HYDROGEN 53 



of hydrogen. Also, that the proportions by weight of the con- 

 stituent elements in the compound produced are always the same 

 (p. 20). If we place a weighed piece of zinc in hydrochloric acid, 

 and wait until the zinc has all disappeared, we can then boil 

 away the water and unused acid, and weigh the white, solid zinc 

 chloride. We find that 65.4 grams of zinc always leave 136.3 

 grams of zinc chloride. The difference, 70.9, is the chlorine, and 

 whatever weights we take, the proportion of zinc to chlorine in the 

 zinc chloride is always in the ratio 65.4 : 70.9. 



Zinc (65.4) + Hydrochloric acid (72.9) -> Hydrogen (2) + Z ^j^^ de 



Hydrogen 2 Zinc 65.4 



Chlorine 70.9 Chlorine 70.9 



Chemically Equivalent Quantities. It will be observed 

 that 65.4 parts of zinc displace 2 parts of hydrogen, whether 

 the acid used is sulphuric acid or hydrochloric acid. The propor- 

 tion is, in fact, the same with every acid. Hence 65.4 parts o 

 zinc and 2 parts of hydrogen are spoken of as chemically equiv- 

 alent quantities. The quantities of the displacing and of the 

 displaced element are in all cases referred to as chemically equiv- 

 alent. 



The term equivalents is applied also to the quantities liberated 

 by decomposition of a compound like the 100 parts of mercury 

 and the 8 parts of oxygen (p. 19). It is likewise used of the pro- 

 portions combining when chemical union takes place, as in the 

 case of phosphorus 31 parts and oxygen 40 parts (p. 35). The 

 proportions of the elements in zinc chloride (see above) are also 

 chemically equivalent. 



Chemically equivalent quantities (or, simply, equivalents) of 

 two substances are exact quantities which enter into or result 

 from a chemical reaction. 



The Order of Activity of the Metals. It will greatly 

 aid us in remembering a number of the facts already given, as 

 well as many others, if we compare carefully with those facts the 



