the insufficiency of present theories of magnetism to explain the various peculiarities of the temporary magnetism of steel; and we have suggested that, in regard to the magnetic properties of its elements, that substance must be considered a heterogeneous mass.

XXIV. On Salt Solutions and Attached Water.
By FREDERICK GUTHRIE*.

II.

Cryogens and Cryohydrates.

BY Cryogen I mean an appliance for obtaining a temperature below 0° C. In this paper it always signifies a freezingmixture. By Cryohydrate I mean the body resulting from the union of water with another body, and which hydrate can only exist in the solid form below 0° C. As this communication is in every respect continuous with the one brought before the Physical Society on November 7th, 1874, and published in the Philosophical Magazine for January 1875, the paragraphs are numbered in sequence with those of the previous communication.

CRYOGENS.

Precision of Temperature of Freezing-mixtures.

§41. In § 39, when I was speaking of the possible uses of cryohydrates for the maintenance of constant temperatures, 1 said:"With the exception of the melting-points of a few organic bodies such as benzol, and the boiling-points of a few liquids such as liquid ammonia, sulphurous acid, and carbonic acid, and the rather ill-defined temperatures to be got by various freezing-mixtures, there are no means in the hands of physicists for obtaining and maintaining with certainty and ease a fixed temperature below 0° C."

In regard to freezing-mixtures, I confess to have been here very much misled by the confident but rather erroneous statements of others, to which I attached faith trebly blind-blind, because no recorded experiments really support them, blinder still because a little thought in the right direction must have shown their fallacy, and blindest of all because the one experiment of my own in this direction (§§ 16 and 17) shows that the minimum temperature of an ice-salt cryogen is reached whether we take the ratios 3 of salt to 1 of ice or 1 of salt to 2 of ice, and so points to the wideness of the margin of ratio which may obtain between the weights of ice and the salt. It will further be shown in * Communicated to the Physical Society, Jan. 18th, 1875.

§§ 44-46 that the temperature of the constituents has nothing to do with the temperature reached, and in § 49 that the degree of hydration of the salt employed is often without effect.

Accordingly I unqualifiedly withdraw the expression "illdefined" as applied to freezing-mixtures. They may be, on the contrary, bodies of precise temperature under widely varying circumstances.

§ 42. The enormous latent heat of water, the fact that the specific heat of ice is only about half that of water, while the specific heats of all salts are far less than that of ice, and therefore, à fortiori, less than that of water, together with the good thermal conductivity of water, all argue that, if constantly stirred, all parts of a freezing-mixture will have the same temperature. The fact that the liquid portion of a freezing-mixture of ice and a solid salt is the cryohydrate of that salt, ensures the identity of the resulting temperature under various conditions of proportion. The constant tendency to the formation of this cryohydrate by contact between the solids is always seeking to depress the temperature; while the solidification of the cryohydrate at an indefinitely small fraction of a degree below the temperature of the freezing-mixture, and the consequent liberation of heat, ensures the temperature against such fall.

Statements therefore, whether previously made by myself or others, that it is advantageous to weigh the salt and ice in definite proportion, that the ice should be dry, that snow is preferable to ice on account of its state of finer division, that additional cold is produced by previously cooling the ice or salt or both, are to be put aside as untrue-untrue, that is, as far as the temperature or heat-tension is concerned. To obtain the greatest quantity of heat-absorption with a given amount of salt, such a quantity of ice must be taken as will form with the salt a cryohydrate. The proportions can be at once gathered from Table X. § 90.

43. Further, the fact that all cryohydrates, with, I believe at present, the sole exception of sulphate of zinc and chloride of magnesium, have far more water than that ordinary hydrate which has most water, shows that it cannot matter whether a salt which affects water of crystallization be employed in the anhydrous state or with its crystalline water.

It may, however, be otherwise with salts containing the elements of water more intimately associated; and, as we shall see in one case at least, an anhydrous compound may melt ice with which it is in contact and heat the so-formed water far above zero, while the compound so formed, when cool, will when mixed with a fresh quantity of ice absorb heat abundantly. I suppose in such cases double decomposition ensues; and though nothing

is eliminated, there is a rearrangement of the elements more i testinal than that effected by the association with water.

§ 44. Effect of the Temperature of the Ingredients on that the Freezing-mixture.-An ounce of finely powdered chloride sodium was cooled in a flask surrounded by a freezing-mixtu till its temperature was - 15° C. It was then stirred with fo ounces of ice, which had been cooled and had the temperatu -10°. As soon as liquefaction began, the temperature -2 was reached; and this degree of cold was never surpassed.

§45. The same degree of cold (-22°) resulted from th mixture of 1 oz. of NaCl at -15° with 4 oz. of ice at 0°, al: when 1 oz. of salt at +12° C. was mixed with 4 oz. of ice -12° C.

§ 46. Indeed the margin of temperature may be greatly ex tended. Thus, 1 oz. of NaCl in powder was heated to incipie redness and thrown upon 5 or 6 oz. of ice at 0°; after a fe minutes constant stirring, the temperature had reached -22°.

4

§ 47. 1 oz. of dry anhydrous Na, SO, was heated nearly redness, and thrown upon 4 oz. of ice at 0°. In a few minut the temperature had sunk to -0°.7. Again, an ounce of anhy drous CuSO4 was heated to about 600° C., and thrown upo 4 oz. of ice; the temperature at once sank to -0°5 (compar § 55).

