The use of ice in cooling depends upon the fact of its requiring a vast quant.i.ty of heat to convert it from a solid into a liquid state, or in other words, to melt it; and the heat so required is obtained from those objects with which it may be in contact. A pound of ice requires nearly as much heat to melt it as would be sufficient to make a pound of cold water boiling hot; hence its cooling power is extremely great. But ice does not begin to melt until the temperature is above the freezing point, and therefore it cannot be employed in freezing liquids, &c., but only in cooling them. If, however, any substance is mixed with ice which is capable of causing it to melt more rapidly, and at a lower temperature, a still more intense cooling effect is the result; such a substance is common salt, and the degree of cold produced by the mixture of one part of salt with two parts of snow or pounded ice is greater than thirty degrees below freezing.

In making ice-creams and dessert ices, the following articles are required:--Pewter ice-pots with tightly-fitting lids, furnished with handles; wooden ice-pails, to hold the rough ice and salt--the pails should be stoutly made, about the same depth as the ice pots, and nine or ten inches more in diameter, with a hole in the side, fitted with a good cork, in order that the water from the melted ice may be drawn off as required. In addition, a broad spatula, about four inches long, rounded at the end, and furnished with a long wooden handle, is necessary to sc.r.a.pe the frozen cream from the sides of the ice-pot, and for mixing the whole smoothly together. When making ices, place the mixture of cream and fruit to be frozen in the ice-pot, cover it with the lid, and put the pot in the ice-pail, which proceed to fill up with coa.r.s.ely-pounded ice and salt, in the proportion of about one part of salt to three of ice; let the whole remain a few minutes (if covered by a blanket so much the better), then whirl the pot briskly by the handle for a few minutes, take off the lid, and with the spatula sc.r.a.pe the iced cream from the sides, mixing the whole smoothly; put on the lid, and whirl again, repeating all the operations every few minutes until the whole of the cream is well frozen.

Great care and considerable labour are required in stirring, so that the whole cream may be smoothly frozen, and not in hard lumps. When finished, if it is required to be kept any time, the melted ice and salt should be allowed to escape, by removing the cork, and the pail filled up with fresh materials. It is scarcely necessary to add, that if any of the melted ice and salt is allowed to mix with the cream, the latter is spoiled. From the difficulty of obtaining ice in places distant from large towns, and in hot countries, and from the impracticability of keeping it any length of time, or, in fact, of keeping small quant.i.ties more than a few hours its use is much limited, and many have been the attempts to obtain an efficient subst.i.tute. For this purpose various salts have been employed, which, when dissolved in water, or in acids, absorb a sufficient amount of heat to freeze substances with which they may be placed in contact. We shall not attempt, in this article, to describe all the various freezing mixtures that have been devised, but speak only of those which have been found practically useful.

Many of the freezing mixtures which are to be found described in books are incorrectly so named, for although they themselves are below the freezing point, yet they are not sufficiently powerful to freeze any quant.i.ty of water, or other substances, when placed in a vessel within them. In order to be efficient as a freezing mixture, as distinguished from a cooling one, the materials used ought to be capable of producing by themselves an amount of cold more than thirty degrees below the freezing point of water, and this the ordinary mixtures will not do. Much more efficient and really freezing mixtures may be made by using acids to dissolve the salts. The cheapest, and perhaps the best, of these for ordinary use, is one which is frequently employed in France, both for making dessert ices, and cooling wines, &c. It consists of coa.r.s.ely powdered Glauber salt (sulphate of sodium), on which is poured about two-thirds its weight of spirit of salts (hydrochloric acid).

The mixture should be made in a wooden vessel, as that is preferable to one made of metal, which conducts the external heat to the materials with great rapidity; and when the substance to be cooled is placed in the mixture, the whole should be covered with a blanket, a piece of old woollen carpet doubled or some other non-conducting material, to prevent the access of the external warmth; the vessel used for icing wines should not be too large, that there may be no waste of the freezing mixture. This combination produces a degree of cold thirty degrees below freezing; and if the materials are bought of any of the wholesale druggists or dry salters, it is exceedingly economical. It is open, however, to the very great objection, that the spirit of salt is an exceedingly corrosive liquid, and of a pungent, disagreeable odour: this almost precludes its use for any purpose except that of icing wines.

