568 



SCIENCE 



[N. S. Vol. XXVII. No. 693 



was observed. The generation of the 

 hydrogen peroxide completely explains the 

 abnormally high E.M.F. developed between 

 two metals so close together in the electro- 

 chemical series. 



The Heat of Dilution of Aqueous Salt Solu- 

 tions: F. L. Bishop. 



At the meeting of the American Physical 

 Society held in Chicago in 1906 the author 

 read a paper describing a new form of 

 calorimeter for certain thermochemieal 

 measurements and gave some preliminary 

 results on the heat of dilution. The ap- 

 paratus has since been perfected, tested 

 and determinations made of the heat of 

 dilution of sodium, barium, and potassium 

 nitrates and potassium chloride when dis- 

 solved in water. 



These results show that the heat of dilu- 

 tion is a linear function of the concentra- 

 tion when the concentration is expressed in 

 weight normal solutions. If we represent 

 by U the heat in calories per gram-molec- 

 ular weight of the substance and by C the 

 concentration, then dU/dC is a constant 

 within the limits of concentration used. 

 Now since C = I/v where v represents the 

 volume of water in which one mol of the 

 substance is contained, we have that 

 dU/dv = a/v^ where a is some constant. 

 This shows that as the concentration ap- 

 proaches zero the heat effect caused by the 

 addition of a definite amount of solvent dv 

 to the solution approaches zero. These 

 results would seem to show that there is 

 a similarity between the heat of dilution of 

 a solution and the Joule-Thomson effect 

 in gases for which, as is well known, the 

 same expression holds true. They also 

 indicate that the osmotic pressure P, like 

 the gas pressure in van der Waals's equa- 

 tion, must be corrected by a term a/v^ 

 representing the attraction of the mole- 

 cules. 



Determination of the Fusion Points of 

 Sodium Nitrate and Potassium Nitrate: 

 Floyd R. Watson, University of Illinois. 

 The object of the investigation is to de- 

 tei-mine a series of definite melting points 

 of salts from 300° to 1,400° centigrade 

 that will be helpful in the calibration 

 of thermo-elements and resistance ther- 

 mometers. 



The determination of the fusion points 

 of the nitrates of sodium and potassium 

 has developed a method that is sensitive, 

 and has opened the way for the more dif- 

 ficult problem of determining fusion points 

 of salts that melt at higher temperatures. 

 The salts were melted in a hard glass 

 tube which was surrounded first by a small 

 air space, then by a hot bath of mixed 

 salts. The heating was effected by gas 

 flame and electric current. Temperatures 

 were found by means of a copper constan- 

 tan thermo-element ; the E.M.F. of the 

 latter being determined by a potentiometer. 

 Readings of E.M.F. were taken to micro- 

 volts by having the bridge wire of the 

 potentiometer ten meters long, and by 

 arranging the bridge resistance to be only 

 a small part of that of the potentiometer 

 circuit. A certificated cadmium cell was 

 used as a standard E.M.F. The thermo- 

 element was calibrated for ice, steam and 

 boiling sulphur points. 



Results of a number of preliminary ob- 

 servations give the temperature of fusion 

 of sodium nitrate as 302° centigrade, and 

 of potassium nitrate as 331°. Apparatus 

 is now being constructed that will allow 

 these results to be verified with a platinum 

 platinum-rhodium thermo-element, and 

 which will also furnish a means of deter- 

 mining fusion points of salts that melt at 

 higher temperatures. 



A New Apparatus for Measuring Electro- 

 lytic Resistance: Arthur Whitmorb 

 Smith, University of Michigan. 



I 



