lO 



NATURE 



[May 5, 1887 



Table VI. — Giving percentage of hydrogen in solutions of 

 various hydrogen salts in which ice melts at given temperatures. 



On considering them, it was at once evident that the 

 lowering of the melting-point of ice followed the concen- 

 tration of the solution, but the law deviated in all cases 

 from that of strict proportionality to the amount of salt 

 dissolved, in some cases to a greater extent than in 

 others. In comparing the effects of different salts in 

 solution on the melting-points of ice, no simple connexion 

 could be traced between their absolute weights and the 

 effects produced ; but on comparing chemically equiva- 

 lent weights, a very close connexion was discovered. 

 This will be evident from the inspection of the tables. 

 In each the first column contains the temperatures at 

 which pure ice melts ; and in the parallel columns, the 

 percentages of chlorine, potassium, or hydrogen in the 

 solutions of the salts indicated at the head of each column, 

 when ice melts in them at the temperature indicated. 

 The figures thus give numbers proportional in each table 

 to the chemically equivalent weights of the different 

 salts. They show at first that, whereas the presence of 

 equal absolute weights in solution produces very different 

 effects, the presence of chemically equivalent weights 

 produces very similar effects. On closer inspection, it is 

 seen that the effects are almost identical where the ele- 

 ments to which the common constituent is united belongs 

 to the same group of the periodic series, and differ 

 sharply where these elements belong to different groups. 

 In the case of the chlorides of sodium and potassium the 

 number expressing the percentage ^ of chlorine in the 

 solution expresses equally the depression of the melting- 

 f nint of ice in terms of the Centigrade scale. The same 

 depression of melting temperature is produced by 10 per 

 cent, less of chlorine united to hydrogen, and by 30 to ^5 

 per cent, more of chlorine when united to magnesium, 

 calcium, or barium. 



The results obtained with sea-water are also given, for 

 comparison. It will be seen that it behaves very approxi- 

 mately as a solution of chloride of sodium containing the 

 same amount of chlorine. 



It is perhaps not very astonishing that unit weight of 

 potassium in saline solution should produce the same 

 effect in lowering the melting-point of ice, whether it is 

 united to CI, to I, or to OH ; but it shows clearly how 

 independent this action is of the character of the body in 

 solution when we find the effect produced by unit weight 

 of hydrogen identical whether it is united to such oppo- 

 site radicles as CI or OK. Table VI. shows further the 

 effect of valence. While a given weight of hydrogen pro- 

 duces the same effect in solution whether it be united to 

 the very different but both univalent radicles CI and OK, 

 its effect is reduced by one-half when united to the 

 bivalent SO4. That valence is not the only factor is 

 shown by comparing the effects of hydrogen and potas- 

 sium when united to the common element, chlorine. 

 Hydrochloric acid in solution produces a markedly more 

 powerful lowering effect on the melting-point of ice than 

 the equivalent amount of chloride of potassium. Of all 



1 All percentages are by iveight. 



the substances that I have experimented on, hydrochloric 

 acid is the most energetic in reducing the melting-point 

 of ice, and with ordinary strong acid and pounded ice 

 there is no difficulty in producing temperatures as low as 

 the freezing-point of mercury. In the case of hydrochloric 

 acid, sulphuric acid, chloride of sodium, and chloride of 

 calcium, I have carried my experiments to low tempera- 

 tures and great concentration. But before passing to 

 them it is well to consider the more dilute solutions with 

 regard to their density. 



That the mere density of the solution in which the ice 

 is melting has no direct connexion with the lov.ering of 

 its melting-point is shown by the following table, in which 

 the specific gravities (at 15° C.) are given of the solutions 

 of different salts which give the same depression of melt- 

 ing-point. 



There are many similarities in the effects produced by 

 greatly increasing the pressure upon pure water and by 

 dissolving salts in it. First, there is an absolute diminu- 

 tion in the volume of the solution as compared with the 

 sum of the volumes of its components ; second, in virtue 

 of this compression by molecular forces it has become 

 less compressible by mechanical means ; third, the 

 temperature of maximum density and the freezing tem- 

 perature are lowered ; and fourth, the former of these 

 two temperatures is lowered more rapidly than the latter. 

 All these effects are produced in kind by increasing 

 the pressure on pure water. Whether, or in how far, 

 they agree in degree must be decided by future experi- 

 ments. 



Experiments wzlh Concentrated i-olutions. — Several 

 series of experiments have been made with hydrochloric 

 acid, chloride of sodium, and chloride of calcium, and 

 also with sulphuric acid. Table VII. gives the results, 

 in the same form as preceding tables, for the chlorides: — 



Table VII. 



Salt dissolved. 



It will be seen that, in proportion as the solution be- 

 comes moreconcentrated, further additions of salt produce 

 a greater effect in lov/ering the melting-point of ice, and 

 at a temperature of — i5°C. equivalent weights of NaCl 

 -f- CaCl2 produce identical results. In Table VIII. 

 the results for hydrochloric acid and sulphuric acid 

 are given in terms of the percentage of hydrogen in the 

 solution. 



