FRIGORIFIC MIXTURES

FRIGORIFIC MIXTURES, are those which experience has taught philosophers to employ for the purpose of producing artificial cold.

(A) The thermometer made use of in this experiment was filled with tierged alcohol, and accurately divided according to Fahrenheit's scale.

Some of these mixtures are enumerated under the title COLD (Encyclop.), and a much more accurate list of them is given, together with the principle upon which they produce their effect, in the article CHEMISTRY, n^o 282. (Suppl.) There is one mixture, however, not mentioned in that list, which was employed by Seguin, and seems, on many accounts, to be the most eligible that has yet been proposed. Considering the muriates (see CHEMISTRY-Index, Suppl.) as a class of salts best suited for the purpose, he gave the decided preference to muriat of lime in crystals; and his method was to mix the crystals, previously pulverised, with an equal weight of uncompressed snow.

By means of this mixture Mr W. H. Pepys junior, of the London Philosophical Society, with the assistance of some friends, froze, on the 8th of February 1799, 56 lbs. averdupe of mercury into a solid mass. The mercury was put into a strong bladder and well secured at the mouth, the temperature of the laboratory at the time being $+33^\{\backslash circ\}$. A mixture consisting of muriat of lime 2 lb. at $+33^\{\backslash circ\}$, and the same weight of snow at $+32^\{\backslash circ\}$ gave $-42^\{\backslash circ\}$ (A). The mercury was put as gently as possible into this mixture (to prevent a rupture of the bladder), by means of a cloth held at the four corners. When the cold mixture had robbed the mercury of so much of its heat as to have its own temperature thereby raised from $-42^\{\backslash circ\}$ to $+5$, another mixture, the same in every respect as the last, was made, which gave, on trial with the thermometer, $-43^\{\backslash circ\}$. The mercury was now received into the cloth, and put gently into this new mixture, where it was left to be cooled still lower than before.

In the mean time five pounds of muriat of lime, in a large pail made of tinned iron, and japanned inside and outside, was placed in a cooling mixture in an earthenware pan. The mixture in the pan, which consisted of 4 lb. of muriat of lime and a like quantity of snow, of the same temperature as the former, in one hour reduced the 5 lb. of muriat in the pail to $-15^\{\backslash circ\}$. The mixture was then emptied out of the earthen pan, and four large corks, at proper distances, placed on its bottom, to serve as rests for the japanned pail which was now put into the pan. The corks answered the purpose of insulating the inner vessel, while the exterior one kept off the surrounding atmosphere, and preserved the air between the two at a low temperature.

To the 5 lb. of muriat of lime which had been cooled, as already noticed, to $-15^\{\backslash circ\}$, and which still remained in the metallic vessel, was now added snow, uncompressed and free from moisture, at the usual temperature of $+32^\{\backslash circ\}$. In less than three minutes the mixture gave a temperature of $-62^\{\backslash circ\}$; a degree of cold which perhaps was never before produced in this country, being $94^\{\backslash circ\}$ below the freezing point of water.

The mercury, which, by immersion in the second cooling mixture to which it was exposed, was, by this time reduced to $-30^\{\backslash circ\}$, was now, by the means employed before, cautiously put into the last made mixture of the temperature of $-62^\{\backslash circ\}$. A hoop, with net-work fastened to its upper edge, and of such a breadth in the rim

rim that the net-work, when loaded with the bladder of mercury, could not reach its lower edge, was at the bottom of the mixture, to prevent the bladder from coming in contact with the vessel; by which means the mercury was suspended in the middle of the mixture. As soon as the bladder was safely deposited on the net-work, the vessels were carefully covered over with a cloth, to impede the passage of heat from the surrounding atmosphere into the freezing materials. The condensation of moisture from the atmosphere by the agency of so low a temperature was greater than could have been expected: it floated like steam over the vessels, and, but for the interposed covering, would have given the mixture more temperature than was desirable.

After one hour and forty minutes they found, by means of a searcher introduced for the purpose, that the mercury was solid and fixed. The temperature of the mixture at this time was — 46, that is, 16° higher than when the mercury was put into it.

Our young philosophers having neglected to sling the hoop and net-work in such a manner as might have enabled them to lift it out of the mixture at once, with the bladder and its contents, were obliged to turn out the whole contents of the pail into a large evaporating capsule made of iron. This was not effected without the mercury striking against its bottom and being fractured, though it received a considerable increase of temperature from the capsule. The fracture was similar to that of zinc, but with parts more cubical. The larger pieces were kept for some minutes before fusion took place, while others were twisted and bent into various forms, to the no small gratification and surprise of those who had never witnessed or expected to see such an effect produced on so fusible a metal.

In experiments of the kind here described, all the exterior vessels should be of earthen-ware or wood, which being bad conductors of heat, prevent the ingredients from receiving heat from the atmosphere and surrounding objects with the same facility that they would through metals; and, for a similar reason, the interior vessels are best of metal, that they may allow the heat to pass more readily from the substance to be cooled into the frigorific mixture employed for that purpose.

Muriat of lime is certainly the most powerful, and at the same time the most economical substance that can be employed for producing artificial cold; for its first cost is a mere trifle, being a residuum from many chemical processes, as the distillation of pure ammonia, &c. and often thrown away: besides, it may be repeatedly used for similar experiments, nothing being necessary for this purpose but filtration and evaporation to bring it to its first state. The evaporation should be carried on till the solution becomes as thick as a strong syrup, and upon cooling the whole will be crystallised: it must then be powdered, put up in dry bottles, well corked, and covered with bladder or cement to prevent liquefaction; which otherwise would soon take place, owing to the great affinity the muriat has for moisture.

The powerful effects produced by the frigorific mixture of muriat of lime and snow, present a wide field for experiments to determine the possibility of fixing some of the gases by intense cold. And we are happy to be informed by Mr Pepp, that, as soon as an

opportunity offers, he and his friends mean to make some experiments with that view, and to communicate the result of them to the editor of the valuable miscellany * from which we have taken this account of his experiment on mercury.