April 28, 1 887] 



NA TURE 



609 



agree that ice, whether formed artificially in the laboratory 

 by freezing sea-water, or found in nature as one of the 

 various species of sea-water ice, retains, in one form 

 or another, and with great tenacity, the salt existing 

 in solution in the water. The question at issue is 

 whether this salt is to be attributed to the solid matter of 

 the ice or to the liquor mechanically adhering to it, from 

 which it is impossible to free it. Most bodies, and 

 especially those which take a crystalline form, are easily 

 purified and freed from all suspected foreign matter, with 

 a view to analysis, by the simple operation of washing 

 and drying. It is impossible to wash the crystals, formed 

 by freezing a saline solution, with distilled water, because 

 they melt at a temperature below that at which distilled 

 water freezes. The effect of the addition of a small 

 quantity of distilled water to a quantity of saline ice is at 

 first the anomalous one, that what was a wet sludge is 

 transformed into a dry crystalline powder. It is of course 

 impossible to dry the ice by heat, and to do so by more 

 intense freezing would be begging the question. The 

 experimental difficulties therefore account for some of 

 the divergence of opinion on the subject. The mixed 

 character of the substances examined has also much to do 

 with it. As a rule it may be said that those investigators 

 who have confined their observations to the labora'ory 

 have concluded that the ice forming when saline solutions 

 of moderate concentration, including sea-water, are frozen, 

 is pure ice, and the salt from which it is impossible to free 

 It entirely belongs to the mother-liquor, while those who 

 have collected and examined sea-water ice in high lati- 

 tudes have come to the opposite conclusion. 



During the Antarctic cruise of the Challenger I made 

 a number of observations on the sea-water ice found in 

 those regions, and relying principally on the fact that the 

 melting temperature of the ice was markedly lower than 

 that of fresh-water ice, and that it was impossible by any 

 of the ordinary means familiar to chemists for freeing 

 crystals from adhering mother-liquor to materially reduce 

 its salinity, I came to the conclusion that the ice forming 

 in freezing sea-water is not a mixture of pure ice and 

 brine, but that it contains the salt found in it in the solid 

 state either as a crystalline hydrate or as the anhydrous 

 salt, but most probably as a hydrate. In dealing with 

 this subject. Dr. Otto Pettersson (" Water and Ice," p. 302) 

 quotes my observations, and also rejects the view that 

 " sea-ice is in itself wholly destitute of salts, and only 

 mechanically incloses a certain quantity of unfrozen and 

 concentrated sea-water." He founds his belief on the 

 fact that numerous analyses of specimens of sea- water 

 ice have shown that the constitution of the saline contents 

 of different specimens of ice difters for each specimen, 

 and is always different from that of the saline contents of 

 sea-water. Were the salinity due to inclosed unfrozen 

 and concentrated sea-water, we " ought to find by chemi- 

 cal analysis exactly the same proportion between CI, MgO, 

 CaO, SO3, &c., in the ice and in the brine as in the 

 sea-water itself." He adduces numerous examples of 

 analyses of specimens of sea-water ice from the Baltic 

 and from the Arctic Seas to show that this is not the 

 case. Calling the percentage of chlorine in each case 

 100, he found in various sea-waters the percentage of SO3 

 vary from 1 1 49 to I rSg. In specimens of sea-water ice 

 it varied from I2'8 to 766, and in brines separating from 

 the ice and remaining liquid at -30^0. it varied from v\\ 

 to ri6. 



This argument appears conclusive. In order to explain 

 all the phenomena observed in connexion with sea-water 

 ice he cites Guthrie's investigations, which went to show 

 that, in freezing saline solutions, under a certain concen- 

 tration, pure ice is formed at a temperature which falls 

 from o C, when the amount of salt dissolved is in- 

 finitely small, to a certain definite temperature when the 

 solution contains a certain definite percentage of salt. 

 Further abstraction of heat then produces solidification 



of the solution as a whole, in the form of a crystalline 

 hydrate, of constant freezing- and melting-point. To such 

 hydrates, Guthrie gave the name of cryohydrates. 

 Pettersson quotes the following as being particularly 

 applicable to the case of sea-water : — 



of 

 NaCl 

 KCl 

 C.iCIo 

 MgSO, 

 NaaSOj 



.•\nd he refers more particularly to the cryohydrate of 

 Na2S04 forming and melting at -o"]. 



Now the bearing of Guthrie's experiments is to show 

 that, while at sufficiently low temperatures, and with 

 suitable concentration, the water will solidify along with 

 one or other of the salts in solution, until this low 

 temperature and high concentration are attained, pure 

 ice must be the result of freezing. 



The abnormal phenomena attending the formation and 

 the melting of ice in saline solutions and sea-water, find 

 a natural explanation in an observation which I have 

 frequently quoted, and which Dr. Pettersson mentions in 

 a footnote at p. 31S, namely, that "a thermometer im- 

 mersed in a mixture of snow and sea-water which is 

 constantly stirred indicates - 1 "8 C." If this is true, it 

 is clear that my melting-point observations proved nothing. 

 On repeating the experiment I found it confirmed, and 

 took the opportunity this winter of investigating the 

 matter more closely. The paper read before the Royal 

 Society of Edinburgh contains the first portion of the 

 results. It deals with the subject under two heads, 

 namely, {a) the temperature at which sea-water and some 

 other saline solutions freeze, and the chemical constitu- 

 tion of the solid and the liquid into which they are split 

 by freezing ; and {b) the temperature at which pure ice 

 melts in sea-water and in a number of saline solutions of 

 different strengths. 



[a) The freezing experiments were limited to sea-water 

 and solutions of NaCl comparable with sea-water. 



Chloride of Sodium. — Four solutions were used, and 

 they were intended to contain 3, 2-5, 2, and i'5 per cent. 

 NaCl respectively. Forty grammes of this solution, in a 

 suitable beaker, were immersed in a freezing mixture of 

 such composition as to give a temperature from 1" to 

 2'-5 C. below the freezing temperature expected. The 

 temperature at which ice began to form (if necessary 

 after adding a minute splinter of ice) was noted, and the 

 freezing was allowed to continue with constant stirring till 

 the temperature had fallen o''2 C. A specimen of the 

 mother-liquor was removed, and the chlorine in it 

 determined ; the chlorine in the original solution had 

 been determined before. The beaker was then removed 

 from the freezing bath and allowed to melt. The 

 temperature in all cases rose during melting exactly as it 

 had fallen during freezing. In the following table are 

 given the means of the temperature at which ice began to 

 form in the original solution, and that of the liquid when 

 the sample of brine was taken, and the means of the 

 chlorine found in the original solution and in the brine 

 sample 



Mean Freezing Temp. 

 Mean per cent. CI. 



■i'-875 C. -i°-63 

 I 87 I 60 



■ i'-3o 

 I 30 



-o'-97S 

 0-98 



It will be seen that, in the dilute solutions experimented 

 with, the percentage of chlorine expresses, in terms of 

 the Centigrade scale, the lowering of the freezing-point 

 of the solution. 



Sea-Water — Similar experiments were made with sea- 

 water of difterent degrees of concentration. In sea-water 

 from the Firth of Clyde containing r84 per cent, of 



