;o6 



NATURE 



[January 30, 1902 



of conductivity at o". These determinations were made 

 by Mr. )\. I). Hall. {4) Cryoscopic determination of the 

 molecular weights. The results of (i) and (2), and also of (3) 

 and (4), were comparable, being under similar conditions. 



The conductivity determinations were made by means of the 

 usual Kohlrausch method with a telephone. The measurements 

 at o" were made in baths of melting ice. Those near the boiling 

 point were not carried out at the boiling point, but at 95 ', as 

 small gas bubbles were apt to form at the electrodes at 100". 

 The freezing-point determinations were made with a regular 

 Beckmann's apparatus of large size, about 40 grammes of water 

 being used in each case. The solutions were cooled only from 

 two to three tenths of a degree below their freezing points, and 

 the crystallisation was inaugurated by means of a point of ice. 

 The boiling-point determinations were made with a Beckmann's 

 apparatus of about double the ordinary size, and thermometers 

 graduated to o°oi. It was at first thought best to surround 

 the thermometer with a platinum cylinder in the boiling tube, 

 as recommended by Jones, but fluctuations in the boiling point 

 were found to result, apparently due to the solution within 

 the cylinder being more concentrated than that without. 



The water used was distilled in a block-tin condenser and had 

 its conductivity reduced to 2 x 10-'' by drawing air through it 

 free from carbon dioxide. In the results given, the conductivity 

 of the water at the proper temperature has been deducted. 

 Water of crystallisation was determined and allowed for in 

 making up solutions, these being based on the amount of 

 anhydrous salt present. 



Conductivity measurements at o" and at 95" are given for NaCl, 

 KCl, KBr, KI, MgCU, BaCl.,, HgCU, KCIO,, KNO3, AgNO,, 

 MgSOj, ZnSOj, MnSOj, CdSOj, NiSOj, C0SO4, FeSOj and 

 CUSO4. The volume in litres containing a gramme equivalent 

 was varied from ^ to 8192 in the case of the determinations at 

 0°, and from \ to 2048 in the case of those at 95". The results 

 show an increase of the equivalent conductivity with dilution 

 and the same trend in the curves at the two temperatures, but 

 they are not parallel. For example, the curve between 

 equivalent conductivity and the cube root of the volume is 

 nearly a straight line for MgSOj at 95°, but much more curved 

 at o". Curves of salts belonging to any one group all have the 

 same trend. 



The freezing-point determinations include NaCl, MgSOj, 

 ZnSOj, MnSOj, CdSOj, NiSO^, CoSOj, FeSOj and CuSOj. 

 The results are summarised below : — 



Sodium Chloride. — For about 0-2 normal solution the 

 molecular weight was found to be 32-6, equivalent to 79*4 per 

 cent, ionisation ; for an approximately normal solution (the 

 strongest used) the molecular weight was 31-7, equivalent to 84 

 per cent, ionisation. According to the conductivity tests the 

 ionisation is about 79 per cent, for a o'2 normal solution and 

 70 per cent, for the normal solution. The results are, there- 

 fore, about the same by both methods for the dilute solution, 

 but whereas the ionisation increases rapidly with dilution 

 .iccording to the conductivity, it remains constant or diminishes 

 according to the cryoscopic method. This result is confirmed 

 by the work of C. Dieterici and of K. W. Wood. 



Magnesiiitit Sulphate. — The limits were about o'l and 1-5 

 normal. The degree of ionisation for the first was 40 per cent, 

 and for the second only 5 per cent. According to the con- 

 ductivity measurements the ionisation should be 44 and 22 per 

 cent, respectively, showing an increasing discrepancy with 

 concentration. 



Zinc Sulphate shows no ionisation in a normal solution, yet 

 the conductivity is nearly the .same as that of MgSOj, and indi- 

 cates 24 per cent, ionisation. The molecular weight in the 

 strong solutions was above the normal. 



Mnni;anous Sulphate shows at first an increase of molecular 

 weight with concentration and then a decrease. The same is 

 true of ZnSOj and CdSOj, and to a slight extent of NiSOj, 

 C0SO4 and Cu.SOj. According to the conductivity of these 

 solutions, the ionisation increases constantly with the dilution, 

 but according to the cryoscopic measurements there is first a 

 decrease and .then an increase with increasing concentration. 

