Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/629

613 ICE 613 Company (Glasgow) utilize as the cooling agent a mixture of certain hydrocarbon gases which are obtained from the distillation of carbonaceous shale. The gas is compressed to a pressure of about 8 atmospheres, and, after being cooled by expansion, is carried off and consumed as fuel. These machines are not specially intended for the production of : ice ; but, as refrigerators, they are successfully employed for preserving meat on board ship. Among machines of the second group there is a great variety of construction, because of the great differences which exist in tli3 properties of the liquids used. Thus water, sulphuric ether, bisulphide of carbon, ammonia, methylic ether, sulphurous acid, and other substances have been employed as refrigerating agents. In all cases, it is the so-called latent heat of vaporization that is utilized ; and did the efficiency of the method depend only on this, ; water would undoubtedly be the best material on account of the great latent heat of its vapour. But as important | from a practical point of view are the vapour pressures that come into play throughout the range of temperature employed. Thus at 10 C. the pressure of water vapour is so small, only 012 of an atmosphere (and at lower tern- peratures of course it is still smaller), that, to make the evaporation of water an efficient means of refrigeration, the process must be conducted under a very much diminished pressure. As early as 1755, Dr Cullen managed to freeze water by its own evaporation in a vacuum ; but this method, though greatly developed by Nairne, Leslie, and Vallance, can be applied to the production of ice iu small quantities only. The same objection applies, of course, to sulphuric ether, bisulphide of carbon, or any substance which boils under ordinary atmospheric pressure at a temperature above that of the air. Ether boils at 34 8 C., and bisulphide of carbon at 46 - 2 C. ; and their vapour pressures at 10 C. are respectively &quot;377 and &quot;267 of an atmosphere. They thus volatilize much more readily than water, and require a comparatively slight vacuum to render their evaporation sufficiently rapid for refrigerating purposes. In the ether machine, which may be taken as a type, the ether, on being vaporized in the refrigerator under a partial vacuum, is drawn over anl compressed to the liquid state in the con denser, which is kept cool by circulating water. From the condenser it is then led back to the refrigerator, to be re- evaporated. Perkins s machine (1834), Twining s patent of 1850, Harrison s machine (1857), Siebe s machine (1862), and Siddeley and Mackay s apparatus are ether-machines ; and all except the first, which is hardly adapted for exten sive freezing, surround the refrigerator with brine, which when cooled flows easily around and between the cases containing tha water to be frozen. Van der &quot;Weyde (1869) substituted naphtha, gasolin, or chimogene for the ether ; and in Johnston an;l Whitelaw s machine bisulphide of carbon is used somewhat similarly. The great difficulty in machines of the ether type is to prevent leakage, so as to keep the partial vacuum really efficient ; and moreover ether, which is in most respects superior to all the other substances employed, has an awkward tendency, under the influence of frequent condensations and rarefactions, to transform itself into less volatile isomers. The great characteristic of ice- machines which employ ammonia, methylic ether, or sulphurous acid, as compared with those of the ether type, is that they work at increased instead of diminished pressures, since these substances are gaseous at ordinary temperatures and pressures, and require for their liquefaction either the production of a low tem perature or the application of a high pressure. For facility of reference the boiling points and vapour pressures at three different temperatures for these substances are given in the following table. Name of Sulwtane Boiling Point. Vapour-Pressures esti mated in Atmospheres. At 10. At 20. At 30. Ammonia -38 50C. -23 65 -10 -08 6 1 3-5 2-3 8-5 4-8 3-2 11-6 6 5 4-5 Methylic ether Sulphurous acid The best known of the ammonia machines is Carry s (1859), the principle and construction of which are remark ably simple. Two strong metal vessels, the boiler and refrigerator, are connected above by a tube. In the boiler a saturated solution of ammonia is raised to 130-150 C. The ammonia is driven over under high pressure into the refrigerator, round which cold water circulates, and in which the ammonia is condensed to a liquid. The boiler is then placed in cold water, and as its temperature falls the pressure in the apparatus is relieved and the liquid ammonia in the refrigerator vaporizes rapidly, thereby producing intense cold, and redissolves in the boiler. The temperature to which the boiler must be raised at first is determined by the condition that the pressure in the boiler must correspond to the pressure of the ammonia vapour at the temperature of the condenser. Now the pressure of ammonia vapour increases from 8| atmospheres at 20 C. to IIJ, at 30 C. ; and this higher pressure is extremely difficult to keep up in such an apparatus as Carre s, because of inevitable leakage. In warm countries, accordingly, the ammonia-machine is practically useless because of the high pressures required ; and in temperate climates, where natural ice can be stored throughout summer, an ice-machine is Hot in sucn great demand. One great drawback to the efficient working of Carre s machine is the difficulty of keeping the refrigerating liquid free of water only 75 per cent, of it being ammonia. To remedy this defect Iveece invented his machine (1869). The essential part of this ingenious apparatus is an upright cylinder in which a descending current of strong ammonia solution, drawn originally from the boiler, is met by an ascending current of steam. The ammonia is thus separated from the water, and is driven off into a rectifier, from which, after being freed from any small quantity of water it may have carried along with it, it passes into a condenser where it is kept liquid by its own pressure. It is then allowed to collect in the refrigerator, where at the required moment the pres sure is relieved, permitting the ammonia to vaporize and escape to a separate chamber to be redissolved. Brine flowing through a coiled tube within the refrigerator is used as the vehicle for the cold produced ; or even mere water may suffice if the object is simply to get a diminished temperature without freezing. Linde s ice-making machine, some twenty-two of which were in operation at the Diissel- dorf Exhibition of 1880, is the latest form of ammonia machine ; and its inventor claims for it superiority over all others as an economical refrigerator. The danger of explosion, one of the great disadvantages of ammonia, is obviated by carrying the liquefied gas through narrow iron tubes and by employing only a small quantity of the substance at one time. Blocks of ice are formed between the spokes of a revolving drum, which, cooled internally by the evaporating liquid, dips into a tank of water. Methylic ether is in some respects better than ammonia, having a higher boiling point, and requiring smaller pres sures, without the necessity of heating. In Tellier s machine (described in the Annales de Chimie et de Physique for 1874), which is specially suitable for use on board ship, the methylic ether evaporates in a closed metallic vessel, the sides of which are in immediate contact with the water to be frozen or chilled. Sulphurous acid, first successfully employed as a refriger-