(30) A liquid invented by Raoul Pictet, of Geneva, for use as a disinfectant, answers well as a freezing mixture for hardening microscopical specimens. Sulphur dioxide and carbon dioxide, having been mixed and cooled, are compressed until they are liquid, and stored in siphons. When liberated, they rapidly evaporate, with great reduction of temperature. By this means mercury may be frozen, and animal or vegetable tissues rendered solid in a few seconds. It is as easily managed and more effective than ether,the odour being the principal objection.
(31) According to Cailletet and Colar-deau, flocculent carbonic acid is capable of cooling bodies down to - 60° C. at the ordinary pressure of the atmosphere, and down to - 76° C. in a vacuum. If the solid carbonic acid is mixed with ether, the temperatures are - 77° C. and - 103° C. If chloride of methyl is used instead of ether, there is obtained a temperature of - 82° C. at the pressure of the atmosphere, and of - 106° C. in a vacuum, which is equal to 190° below freezing, or 158° below zero on Fahrenheit's scale.
(32) The most commonly used mixture for obtaining, on a small scale, temperatures between - 20° and - 40° C. is that of snow and commercial hydrochloric acid. Since diluted sulphuric or nitric acid can by similarly used, it was thereby suggested that one might utilise for this purpose the mixture of equal volumes of strong nitric and sulphuric acid which had been employed in a Grove battery and for which there was little further use. When first made, the mixture of acids has a specific gravity of about 1.63, and when spent about 1.57.
Bachman undertook to ascertain in what mixture this spent acid can be best employed for obtaining a freezing mixture. The temperature of the atmosphere in which the trials were made ranged from - 2° to + 2° C, and in each instance the acid was brought to the temperature of the air before mixing with the snow. Diluting the acid with differing amounts of water and mixing these with snow, it was found that the undiluted acid and that diluted with one-tenth of the volume of water gave equal diminution of temperature. Any large addition of water lessened the cooling effect. The following results were obtained: -
100 acid and 225 snow gave fall of 31°
„ 285 „ „ „
„ 340 „ „ „
„ 200 ,, „ „
As there was so little difference in the result when the snow was used within so wide limits, it was found most satisfactory to mix the snow with the acid until it attains the consistency of a thin mush, thus dispensing with all weighing. It is to be noted, as is explained by the above, that when the snow is wet, the temperature to be obtained with it is almost as low as when it is dry, which is far from being the case when hydrochloric acid is used. It will also be seen from the above figures that when working at a tempera-ture near zero, the "spent acids" answer as well as, if not better than hydrochloric acid; but when endeavouring to obtain lower temperatures than - 30° C. by previously cooling the acid, it was found that better results were obtained with hydrochloric acid. When snow is not available, there is equal satisfaction in employing shaved ice for this purpose.
Instead of using sulphurous acid, as in his previous machines, Pictet uses mixture of sulphurous acid and carbonic acid, which has received the name of "liquide pictet." The boiling point of this liquid under atmospheric pressure is at - 19° C, and at a temperature of + 50° C. the pressure of the gas is only half that of pare sulphurous acid. The inventor has some theory, according to which there takes place an actual chemical combination of the molecules of the two gases when they are being liquefied under pressure; and it is due to this property that the work expended in compression is much smaller than in any other working agent. The " liquide pictet" is not inflammable, and can even be used for the extinction of fires. It has the further advantage of leaving a greasy dew upon the surfaces of the cylinder, piston rod, valves, etc, rendering special lubrication unnecessary. The generator consists of a system of seamless copper pipes communicating with a chamber, at the bottom of which the liquid enters, whilst the gas is drawn off from the upper part. The arrangement of pipes is such as to facilitate an efficient circulation throughout the whole of the generator.
The principle of the apparatus is extremely simple, and can best be illustrated by the well-known lecture experiment intended to show the absorption of heat when liquids of low boiling point are evaporated in vacuum. If of two bulbs which are connected by a pipe, one be filled with a mixture of water and ammonia, and heated, the ammonia will distil over, and may be condensed in the other bulb if this be artificially cooled. Some water will also pass over, but eventually the liquid in the second bulb will be richer in ammonia than that in the first bulb, and if the source of heat be withdrawn, the ammonia will boil over and return to the first bulb, whilst the second bulb will be cooled far below the temperature of the cooling water previously applied.
The construction of the apparatus itself is of the utmost simplicity, and will be readily understood from Fig. 15. There are two horizontal tubes C D closed at the ends and placed horizontally the one above the other. They are joined by a short vertical tube F at either end, which reaches about half way down into the cavity of the lower tube. This part of the apparatus is called by the investor the " combiner," because in it the water and ammonia combine again when the ammonia vapour passes over from the "receiver" R through the connecting tube shown on the top. The receiver is also an iron tube, and is placed in the cold room, or in any other position where the effect of the apparatus is required. The combiner and receiver need not be placed in close proximity as shown; there may be any distance between them, so that the combiner, for instance, can be placed in the basement of a building close to the hot-water apparatus if the building be heated on the Perkins system, and the receiver in any other part of the building, but at a slightly higher level.
In such cases the lower tube of the combiner is provided with two Perkins's hot-water tubes E,indicated by dotted circles, and the same apparatus which is used to heat the building can be employed for supplying heat to the combiner, the hot water being simply circulated through the tubes E. At Perkins's factory this arrangement is actually in use, the hot water being diverted either into the heating pipes of the factory or into the combiners ot his cold air apparatus by a series of stop cocks, which are manipulated once or twice a day by an attendant. Where a heating apparatus is not available, the combiner is heated by gas issuing from the tube B, which is provided with atmospheric burners. The whole apparatus is perfectly sealed, so that no loss of ammonia can occur, the same charge serving over and over again. The process is as follows: The combiner is charged with a mixture ol pure ammonia and water, the strength being about that of the ordinary commercial ammonia. After a short application of heat the air is expelled by opening a stop cock, not shown, the escaping gases being led through a water seal, bo as to avoid the unpleasant smell of ammonia.
The loss of ammonia in blowing out the. apparatus is very trifling, and after this operation is performed the apparatus contains a very small quantity of air, ami if left to itself there would be a vacuum of about 25 in. in it. On applying heat in one form or another to the lower tube of the combiner, the ammonia begins to distil over, and is condensed on the way ' to the receiver by the action of the water jacket surrounding the connecting tube. A certain amount of water also distils over with the ammonia; but as the specific gravity of ammonia is very much less than that of water, the latter accumulates at the bottom of the receiver, and is conveyed back into the combiner by means of a siphon S, provided with a trap T within the receiver. The construction of the trap will be obvious from our diagram. It consists of a short vertical cylinder surrounding the upper end of the siphon tube, and provided with small holes at the bottom. Owing to the position of these holes, only water can find its way into the short cylinder T, and hence the siphon draws off the water without allowing the ammonia to follow.
150 lb. per sq. in. at the most, and the temperature within the combiner reaches about 270° F. When, after a few hours' boiling, all the ammonia has distilled over, the source of heat is withdrawn, and the combiner is cooled by an external application of water, which is allowed to How over its surface. The pressure immediately falls, and finally a vacuum of about 25 in. is produced, after which the ammonia in the receiver begins to boil and distil back into the combiner. In order to promote an intimate miiture of the ammonia which is passing over with the water left in the combiner, the connecting tubes F project some distance into the cavity of the lower tube C, as already mentioned. By this means the ammonia enters the liquid well within its mass, and is quickly absorbed.
Perkins's cooling apparatus.