at

the Hoffmann furnaces in England. From these it appears that the saving is even greater than he claims. At the works of Mr. Betty, Kensington, the following represents the comparative cost to the proprietor of brick per thousand, by the old and new methods of burning. These numbers are given in the Practical Mechanic's Journal of October 1, 1865, by the superintendent of the works:

Cost of production of one thousand brick.

Giving a ratio of economy as one to five.

The following is the result of observations made at Durham by Mr. G. Furness:

Comparative cost of production of 222,000 brick in old-fashioned Scotch brick-kilns and in Hoffmann's furnaces.

Scotch furnaces-17 tons 14 cwt., at 15s. the ton....£59 15s. 5d.=$289 29 Hoffmann's furnaces-27 tons 12 cwt., at 12s. 6d.

Giving a ratio of economy as one to three and four-tenths. Such results as this, added to the very superior quality of the brick obtained by this process, are sufficient to insure the general adoption by manufacturers of this exceedingly important improvement. Already, more than four hundred Hoffmann furnaces are in operation in Germany, and about thirty in England. A single establishment in Vienna, that of Mr. Henry Drasche, employs nineteen of them, having each a capacity to produce eight million of brick per annum. Mr. Drasche employs forty-five hundred workmen, and turns out annually one hundred and ninety-eight milllion of brick; but, besides this, he produces a vast multitude of objects of ornament in terra cotta, designed for the decoration of buildings and grounds-a class of works very favorite in Austria, both for their beauty and for their cheapness.

Mr. Drasche exhibited one of the most attractive collections of bas reliefs, statues, vases, architectural and other ornaments in the Exposition; all of them formed in terra cotta. His display was as remarkable for the great number of beautiful objects which it contained as for the taste with which they had been designed. They were bought up by

visitors with eagerness, and only a few weeks had elapsed after the opening of the Exhibition before nearly every object in the whole collection bore the mark which, in all quarters, grew more and more familiar every day, "sold." All these beautiful productions were baked in the Hoffmann furnaces of Mr. Drasche's establishment.

CHAPTER XII.

ARTIFICIAL PRODUCTION OF COLD.

GENERAL OBSERVATIONS-USEFUL APPLICATIONS OF COLD-FREEZING MIXTURESREDUCTION OF TEMPERATURE BY EVAPORATION-ARTIFICIAL PRODUCTION OF ICECARRE'S SULPHURIC ACID FREEZING APPARATUS-CARRÉ'S AMMONIACAL FREEZING APPARATUS-Cost of ICE PRODUCED IN THIS FORM OF APPARATUS-CARRÉ'S CONTINUOUS FREEZING APPARATUS-USEFUL APPLICATION of refrigERATING APPARATUS-TWINING'S AMERICAN ICE MACHINE-ECONOMY OF PRODUCTION OF ICE BY TWINING'S APPARATUS.

I. GENERAL OBSERVATIONS.

The importance to the arts of industry and to the promotion of human comfort of being able to control temperatures, as, for instance, in an apparatus, in an apartment, or, in fact, in any given space, large or small, is too obvious to need illustration. As respects the higher temperatures, it may, indeed, be said that without the power to create such temperatures artificially, and to carry them to a degree of elevation immensely superior to any which nature, under ordinary circumstances, anywhere presents over the habitable surface of the earth, the industrial arts as now understood could not exist, and civilization itself would be impossible.

The useful applications of cold are less numerous and less obvious. In most climates, in fact in nearly all beyond the limits of the tropics, cold is regarded rather as an enemy to be repelled, than as an ally to be courted. Its most familiar applications are to check or prevent the putrefaction of organic substances, or to subserve the uses of luxury. For neither of these purposes is it usually necessary to secure a temperature very greatly depressed below that of the ambient air. Nor, if it were so, would the means of accomplishing the object be generally within the reach of those who would desire to profit by them. To command a superior temperature is easy. Combustion furnishes heat in quantity practically exhaustless, and though skill may be of use in securing its economical production or application, none whatever is necessary in order to set the process in operation. But the means of creating artificially a temperature extremely depressed are neither simple nor familiar, and to employ them successfully at all requires a species of knowledge and a degree of scientific skill which are rarely found except with the experimental chemist. There are certain industries, however, of which the process of refrigeration forms a part, which do not require a temperature inferior to that which prevails in the atmosphere, or in the running waters of the earth at the same time.

