276 Dr. C. R. A. Wright on the Determination of 



nations were first made of the E.M.F. set up in various forms 

 of Daniell cell when generating currents of magnitude not 

 exceeding 8 microamperes per square centimetre (and usually 

 when generating no current at a\l),the zinc- and copper-sulphate 

 solutions being in any given case of the same specific gravity*. 

 The average values being thus fixed, the observations were 

 then repeated, using solutions not of the same specific gravity. 

 It is particularly noteworthy in this connexion, that the E.M.F. 

 of a Daniell cell was found to be sensibly independent of the 

 strength of the solutions when both are of the same specific gra- 

 vity — i. e. the deviations observed from equality were less than 

 the experimental errors. 



The cells employed were constructed of two small beakers 

 — one containing zinc-sulphate solution and plates of zinc 

 (scraped bright, covered with electro-deposited metal, or 

 amalgamated with pure mercury), and the other copper-sul- 

 phate solution and similar copper plates. The two beakers 

 were connected, in the way described by Raoult (Ann. de Chim. 

 et de Phys. 4th series, ii. p. 317, and iv. p. 392), by means of 

 an inverted Y-tube, the ends of which, dipping into the two 

 beakers respectively, were covered over with thin bladder, the 

 tube being filled with the zinc-sulphate solution. Each one 

 of the zinc and copper plates used was soldered to a platinum 



* Although two solutions of zinc and copper sulphate, respectively of 

 the same specific gravity, are not absolutely chemically equivalent to one 

 another (i. e. do not contain precisely equivalent percentages of the two 

 salts), yet the difference in specific gravity between any two solutions of 

 equivalent strengths is so small that, for all practical purposes, it may be 

 assumed that when the specific gravity is the same the solutions are of 

 equivalent strengths. Direct determinations of the specific gravities of 

 various solutions of equivalent strengths (made by dissolving known 

 weights of the air-dry pure salts to known weights of aqueous solutions), 

 gave the following results : — 



Percentage of 



Specific gravity 



Equivalent percentage 



Specific gravity 

 at 18°. 



CuS0 4 , 5H 2 0. 



at 18°. 



of ZnS0 4 , 7B.JO. 



1 



1-006 



1-15 



1-007 



3-75 



1-023 



4-3 



1-026 



7-5 



1047 



8-6 



1053 



15 



1-098 



17-2 



1-107 



225 



1-156 



25-9 



1-163 



30 



1-214 



34-5 



1-223 



The 30-per-cent. solution of copper sulphate was slightly supersaturated, 

 and deposited crystals on standing in a closed vessel; saturated zinc- 

 sulphate solution has a specific gravity upwards of 1-4. It is notev orthy 

 that these figures indicate that when given bulks of water and of either 

 zinc- or copper-sulphate solution are mixed, an increase in bulk occurs, thus 

 agreeing with J. Thomsen's result that dilution of a solution of either salt 

 is accompanied by heat-absorption (§ 113). 