[FAIR AND SOFTLY GO SURE AND FAR.]

2143. Further Directions.

Actual quanties--one pound of chloride of ammonium, or sal ammoniac, finely powdered, is to be _intimately_ mixed with two pounds of nitrate of potasium or saltpetre, also in powder; this mixture we may call No. 1. No. 2 is formed by crushing three pounds of the best Scotch soda. In use, an equal bulk of both No. 1 and No. 2 is to be taken, stirred together, placed in the ice-pail, surrounding the ice-pot, and rather less cold water poured on than will dissolve the whole; if one quart of No. 1, and the same bulk of No. 2 are taken, it will require about one quart of water to dissolve them, and the temperature will fall, if the materials used are cool, to nearly thirty degrees below freezing. Those who fail, may trace their want of success to one or other of the following points:--the use of too small a quant.i.ty of the preparation,--the employment of a few ounces; whereas, in freezing ices, the ice-pot must be entirely surrounded with the freezing material: no one would attempt to freeze with four ounces of ice and salt. Again, too large a quant.i.ty of water may be used to dissolve the preparation, when all the excess of water has to be cooled down instead of the substance it is wished to freeze. All the materials used should be pure, and as cool as can be obtained. The ice-pail in which the mixture is made must be of some non-conducting material, as wood--which will prevent the access of warmth from the air; and the ice-pot, in which the liquor to be frozen is placed, should be of pewter, and surrounded nearly to its top by the freezing mixture. Bear in mind that the making of ice-cream, under any circ.u.mstances, is an operation requiring considerable dexterity and practice.

2144. To Make Dessert Ices, both Cream and Water.

2145. Strawberry Ice Cream.

Take one pint of strawberries, one pint of cream, nearly half a pound of powdered white sugar, the juice of a lemon; mash the fruit through a sieve, and take out the seeds: mix with the other articles, and freeze. A little new milk added makes the whole freeze more quickly.

2146. Raspberry Ice Cream.

The same as strawberry. These ices are often coloured by cochineal, but the addition is not advantageous to the flavour. Strawberry or raspberry jam may be used instead of the fresh fruit, or equal quant.i.ties of jam and fruit employed. Of course the quant.i.ty of sugar must be proportionately diminished.

2147. Strawberry Water Ice.

One large pottle of scarlet strawberries, the juice of a lemon, a pound of sugar, or one pint of strong syrup, half a pint of water.

Mix,--first rubbing the fruit through a sieve,--and freeze.

2148. Raspberry Water Ice.

Raspberry Water Ice is made in precisely the same manner as Strawberry-water ice.

2149. Lemon-Water Ice.

Lemon juice and water, each half a pint; strong syrup, one pint: the rind of the lemons should be rasped off, before squeezing, with lump sugar, which is to be added to the juice; mix the whole; strain after standing an hour, and freeze. Beat up with a little sugar the whites of two or three eggs, and as the ice is beginning to set, work this in with the spatula, which will be found to much improve the consistence and taste.

2150. Orange-Water Ice.

Orange-Water Ice is made in the same way as Lemon-water ice.

2151. Nitrate of Ammonium as a Freezing Mixture.

Another substance, which is free from any corrosive action or unpleasant odour, is nitrate of ammonium, which, if simply dissolved in rather less than its own weight of water, reduces the temperature about twenty-five degrees below freezing. The objections to its use are that its frigorific power is not sufficiently great to freeze readily; and if it be required to form dessert ices, it is requisite to renew the process, at the expiration of a quarter of an hour, a second time, and, if the weather is very hot, and the water used is rather warm, even a third or fourth time. Again, nitrate of ammonium is a very expensive salt; even in France, where it is manufactured expressly for this purpose, it is sold at the rate of three francs a pound; and in England it cannot be obtained under a much higher price.

One great recommendation, however, attends its use, namely, that it may be recovered again, and used any number of times, by simply boiling away the water in which it is dissolved, by a gentle fire, until a small portion, on being removed, crystallizes on cooling.

