A much greater degree of cold is obtained from the action of the salts with ice or snow, than when these sub stances are simply dissolved in water. A mixture of snow and common salt reduces the thermometer to zero on the scale of Fahrenheit. As this was the greatest degree of cold known at the time when that artist pro posed his division of the thermometrical scale, it was conceived to be a very fit point for the commencement of tile graduation. It has been ascertained, however, that several other salts have a much greater refrigerat ing effect, and that some of them produce a cold which is upwards of 70° below that point. The reason why tho salts cause a greater reduction of temperature with ice or snow than with water, is evidently the great absorp tion of caloric which takes place during the liquefaction of the ice or snow, in consequence of the increased capacity in a liquid state.
I laving considered the various methods of producing cold by liquefaction, we shall now subjoin a list of the most powerful of the freezing mixtures, the materials to be employed being taken by weight.
The mixtures in which the proportion of the ingre dients are omitted, have been proposed by Lowitz : the materials are probably meant to be employed in equal quantities.
The salts employed should neither be in an efflores cent state, nor contain an excess of water of crystalliza tion; and after being reduced to powder, they should be mixed as quickly as possible with the other materials. In those mixtures in which snow is used, the snow an swers the purpose best when it has fallen during frosty weather, and is in a dry pulverulent state. In all cases, the proper proportion of the materials should be care fully observed ; an excess of any of them is so much superfluous matter to be cooled down, and must diminish the effect.
A cold still more intense than can be procured by any of the freezing mixtures mentioned in the preceding Ta ble has lately been obtained by Mr Hutton, a gentleman in Edinburgh. Ile has not thought proper to commu nicate to the public the processes of refrigeration which he has employed ; but the results of the cold he obtain ed, indicate a much greater reduction of temperature than has hitherto been known.
4; Directing my attention to this subject," says Mr Hutton, " in the summer of 1810, a method occurred to me, by which I imagined a greater degree of cold might be produced than had hitherto been obtained. Although the power of this method appeared in theory almost in definite, yet it was easy to foresee, that in practice many circumstances might at first concur to set limits to its application ; from the nature of these circumstances, how ever, it was to be expected that some of them might be considerably modified, and many of them might in time be altogether removed, and thus the practice made in sonic degree to approximate to the theory.
At the time this method occurred to me, the pressure my professional avocations did not allow me to prose cute it ; but, as I anticipated some leisure in the follow ing autumn, I immediately began to provide, at any lei sure moments I had, such apparatus as I considered ab solutely necessary, or was most likely to be useful. The ;'tale dependance, however, which is to be placed on ge neral reasoning on such subjects, and the apprehension that the method might have been previously tried, and found insufficient by others, prevented me from providing any very extensive apparatus.
My first experiment was tried in the following autumn. The thermometer was filled and sealed by myself. The tube was previously tried by the common method, and found, as nearly as such tubes are commonly to be met with, of equal calibre throughout. The spirit with which it was filled, was prepared by Richter's process, and af terwards re-distilled by itself.—Its specific gravity at 62° was 798.—The points 60° and 100`' were determined by a mercurial thermometer, which had been made with the usual precautions ; the interval was divided into four spa ces, each of which, of course, corresponded to 10° ; the part of the stem below 6U° measured nearly 18 of these spaces. A mark was made at every space, till, on arri ving at the end of the 17th, the graduation could not be carried farther. This point, of course, corresponded to + 60° — of Fahrenheit's scale.
This thermometer was exposed to the cold produced by the method alluded to, and after some time was exa mined, when the alcohol was found to have passed all the marks, and was obviously sunk within the ball of the thermometer. A slight degree of discoloration was ob servable. The thermometer was replaced, and examined about five minutes afterwards, when the ball of the ther mometer was found broken, and crystals adhered to the fragments.
I next took a glass tube, about 3-lOths of an inch in diameter, and sealed at one end ; into this I poured alco hol till it stood in the tube 4-10ths of an inch deep, and then exposed it to the cold, produced as before ; after some time, it was so completely solid, that on inverting the tube it did not drop, and only a very minute stream was perceived to glide slowly down the inside of the tube ; when this stream had reached nearly the middle of the tube, the whole suddenly fell out, and, pitching in a glass, was broken into several pieces, which quickly melted.