910 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. 



identity of the resulting temperature under various conditions of pro- 

 portion. The constant tendency to the formation of this cryohydrate 

 by contact between tlie solids is alway seeking- to depress the tempera- 

 ture; while the solidification of the cryohydi-ate at an indefinitely small 

 fraction of a degree below the temi)erature of the freezing-mixture and 

 the consequent liberation of heat insure the temperature against such 

 faU." 



"Statements, therefore," says Professor Guthrie, " whether previously 

 made by myself or others, that it is advantageous to weigh the salt and 

 ice in definite proportions, that the ice should be dry, that snow is prefer- 

 able 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 ten- 

 sion 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." 



Within very wide limits as to quantity, the temperature of a freezing-mix- 

 ture may he very independent of the temperature both of the salt and of 

 the ice. Professor Guthrie established this as follows : an ounce of finely- 

 powdered chloride of sodium was cooled in a flask surrounded by a freez- 

 ing-mixture till its temperature was —15° C. It was then stirred with 

 four ounces of ice, which had been cooled and had the temperature —10°. 

 As soon as liquefaction began, the temi^erature —22° was reached j and 

 this degree of cold was never surpassed. 



The same degree of cold ( —22°) resulted from the mixture of 1 ounce 

 of sodium chloride at —15° with 4 ounces of ice at 0°; also, when 1 

 ounce of salt at +12° C. was mixed with 4 ounces of ice at —12° C. 



Indeed, the margin of temperature may be greatly extended. Thus, 

 1 ounce of sodium chloride in powder was heated to incipient redness 

 and thrown upon 5 or C ounces of ice at 0° ; after a few minutes' constant 

 stirring, the temperature had reached —22°. 



One ounce of dry anhydrous sodium sulphate was heated nearly to 

 redness and thrown upon 4 ounces of ice at 0°. In a few minutes the 

 temperature had sunk to — 0°.7. Again, an ounce of anhydrous copper 

 sulphate was heated to about G00° C. and thrown upon 4 ounces of ice ; 

 the temi^erature at once sank to — 0°.5; whereas, if mixed at ordinary 

 temperatures, the reduction would only have been to —2° 0. 



H.— SPECIAL EXAMPLES OF CRYOHYDEATES. 



Some examples of special interest among cryohydrates may be noticed. 

 Ice and chloride of ammonium solidify at —15° C, taking the form of a 

 brilliant white, apparently flocculent mass, lighter than the unsolidified 

 liquid. Decanting liquid separated after awhile, the solidifymg parts 

 were seen to be minute crystals, very much resembling ice-flowers, but 

 opaque. "The sides of the beaker become studded with transparent 



