22 SMITH'S INTERMEDIATE CHEMISTRY 



out chemical changes in closed vessels (Fig. 14), which permit 

 neither escape nor access of material, we find that the weight does 

 not alter. 



Physics in Chemistry. It will be seen that one cannot 

 accomplish anything in chemistry without acquiring and using 

 some knowledge of physics. We measure quantities by means of 

 the physical attributes, weight and volume. We produce chemi- 

 cal change by arranging the physical conditions, for example, by 

 mixing, heating, or using an electric current. Physical means are 

 the only means we possess for producing, stopping, or modifying 

 chemical changes. Again, we ascertain whether a chemical 

 change has taken place or not by observing the physical properties 

 of the materials before and after the experiment. Thus, we noted 

 that the red, powdery oxide of mercury, when heated, gave a 

 liquid metal and a gas. All the phenomena of chemistry are 

 physical. A phenomenon is literally something that is seen or, 

 more generally, something that affects any of the senses. Ob- 

 serving physical phenomena is, therefore, our sole means of study- 

 ing chemical changes. Chemical work is, in fact, entirely de- 

 pendent upon the skilful use of physical agencies, and upon the 

 close observation of physical phenomena for its success. 



It is only the inference, following the experiment and the obser- 

 vation, that is strictly chemical. If one substance gives two 

 different substances, or if two substances give one different sub- 

 stance, for example, we infer that a chemical change has occurred. 

 We then try to recognize the substances by their properties and 

 name them. 



Changes like that of ice into water, or of water into steam, and 

 vice versa, are not regarded as chemical changes. These are 

 called changes of state (see p. 64). 



Law: Explanation: Scientific Method. There is a widely 

 spread impression that a science, like chemistry is a part of 