The ideas conveyed by the words cold and hot are related to our senses merely, and not to anything absolute in the condition of bodies. It is a familiar experiment to place upon the table three vessels of water, one of them at the temperature of freezing, another as hot as the hand can bear, and a third at the temperature of the weather. The experimenter at first immerses one hand in the hot fluid and the other in the cold; after the lapse of a minute or two he places both together in the water of mean temperature, and he is conscious at once of the paradox of perceiv ing the same liquid to be apparently both hot and cold at the same time. In the working of a steam-engine the condenser supplied with water from natural sources is cold relatively to the steam to be condensed. For all ordinary distillations the natural temperature furnishes sufficient refrig eration.

For certain purposes the natural temperature of the colder season suffices, but not that of the warmer. For such, to a limited extent, it is practicable, by careful contrivances, to preserve the winter temperature throughout the year. This is done by collecting ice in the season of its abundance, and storing it away in magazines with non-conducting walls, sunk usually beneath the surface of the earth. It is by means of ice thus preserved that the low temperature required in refrigerators for domestic uses is maintained; and such ice furnishes also the essential element in the most simple and best known of freezing mixtures-that which is employed by confectioners-a mixture of ice-powder and common salt. This mixture produces a depression of temperature nearly 18° C below that of freezing water. It is the temperature which was adopted by Fahrenheit as the zero of his thermometer.

The cause of the cold produced by freezing mixtures is to be sought in the absorption of heat which accompanies the transformation of bodies of every kind from the solid to the liquid state. This heat becomes latent; that is to say, it does not raise the temperature of the substance into which it passes. On the contrary, when the substances mingled are such (as for the purposes of a freezing mixture they must be) as when in union to remain liquid at a temperature much below that at which they solidify when pure, they will, in liquefying, draw upon their own sensible heat and upon that of bodies in contact with them for the latent heat necessary to their liquefaction; and it is thus that they produce the refrigerating effect from which they derive their name. Saltpetre and common salt, or saltpetre and sal ammoniac, added to three times their weight of water, will depress the temperature of the solution 22° C. If to this solution be added once and a half as much sulphate of soda as of either of the salts previously used, the temperature will sink three or four degrees lower. Equal parts of carbonate of soda, nitrate of ammonia, and water will produce a depression of 32° C. And three parts of phosphate of soda with two of nitrate of ammonia and a little more than one of water, will sink the thermometer nearly 40° C. This depression of temperature is to be understood only of the solution

itself, on supposition that it draws no heat from the substances in contact with it. The latent heat of liquefaction for a given weight of material liquefied is but a determinate amount. When, therefore, a solution is employed as a means of refrigerating other things, the degree of cold will be less considerable in proportion as the quantity of matter to be chilled is greater. And in general it will be found that the useful effect produced will not be sufficient compensation for the trouble and expense of the operation, if it is proposed to apply the process on a very large scale.

As heat becomes latent whenever a body passes from a solid to a liquid state, so a much larger amount is similarly absorbed when a liquid becomes a vapor. There are a large number of liquids, moreover, which evaporate so freely as to produce a very sensible degree of cold without any special arrangements to favor this result. If one hand be moistened with water while the other remains dry, the moistened hand will be very perceptibly colder than the other. If the fluid used to moisten the hand be alcohol instead of water, the sensation will be much more marked; and if, instead of alcohol ether be substituted, it will soon become intolerable. A current of air blowing upon the moistened surface will accelerate the evaporation and increase the intensity of the cold. And a removal or diminution of the atmospheric pressure upon the surface of the liquid will have a similar effect. Pressure and variation of pressure exercise indeed a most marked influence upon the formation of vapor. The particles of most liquids are constantly tending to assume the gaseous form. To a certain extent this tendency is efficient at every temperature to which observation has been carried; but there are two causes by which it is usually held more or less in check, viz: the force of cohesion and the pressure superincumbent on the surface. This tendency diminishes, it is true, as the temperature is more depressed; but with none of the more volatile liquids, nor even in the case of water, has there been experimentally found a point at which it wholly ceases to exist, or at which the fluid becomes absolutely fixed. Evaporation goes on from the surface of snow at the temperature of 0° C, in the open air and in a perfectly still night, at the rate of nearly thirty grams, or about an English ounce, per square metre of surface exposed per hour. At the zero of Fahrenheit, 32° F below freezing, evaporation goes on at the rate of something like seven and a half grams per square metre per hour; and even at 320 F it continues still to be sensible, amounting to not less than a quarter of a gram per square metre per hour. Trivial and insignificant as this slight evaporation may seem, it nevertheless, when extensive surfaces are considered, produces large effects. At the extremely low temperature last named, there rises hourly, from every acre of surface exposed, more than a kilogram of water in the form of invisible vapor; and from every square mile between six hundred and seven hundred kilograms, or from thirteen hundred to fifteen hundred pounds. The elastic force of the vapor thus formed