

 other minerals are heated, this water is given off, but the crys- 

 talline structure is thereby destroyed, and the substance be- 

 comes a powder. Plaster t)f Paris is made by driving the water 

 of crystallization out of gypsum. 



116. Alloys. Homogeneous mixtures of metals are called 

 alloys, but in a way these are also solutions. In making alloys, 

 the metals are usually brought to the liquid state, though they 

 may slowly mix even when solid. If gold is placed in close 

 contact with lead and left for a time, particles of it may be 

 found in the lead. Gold readily dissolves in mercury, forming 

 a true solution. When working with mercury, care must be 

 taken not to get it on gold rings and the like. A large number 

 of alloys have important uses. Brass is a mixture of copper 

 and zinc. Bronze consists mostly of copper, tin and zinc. 

 German silver is made of copper, nickel, and zinc and has no 

 silver in it. Type metal is composed of lead, tin, and antimony. 

 Alloys very frequently have melting points far below the melt- 

 ing points of the individual metals composing them. Fusible 

 metals used as plugs for automatic fire extinguishers and the 

 like are made of various mixtures of lead, antimony, tin, bis- 