2152. Washing Soda as a Freezing Mixture.

If, however, nitrate of ammonium in coa.r.s.e powder is put into the cooler, and there is then added twice its weight of freshly crushed washing soda, and an equal quant.i.ty of the coldest water that can be obtained, an intensely powerful frigorific mixture is the result, the cold often falling to forty degrees below freezing. This is by far the most efficacious freezing mixture that can be made without the use of ice or acids. But, unfortunately, it has an almost insuperable objection, that the nitrate of ammonium is decomposed by the soda, and cannot be recovered by evaporation; this raises the expense to so great a height, that the plan is practically useless.

[ALL IS NOT GOLD THAT GLITTERS.]

2153. Sal Ammoniac as a Freezing Mixture.

If the ordinary sal ammoniac of the shops is used, it will be found both difficult to powder, and expensive; in fact, it is so exceedingly tough, that the only way in which it can be easily divided, except in a drug mill, is by putting as large a quant.i.ty of the salt into water which is actually boiling as the latter will dissolve; as the solution cools, the salt crystalizes out in the solid form, and if stirred as it cools, it separates in a state of fine division. As this process is troublesome, and as the sal ammoniac is expensive, it is better to use the crude muriate of ammonium, which is the same substance as sal ammoniac, but before it has been purified by sublimation. This is not usually kept by druggists, but may be readily obtained of any of the artificial manure merchants, at a very moderate rate; and its purity may be readily tested by placing a portion of it on a red-hot iron, when it should fly off in a vapour, leaving scarcely any residue.

2154. Coldness of the Materials used.

It is hardly necessary to add, that in icing wines, or freezing, the effect is great in proportion to the coldness of the materials used; therefore, every article employed, viz., the water, tubs, mixtures, &c., should be as cool as possible.

2155. Blackbirds.

The c.o.c.k bird is of a deep black, with a yellow bill. The female is dark brown. It is difficult to distinguish male from female birds when young; but the darkest generally are males. Their food consists of German paste, bread, meat, and bits of apple. The same treatment as given for the thrush (_See par._ 2456) applies to the blackbird.

2156. Food of Blackbirds.

The natural food of the blackbird is berries, worms, insects, sh.e.l.led snails, cherries, and other similar fruit; and its artificial food, lean fresh meat, cut very small, and mixed with bread, or German paste.

2157. Thrushes.

A c.o.c.k may be distinguished from a hen by a darker back, and the more glossy appearance of the feathers. The belly also is white. Their natural food is insects, worms, and snails. In a domesticated state they will eat raw meat, but snails and worms should be procured for them. Young birds are hatched about the middle of April, and should be kept very warm. They should be fed with raw meat, cut small, or bread mixed in milk with hemp seed well bruised; when they can feed themselves give them lean meat cut small, and mixed with bread or German paste, plenty of clean water, and keep them in a warm, dry, and sunny situation.

2158. Canaries.

To distinguish a c.o.c.k bird from a hen, observe the bird when it is singing, and if it be a c.o.c.k you will perceive the throat heaving with a pulse-like motion, a peculiarity which is scarcely perceptible in the hen. Feed young canaries with white and yolk of hard egg, mixed together with a little bread steeped in water. This should be pressed and placed in one vessel, while in another should be put some boiled rape seed, washed in fresh water. Change the food every day. When they are a month old, put them into separate cages. Cut the claws of cage-birds occasionally, when they become too long, but in doing so be careful not to draw blood.

2159. Treatment of Canaries.

Care must be taken to keep canaries very clean. For this purpose, the cage should be strewed every morning with clean sand, or rather, fine gravel, for small pebbles are _absolutely essential_ to life and health in cage-birds: fresh water must be given every day, both for drinking and bathing; the latter being in a shallow vessel; and, during the moulting season, a small bit of iron should be put into the water for drinking. The food of a canary should consist princ.i.p.ally of _summer_ rape seed that is, of those small _brown_ rape seeds which are obtained from plants sown in the spring, and which ripen during the summer; large and _black_ rape seeds, on the contrary, are produced by such plants as are sown in autumn and reaped in spring. A little chickweed in spring, lettuce leaves in summer, and endive in autumn, with slices of sweet apple in winter, may be safely given; but bread and sugar ought to be generally avoided. Occasionally, also, a few poppy or canary seeds, and a small quant.i.ty of bruised hemp seed may be added, but the last very sparingly.