OXYGEN. 143 



is exposed to air, and also during a number of chemical decomposi- 

 tions. Ozone differs from ordinary oxygen by possessing a peculiar 

 odor, by being an even stronger oxidizing agent than common oxygen, 

 by liberating iodine from potassium iodide, etc. This latter action 

 may be used for demonstrating the presence of ozone by suspending 

 in the gas a paper moistened with a solution of potassium iodide and 

 starch. The iodine, liberated by the ozone, forms with starch a dark- 

 blue compound. Theoretically, we assume that ozone contains three, 

 common oxygen but two, atoms in the molecule, which is substan- 

 tiated by the fact that three volumes suffer a condensation to two 

 volumes when converted into ozone, which would indicate that three 

 molecules of oxygen furnish two molecules of ozone, thus : 



30 2 = 20 3 ; or 3 [O = O] =2 



[A] 



Ozone is obtained in a pure condition by passing the impure gas through a 

 tube cooled by liquid oxygen. It is then a blue liquid which boils at 110 C. 

 ( 166 F.), forming a blue gas. Atmospheric air, in which part of the oxygen 

 has been converted into ozone by the electrical method, is used for bleaching 

 purposes, purification of starch, resinifying oils, purifying water of germs and 

 organic matter, etc. 



Ozone occurs in small quantities in country air, but is rarely noticed in 

 cities, where it is decomposed too quickly by the impurities of the atmospheric 

 air. It has been assumed that ozone acts advantageously, as it has a tendency 

 to destroy matters which are unwholesome. Too little, however, is known of 

 the subject to justify a positive opinion in regard to it. 



Thermo-chemistry. It is stated in Chapter 5 that the free or available 

 chemical energy in a chemical change usually appears as heat. This heat can 

 be measured in calories in an apparatus called a calorimeter (see page 48). 

 The equations ordinarily used to represent chemical changes do not express 

 energy changes, but simply what kinds of substances are concerned in the 

 change, and what new substances are formed. For example, the expression, 

 2H -}- O = H 2 O, when translated means that when hydrogen and oxygen unite 

 water is formed, but it says nothing about the fact that a great amount of chem- 

 ical energy is liberated as heat. Likewise the expression, HgO = Hg + O, 

 which means that when mercuric oxide undergoes decomposition (by heat) 

 mercury and oxygen are formed, says nothing about the fact that during the 

 change, heat energy is "absorbed and transformed into chemical energy. For 

 the purpose of showing the energy change involved, use is made of thermal 

 equations. The amount of heat energy in calories represented in thermal equa- 

 tions as liberated or absorbed refers to certain weights of the substances in- 

 volved in the chemical change. These weights are the number of grammes cor- 

 responding to the chemical symbols of the substances. For instance, the thermal 

 equation for the formation of water is written, 2H + O = H 2 O + 67,883 cal., 

 which means that when 2 grammes of hydrogen unite with 15.88 grammes of 

 oxygen to form 17.88 grammes of water (corresponding to the symbol H 2 O), 



