500 



On the Theory of Osmotic Pressure. 



Direction of the Deviations from Constancy with Stronger 

 Solutions. 



According to the theory of osmotic pressure the propor- 

 tionality between pressure (lowering of freezing-point) and 

 amount of substance dissolved will hold good, as has been re- 

 marked, only for very weak solutions. With stronger solutions 

 the dissolved molecules will be brought within the sphere of 

 each other's attraction and will, therefore, exert an abnormally 

 small pressure ; that is ; they will lower the freezing-point of 

 the solvent to an abnormally small extent*. 



But in every case at present known exactly the reverse is 

 the case. Stronger solutions exhibit an abnormally large 

 depression, and the freezing-points fall at an increasing instead 

 of a diminishing rate. The table given above illustrates to 

 what extent this takes place in the case of solutions of sul- 

 phuric acid in water, the molecular depression increasing 

 from 2 o, 08 to 6°*54 (213 per cent.), and the examination of 

 six other curves from the sulphuric-acid series, and two of a 

 series with calcium nitrate, supplies us with eight other in- 

 stances of a similar character. The results may be summarized 

 thus : — 



Solvent. 



Dissolved substance. 



Molec. depression. 



Increase. 



100 H 2 S0 4 



1 to 33 H 2 



1°072 to 1°253 



17 percent, 



100 H.SO, 



1 to 5-8 S0 3 



0-422 to 634 



50 



100 (H,S0,H,O) 



1 to 50 H 2 



0-02 to 0-31 



1,450 



100 (H.SO^O) 



1 to 50 HoSO t 



001 to 0-197 



1,870 „ 



100 (H.,S0 4 4H.,0) 



1 to 400 H.,Ot 



0-078 to 0-108 



39 „ 



100(H.,SO 4 4H,O) 



5to60H 2 SO 4 + 



0-27 to 0-42 



55 „ 



100(0a(NO 3 ) 2 4H,O) 



1 to 150 H.,0 



0-007 to 0-326 



45,600 „ 



100(Ca(NO 3 ) 4 4H,O) 



1 to 33Ca(N0 3 )„ | 



0015 to 0-179 



11,000 „ 



It is true that these results have only recently been placed 

 tat the disposal of the supporters of the osmotic-pressure 

 theory, but it is otherwise with the following values in the case 

 of alcohol (Kaoult, Compt. Rend. xc. p. 863) and calcium 

 chloride (Hammerl, Wien. Sitzungsber. lxxviii. p. 59) §: — 



* The same will hold good if we attribute dissolution to the balancing 

 of the physical attractions of (1) solvent for solvent molecules, (2) dissolved 

 for dissolved molecules, and (3) solvent for dissolved molecules. The 

 stronger the solution the greater will be (2), and, consequently, the less 

 will (1) be counterbalanced. 



t With 60 H 2 the depression is o, 067, so that it first decreases and 

 then increases. 



X With 1 mol. H 2 S0 4 it is 0°-40. 



§ In each case I give the values as determined from my own results, 

 but these are in good accord with those referred to above. 



