Polished or diamond cotton is a lustrous looking article of a soft silky nature, formed by plating the threads with a liquid emulsion of a waxy and starchy substance, and polishing the threads with rapidly revolving brushes.
Mercerized Cloth.—In late years a distinct novelty has appeared on the shelves of the dry goods stores. Beautiful, filmy fabrics, and lustrous embroidery thread, not of silk, but so close to it in appearance as to be scarcely distinguishable, have gained much popularity and attained a large sale. They are known as mercerized goods. About the middle of the century John Mercer, of England, found that when cotton goods were treated with chemicals (either alkalies or acids), a change was produced in the fibre which caused it to shrink and become thicker, and which imparted also an increased affinity for dyes. He took out British patent No. 13,296, of 1850, for his invention, but practically nothing further was done with the process. Recently the important step of Thomas and Prevost of mercerizing under tension gave some new and wonderful results. United States patents No. 600,826 and No. 600,827, of May 15, 1898, disclose this process. The cloth or thread, while being treated chemically, is at the same time subjected to a powerful tension that causes the fibres (softened and rendered glutinous by the chemicals) to be elongated or pulled out like fibres of molten glass, giving it the same striated texture and fine luster that silk has, and by substantially the same mechanical agency, for it is the elongation of the plastic glutinous thread from the silk worm that gives the thread its silky luster, by a process which has a familiar illustration in the molecular adjustment that imparts luster to spun glass or drawn taffy.
Standing in the light of the Twentieth Century, and looking back through past ages, we find the art of spinning and weaving in an ever present and unbroken thread of evidence all along the path of history—through wars and famine, floods and conflagrations; through the progress and decay of nations, through all phases of change, and the vicissitudes of centuries, it has never been relegated to the domain of the lost arts, but has remained a persisting invention. It has been a paramount necessity to the human race, indissolubly locked up with its continuity and welfare, and will ever continue to supply its work in maintaining the greater fabric of human existence.
[CHAPTER XXXII.]
Ice Machines.
[General Principles]—[Freezing Mixtures]—[Perkins’ Ice Machine, 1834]—[Pictet’s Apparatus]—[Carré’s Ammonia Absorption Process]—[Direct Compression and Can System]—[The Holden Ice Machine]—[Skating Rinks]—[Windhausen’s Apparatus for Cooling and Ventilating Ships].
Very few people have any correct conception of the principles of ice-making. Most persons have heard in a vague sort of way that chemicals are employed in its manufacture, and many a fastidious individual has been known to object to artificial ice on the ground that he could taste the chemicals, and that it could not therefore be wholesome. Such is the power of imagination, and such the misconception in the public mind. Nothing could be more erroneous, nor more amusing to the physicist, since no chemicals ever come in contact with either the water or the ice. An intelligent understanding of the operations of an ice machine involves only a correct appreciation of one of the physical laws governing the relation of heat to matter, and the forms which matter assumes under different degrees of heat. We see water passing from solid ice to liquid water and gaseous steam, by a mere rise in temperature, and conversely, by abstraction of heat, steam passes back to water, and then to ice.
When one’s hands get wet they get cold. A commonplace, but convenient proof of this is to wet the finger in the mouth and hold it in the air. A sensible reduction of temperature is instantly noticeable. A more pronounced illustration is to wet the hands in a basin of water, and then plunge them in the blast of hot, dry air coming from a furnace register. Instead of warming the hands, as many would suppose, this will, as long as the hands are wet, produce a distinct sensation of increased cold. It is due to rapid evaporation, which in changing the water from a liquid to a gaseous form, abstracts heat from the hands.