 An approximately N/4 solution of MnSOj gave a molecular 

 weight of I25'2, or 21 per cent, ionisation, the conductivity 

 method giving 35 per cent. In a solution giving a molecular 

 weight of 146-5 the ionisation is 3 per cent., whereas conduc- 

 tivity indicates 20 per cent. 



Cadmium Sulphate, though a good conductor, shows no 

 ionisation except in the most dilute solution {3-071 gm. CdSOj 



NO. 1683, VOL. 65] 



in 100 gm. of water), which gave 12 per cent, ionisation, the 



conductivity indicating 30 per cent. 



Nickel Sulphate appears to be un-ionised when the strength is 

 10 per cent., but the conductivity shows 22 per cent, ionisation. 

 In the most dilute solution the two methods gave about the 

 same result. 



Cobalt Sulphate. — The freezing point shows no ionisation 

 when the solution is 5 per cent, or stronger, whereas the con- 

 ductivity indicates 26 per cent, when the observed molecular 

 weight is 155-2. In the most dilute solution the molecular 

 weight was 131-8, corresponding to iS per cent, ionisation, the 

 conductivity indicating 34 per cent. 



Ferrous Sulphate also is un-ionised in 6 per cent, solutions or 

 above, according to cryoscopic determinations, yet the conduc- 

 tivity indicates 24 per cent, ionisation when the observed mole- 

 cular weight is 154-8 {i.e. above the normal), and ionisation 

 should be absent. The most dilute solution showed a molecular 

 weight of 135-8, or 12 per cent, ionisation, the conductivity 

 indicating 30 per cent. 



Copper Sulphate is like the last two salts. When the 

 observed molecular weight is l63"9, corresponding to no ionisa- 

 tion, the conductivity indicates about 22 per cent. In the most 

 dilute solutions tested the molecular weight corresponds to 38 

 per cent, ionisation and the conductivity to 32 per cent. 



The results obtained with copper sulphate are as follows, 

 similar results for the other salts being given in the original 

 paper : — 



Copper Sulphate {CuSOt). Molecular Weight, 1597. 



The agreement, therefore, of the methods, viz. conductivity 

 and freezing points, is poor, even in the dilute solutions. 

 Arrhenius originally gave figures from cryoscopic measurements 

 indicating no ionisation for MgSOj, FeSOj, CuSUj, ZnS04, 

 CdSOj and Cdl.,, whereas ionisation was indicated by conduc- 

 tivity. He sought to explain this, in the case of the sulphates, 

 by polymerisation of the un-ionised molecules, basing this 

 assumption on the fact that Hittorf found the migration numbers 

 of MgSOj and ZnSOj to show a considerable variation with 

 concentration. This was also true of CdL, for which Hittorf 

 consequently assumed double molecules, and applied the same 

 explanation to other salts of the magnesia series. This at first 

 seems to justify the position taken up by Arrhenius. However, 

 the latter has not applied the explanation to all salts of the 

 magnesia series, but has assumed polymerisation simply for 

 those salts that did not behave according to his theory. MgCI, 

 is a case in point. Similarly, Hittorf found the migration 

 numbers of CaCU, BaCl.^, Ca{XO:,)., and Ba(N03)2 strongly 

 dependent on concentration, but Arrhenius did not assume 

 polymerisation, for these salts agree better with his theory. To 

 assume polymerisation in the case of MgCl.,, CaCl.. and BaCI._, 

 would render it difticult to explain the results of Jones and 

 Chambers, who found a minimum for the molecular lowering 

 between o-i and 02 normal, and that the lowering in concen- 

 trated solutions was as great as, -or greater than, that correspond- 

 ing to complete ionisation. These authors attempt to explain 

 this by assuming that the salts form hydrates. Thus another 

 theory is brought in to account for abnormally low freezing 

 points, to explain which the ionisation theory was itself originally 

 introduced. Results of a similar kind have been observed byC. 

 Dieterici. 



The boiling-point determinations given by the author refer to 

 NaCl, KCl, KBr, KI, MgCl.,, BaCU, HgCl.,, KCIO3, KNO., 

 AgNO,, MgSOj.ZnSO,, MnSOj, GdS'Oj, NiSOj, C0SO4, FeSO, 

 and CuSOj. 



In the case of XaCl, KCl, KBr and KI the molecular 

 weights continually diminish with incre.ise of concentration, 

 finally becoming less than half the theoretical values, whereas 

 the molecular conductivity increases regularly with the dilution. 



