582 'PRINCIPLES OF CHEMISTRY 



determining the relation between its weight and that of its salt- 

 giving oxide, as by this we know the quantity of the metal which 

 combines with 8 parts by weight of oxygen, and this will be the 

 equivalent, because 8 parts of oxygen combine with 1 part by weight of 

 hydrogen. One method is verified by another, and all the processes 

 for the accurate determination of equivalents require the greatest care 

 to avoid the absorption of moisture, further oxidation, volatility, 

 and other accidental influences which affect exact weighings. The 

 description of the methods necessary for the attainment of exact 

 results belongs to the province of analytical chemistry. 



For univalent metals, like those of the alkalis, the weight of the 

 equivalent is equal to the weight of the atom. For bivalent metals 

 the atomic weight is equal to the weight of two equivalents, for n-valent 

 metals it is equal to the weight of n equivalents. Thus aluminium, 

 AJ = 27, is trivalent, that is, its equivalent = 9 ; magnesium, Mg = 24, 

 is bivalent, and its equivalent = 12. Therefore, if potassium or sodium, 

 or in general a univalent metal, M, give compounds M 2 O, MHO, 

 MCI, MNO 3 , M 2 8O 4 , &c., and in general MX, then for bivalent 

 metals like magnesium or calcium the corresponding compounds 

 will be MgO, Mg(HO) 2 , MgCl 2 , Mg(NO 3 ) 2 , MgS0 4 , &c., or in general 

 !MX 2 . 



By what are we to be guided in ascribing to some metals uni- 

 valency and to others bi-, ter-, quadri-, w-valency 1 What obliges 

 us to make this difference ? Why are not all metals given the same" 

 valency for instance, why is not magnesium considered as univalent ? 



on a large scale, together with the employment of electricity for obtaining high 

 temperatures, &c., makes me regret that the plan and dimensions of this book, and the 

 impossibility of giving a concise and objective exposition of the necessary electrical facts, 

 prevent my entering upon this province of knowledge, although I consider it my 

 duty to recommend its study to all those who desire to take part in the further develop- 

 ment of our science. 



There is only one side of the subject respecting the direct correlation between thermo- 

 chemical data and electro-motive force, which I think right to. mention here,- as it 

 justifies the general conception, enunciated by Faraday, that the galvanic current is an 

 aspect of the transference of chemical motion or reaction along the conductors. 



Prom experiments conducted by Favre, Thomsen, Garni, Berthelot, Cheltzoff, and 

 others, upon the amount of heat evolved in a closed circuit, it follows that the electro-, 

 motive force of the current or its capacity to do a certain work, E, is proportional to the 

 whole amount of heat, Q, disengaged by the reaction forming the source of the current. 

 If E be expressed in volts, and Q in thousands of units of heat referred to equivalent 

 weights, then E = 0'0436Q. For example in a Daniells battery E = 1'09 both by experi- 

 ment and theory, because in it there takes place the decomposition of CuSO 4 into Cu + O 

 together with the formation of Zn + O and ZnO + SO 3 Aq, and these reactions correspond 

 to Q = 25-06 thousand units of heat. So also in all other primary batteries (e.g. Btmsen's, 

 Poggendorff's, &c.) and secondary ones (for instance, those acting according to the 

 reaction Pb + H,S0 4 + Pb0 2 , as Cheltzoff showed) E = 0'0436Q. 