Evaporation may be effected in two ways. The common one is by applying extraneous heat, as under a tea kettle, in which case the evaporated vapor is hot by virtue of the heat absorbed from the fire. The other way is to reduce pressure or produce a partial vacuum over the liquid without any application of heat, in which case the vapor is made cold. As early as 1755 Dr. Cullen observed this, and discovered that the cold thus produced was sufficient to make ice. An incident of evaporation is the passing from the limited volume of a liquid to the greatly increased volume of a gas. Water, for instance, when it changes to a vapor, increases in volume about 1,700 times; that is, a cubic inch of water makes about a cubic foot of steam, and when evaporation takes place from a reduction of pressure, this involves a dissipation of heat throughout the increased volume, and the corresponding production of cold. When, however, matter changes from a liquid to a gas, or from a solid to a liquid, a peculiar phenomenon manifests itself, in that a great amount of heat is absorbed and, so far as the evidence of the senses goes, disappears in the mere change of state. It is called latent heat. In such case the heat becomes hidden from the senses by being converted into some other form of intermolecular force not appreciable as sensible heat, and producing no elevation of temperature. In illustration, if a pound of water at 212° F. be mixed with a pound of water at 34° (both being matter in the same state), there results two pounds of water at the mean temperature of 123°. If, however, a pound of water at 212° be mixed with a pound of ice at 32° (matter in another state), there will not be two pounds of water at the mean temperature of 122°, as might be expected, but two pounds at 51° only, an amount of heat sufficient to raise two pounds of water 71° being absorbed in the mere change of ice to water without any sensible raise in temperature. This absorbed heat is called latent heat, and it plays an important part in artificial freezing. A familiar illustration of the absorption of heat in changing from a solid to a liquid is found in the admixture of salt and ice around an ice-cream freezer. These two solids, when brought together, liquefy rapidly with an absorption of heat that produces in a limited time a far greater degree of cold than that which could be obtained from the ice by mere conduction, since the reduction of temperature proceeds faster by liquefaction than can be compensated for by the absorption of heat from the air. Were this not true, ice cream could not be frozen by a mixture of salt and ice. Many such freezing mixtures are known, and a few have been made commercially available, but they cannot be economically employed in ice-making, and it is therefore only necessary to consider the development of the more common principle of evaporation and expansion, in which the change from a liquid to a gas occurs. The volatile liquid which was first employed was water, but as it did not vaporize as readily as some other liquids, more volatile substitutes were soon found, among which may be named ether, ammonia, liquid carbonic acid, liquid sulphurous acid, bisulphide of carbon and chymogene, which latter is a petroleum product lighter and more volatile than benzine or gasoline. As these liquids were expensive, it is obvious that their vaporization could not be allowed to take place in the open air, since the reagent would thus be quickly dissipated and lost, and hence means were devised to condense and save this valuable volatile liquid to be used over again. The vaporization of the volatile liquid to produce cold, and its re-condensation to liquid form to be used over again in an endless cycle of circulation, seems to have been first effected by Mr. Perkins, of England, whose British patent No. 6,662, of 1834, affords a simple and clear illustration of the fundamental principles of most modern ice machines. His apparatus is shown in [Fig. 294]. A tank a is filled with water to be frozen or cooled. A refrigerating chamber b, submerged in the water, is charged internally with some volatile liquid, such as ether. When the piston of suction pump c rises a partial vacuum is formed beneath it, and the volatile liquid in b being relieved of pressure, evaporates and expands into greater volume, the vapor passing out through pipe f and upwardly opening valve e. This vapor is rendered intensely cold by expansion, and this cold is imparted to the water in tank a to freeze it. A more scientific statement, however, is that the cold vapor absorbs the heat units of the water, and taking them away with it, lowers the temperature of the water to the freezing point. When the piston of pump c descends, valve e closes, and the vapor, laden with the heat units absorbed from the water, is forced through the downwardly opening valve e′, and through pipe g to a cooling coil d, around which a body of cold water is continually flowed. This water, in turn, takes the heat units from the vapor, and passes off with them in a constant flow, while the vapor of ether is condensed into a liquid again by the cold water, and passing through a weighted valve h, goes into the evaporating or refrigerating chamber to be again vaporized in an endless circuit of flow. It will be seen that the heat units from the water in tank a are first handed over to the cold ether vapors passing out from chamber b, and by this vapor are then transferred to the flowing body of water surrounding the coil d. The result is that the heat units carried off by the water flowing around coil d are the same heat units abstracted from the water in tank a, which water is thus reduced to congealation.