226 Prof. E. F. Herroun on the Divergence of 



is the metal attacked, the actual E.M.F being about '008 to 

 '012 volt. On the other hand, pure tin will not remove lead 

 from solutions of its nitrate or acetate or cold solutions of its 

 chloride, though if an acid solution of lead chloride be boiled 

 with tin, lead is deposited to a slight extent. Similarly the 

 above lead -tin cell, using an acid solution of stannous chloride, 

 will be found to diminish in E.M.F. as its temperature is 

 raised, so that between 50° and 60° 0. it possesses no E.M.F-, 

 while above that temperature the current flows in the reversed 

 direction, tin being the metal attacked. This explains why an 

 alloy of lead and tin, when treated with hot hydrochloric acid, 

 yields a solution of stannous chloride, leaving the lead un- 

 attacked. That the non-production of lead chloride cannot 

 be due to the prospective absorption of heat on dissolving 

 after its formation appears sufficiently evident ; and I think 

 this reversed action at the higher temperature is to be ex- 

 plained by the heat of formation of the compound SnCl22HoO 

 being greater than that of even solid lead chloride (viz. 86,510 

 and 82,550 respectively) ; and although the former would be 

 dissolved with a certain absorption of heat dependent on the 

 strength of solution and the temperature, this amount would 

 be less than the 5370 calories absorbed at low temperatures 

 and in dilute solutions, and, further, being due to the merely 

 physical process of solution, may at the high temperature be 

 supplied largely at the expense of sensible heat, instead of being- 

 deducted from the energy of the chemical change. 



Nickel is a metal in the formation of the anhydrous 

 chloride of which 74,530 calories are evolved, while in aqueous 

 solution it furnishes 93,700 calories. If, therefore, the heat of 

 formation of the dry salt plus the heat of hydration be 

 regarded as measuring the total chemical energy and as 

 occurring together, nickel should be able to replace tin' or 

 lead from their chloride solutions. But, as a matter of fact, 

 instead of this being the case, pure electro-deposited tin is 

 capable of completely displacing nickel from a solution of its 

 chloride containing free hydrochloric acid, when allowed to 

 remain in contact for a considerable time ; and on boiling the 

 mixture of tin and deposited nickel in hydrochloric acid, tin 

 and not nickel is dissolved. Nitric acid, however, rapidly 

 attacks the nickel. 



This result is at once explained on reference to the heat of 

 formation of anhydrous nickel chloride, which, as above stated, 

 is given by Thomson as 74,530, while the heat of formation 

 of stannous chloride is 81,140 in aqueous solution, or 80,790 

 if anhydrous. The heat which would on the whole be ab- 

 sorbed by this replacement in aqueous solution would doubt- 

 less be supplied by surrounding temperature. 