§ 48. From the above experiments, and from the theoretica considerations touched upon in § 42, I conclude that, within ver wide limits as to quantity, the temperature of a freezing-mixtur may be very independent of the temperature both of the sal and of the ice.

$49. Effect of Crystalline Water in the salt on the Tempera ture of the Freezing-mixture-From § 26 it appears that crystal lized sulphate of soda (Na2 SO4+7H2 O) gives with ice a tempe rature of -0°.7. Deprived of water, the anhydrous salt N, SO gave with ice the temperature also of -0°.7. Sulphate of cop per in the anhydrous state produced, when mixed with ice, temperature of -1°7, while with the ordinary crystallized hydrate the temperature was -2°.

§ 50. Having examined six or seven other salts as to the tem perature of their freezing-mixtures when employed both with and without their crystalline water, I do not scruple to assert tha water of crystallization, properly so called, takes no effect upon the temperature of the freezing-mixture; and I believe the same is true of that water which has been called constitutional. But where a profound rearrangement of the elements of water ensues, the effect may be different, as appears in the next paragraph. § 51. The chloride of aluminium, AlCl3, as offered in com

merce, fumes strongly in the moist air, so great is its avidity for water. Thrown upon several times its own weight of ice, the two liquefy and the temperature stands above 0°. The strongest commercial solution of chloride of aluminium, however, when at the temperature of the air or at 0° or at 100° C., will reduce the temperature to -13° when poured upon three or four times its own weight of ice. I suppose the anhydrous chloride may be viewed as separating the atoms of the water-molecule, as is supposed to be the case with the chlorides of phosphorus.

$52. Hydrochloric Acid as a Cryogen with Ice.-Ordinary commercial hydrochloric acid when poured upon ice may produce as much as 22° of cold. The amount of HCl in the hydrochloric acid, however, is here to be as much considered as it would have to be if we dealt with ice and a solution of NaCl. In the experiments which are given in the succeeding paragraphs as far as $54, the hydrochloric acid employed was formed by saturating water with the gas, while the liquid was kept after absorption had ceased for half an hour at 0° while the acid passed through. The barometric pressure was 770 millims. This may be considered a normal acid.

§ 53. The acid at 0° was poured in various proportions upon ice at 0°. Fifty grms. of ice were used in each experiment. The Table shows within what small limits of ratio the minimum temperature is reached. In the Table the weight of the ice is taken as unity.

We are therefore, when dealing with a cryogen one of whose constituents is a liquid, much more limited in the range of ratio which we may employ to procure the maximum cold than is the case when both are solid.

§ 54. Accordingly, in examining the effect of the alteration in the temperature of one or both of the constituents, it is necessary always to use the same ratio. In the Table below, the ratio used was that which gave the maximum cold when both were at 0° C., namely 1 of ice to 0.4 of the saturated solution of HCl. It must be understood that in those cases where the hydrochloric solution was used below 0° C., it is not meant that the saturation with HCl took place at that lower temperature, but that, having been saturated at 0°, the solution was subsequently cooled to the lower temperature.

Phil. Mag. S. 4. Vol. 49. No. 324. March 1875.

P

is eliminated, there is a rearrangement of the elements more intestinal than that effected by the association with water.

§ 44. Effect of the Temperature of the Ingredients on that of the Freezing-mixture.-An ounce of finely powdered chloride of sodium was cooled in a flask surrounded by a freezing-mixture till its temperature was -15° C. It was then stirred with four ounces of ice, which had been cooled and had the temperature As soon as liquefaction began, the temperature -22° was reached; and this degree of cold was never surpassed.

§ 45. The same degree of cold (-22°) resulted from the mixture of 1 oz. of NaCl at -15° with 4 oz. of ice at 0°, also when 1 oz. of salt at +12° C. was mixed with 4 oz. of ice at -12° C.

§ 46. Indeed the margin of temperature may be greatly extended. Thus, 1 oz. of NaCl in powder was heated to incipient redness and thrown upon 5 or 6 oz. of ice at 0°; after a few minutes constant stirring, the temperature had reached —22°.

§ 47. 1 oz. of dry anhydrous Na, SO, was heated nearly to redness, and thrown upon 4 oz. of ice at 0°. In a few minutes the temperature had sunk to -0°.7. Again, an ounce of anhydrous CuSO, was heated to about 600° C., and thrown upon 4 oz. of ice; the temperature at once sank to -0°.5 (compare § 55).

§ 48. From the above experiments, and from the theoretical considerations touched upon in § 42, I conclude that, within very wide limits as to quantity, the temperature of a freezing-mixture may be very independent of the temperature both of the salt and of the ice.

$49. Effect of Crystalline Water in the salt on the Temperature of the Freezing-mixture-From § 26 it appears that crystallized sulphate of soda (Na, SO4+7H, O) gives with ice a temperature of -0°.7. Deprived of water, the anhydrous salt N, SO gave with ice the temperature also of -0°.7. Sulphate of copper in the anhydrous state produced, when mixed with ice, a temperature of -1°7, while with the ordinary crystallized hydrate the temperature was -2°.

§ 50. Having examined six or seven other salts as to the temperature of their freezing-mixtures when employed both with and without their crystalline water, I do not scruple to assert that water of crystallization, properly so called, takes no effect upon the temperature of the freezing-mixture; and I believe the same is true of that water which has been called constitutional. But where a profound rearrangement of the elements of water ensues, the effect may be different, as appears in the next paragraph. § 51. The chloride of aluminium, AlCl,, as offered in com