Part 4

In the case of green vegetable matters which require to be reduced rapidly to a fine state of subdivision in order to secure even a fairly good sample some special provision must be made. This is the case with stalks of maize and sugar cane, root crops, such as potatoes and beets, and green fodders, such as clover and grasses. The chopping of these bodies into fine fodders by hand is slow and often impracticable. The particles rapidly lose moisture and it is important to secure them promptly as in the preparation of beet pulp for polarization. For general use we have found the apparatus shown in fig. 2 quite satisfactory in this laboratory. It consists of a series of staggered circular saws carried on an axis and geared to be driven at a high velocity, in the case mentioned, 1,400 revolutions per minute. The green material is fed against the revolving saws by the toothed gear-work shown, and is thus reduced to a very fine pulp, which is received in the box below. Stalks of maize, green fodders, sugar canes, beets and other fresh vegetable matters are by this process reduced to a fine homogeneous pulp, suited for sampling and for analytical operations. Such pulped material can also be spread in a fine layer and dried rapidly at a low temperature, thus avoiding danger of fermentative changes when it is desired to secure the materials in a dry condition or to preserve them for future examination. Samples of sorghum cane, thus pulped and dried, have been preserved for many years with their sugar content unchanged.

Such a machine is also useful in preparing vegetable matter for the separation of its juices in presses. Samples of sugar cane, sugar beets, apples and other bodies of like nature can thus be prepared to secure their juices for chemical examination. Such an apparatus we have found is fully as useful and indispensable in an agricultural laboratory as a drug mill for air-dried materials.

It is often desirable in the preparation of roots for sugar analysis to secure them in a completely disintegrated state, that is with the cellular tissues practically all broken. Such a pulped material can be treated with water and the sugar juices it contains thus at once distributed to all parts of the liquid mass. The operation is known as instantaneous diffusion. The pulp of the vegetable matter is thus introduced into the measuring flask along with the juices and the content of sugar can be easily determined. Several forms of apparatus have been devised for this purpose, one of which is shown in fig. 3. This process, originally devised by Pellet, has come into quite general use in the determination of the sugar content of beets. It is observed that it can be applied to other tubers, such as the turnip, potato, artichoke, etc. It is desirable, therefore, that an agricultural laboratory be equipped with at least three kinds of grinding machines; viz., first, the common drug mill used for grinding seeds, air-dried fodders, and the like; second, a pulping machine like the system of staggered saws above described for the purpose of reducing green vegetable matter to a fine state of subdivision, or one like the pellet rasp for tubers; third, a mill for general use such as is employed for making sausages from soft animal tissues.

=10. Grinding Apparatus at Halle Station.=—The machine used at the Halle station for grinding samples for analysis is shown in Fig. 4. It is so adjusted as to have both the upper and lower grinding surfaces in motion. The power is transmitted through the pulley D, which is fixed to an axis carrying also the inner grinding attachment B. Through C₂, C₃, C₄, and C₁, the reverse motion is transmitted to the outer grinder A. By means of the lever E the two grinding surfaces can be separated when the mill is to be cleaned. The dree mill above described is especially useful for grinding malt, dry brewers’ grains, cereals for starch determinations and similar dry bodies. It is not suited to grinding oily seeds and moist samples. These, according to the Halle methods, are rubbed up in a mortar until of a size suited to analysis, and samples such as moist residues, wet cereals, mashes, beet cuttings, silage, etc., are dried before grinding. If it be desired to avoid the loss of acids which may have been formed during fermentation, about ten grams of magnesia should be thoroughly incorporated with each kilogram of the material before drying.

=11. Preliminary Treatment of Fish.=—The method used by Atwater in preparing fish for analysis is given below. The same process may also be found applicable in the preparation of other animal tissues. The specimens, when received at the laboratory, are at once weighed. The flesh is then separated from the refuse and both are weighed. There is always a slight loss in the separation, due to evaporation and to slimy and fatty matters and small fragments of the tissues which adhere to the hands and the utensils employed in preparing the sample. Perfect separation of the flesh from the other parts of the fish is difficult, but the loss resulting from imperfect separation is small. The skin of the fish, although it has considerable nutritive value, should be separated with the other refuse.

The partial drying of the flesh for securing samples for analytical work is accomplished by chopping it as finely as possible and subjecting from fifty to one hundred grams of it for a day to a temperature of 96° in an atmosphere of hydrogen. After cooling and allowing to stand in the open air for twelve hours, the sample is again weighed, and then ground to a fine powder and made to pass a sieve with a half millimeter mesh. If the samples be very fat they cannot be ground to pass so fine a sieve. In such a case a coarser sieve may be used or the sample reduced to as fine and homogeneous a state as possible, and bottled without sifting.

The reason for drying in hydrogen is to prevent oxidation of the fats. As will be seen further on, however, such bodies can be quickly and accurately dried at low temperatures in a vacuum, and thus all danger of oxidation be avoided. In fact, the preliminary drying of all animal and vegetable tissues, where oxidation is to be feared, can be safely accomplished in a partial vacuum by methods to be described in another place. In order to be able to calculate the data of the analysis to the original fresh state of the substance, a portion of the fresh material should have its water quantitively determined as accurately as possible.

DRYING ORGANIC BODIES.

=12. Volatile Bodies.=—In agricultural analysis it becomes necessary to determine the percentage of bodies present in any given sample which is volatile at any fixed temperature. The temperature reached by boiling water is the one which is usually selected. It is true that this temperature varies with the altitude and within somewhat narrow limits at the same altitude, due to variations in barometric pressure. As the air pressure to which any given body is subjected, however, is a factor in the determination of its volatile contents, it will be seen that within the altitudes at which chemical laboratories are found, the variations in volatile content will not be important. This arises from the fact that while water boils at a lower temperature, as the height above the sea level increases, the corresponding diminished air pressure permits a more ready escape of volatile matter. As a consequence, a body dried to constant weight at sea level, where the temperature of boiling water is 100°, will show the same percentage of volatile matter as if dried at an altitude where water boils at 99°. When, therefore, it is desirable to determine the volatile matter in a sample approximately at 100°, it is better to direct that it be done in a space surrounded by steam at the natural pressure rather than at exactly 100°, a temperature somewhat difficult to constantly maintain. However, where it is directed or desired to dry to constant weight exactly at 100°, it can be accomplished by means of an air-bath or by a water-jacketed-bath under pressure, or to which enough solid matter is added to raise the boiling-point to 100°. It is not often, however, that it is worth while to make any special efforts to secure a temperature of 100°. When bodies are to be dried at temperatures above 100°, such as 105°, 110°, and so on, an air-bath is the most convenient means of securing the desired end. The different kinds of apparatus to be employed will be described in succeeding paragraphs.

=13. Drying at the Temperature of Boiling Water.=—The best apparatus for this process is so constructed as to have an interior space entirely surrounded with boiling water or steam, with the exception of the door by which entrance is gained thereto. The metal parts of the apparatus are constructed of copper, and to keep a constant level of water and avoid the danger of evaporating all the liquid, it is advisable to have a reflux condenser attached to the apparatus. It is also well to secure entrance to the interior drying oven, not only by the door, but also by small circular openings, which serve both to hold a thermometer and to permit of the aspiration of a slow stream of dry air through the apparatus during the progress of desiccation. The gaseous bodies formed by the volatilization of the water and other matters are thus carried out of the drying box and the process thereby accelerated. The bath should be heated by a burner so arranged as to distribute the flame as evenly as possible over the base. A single lamp, while it will boil the water in the center, will not keep it at the boiling-point on the sides. The temperature of the interior of the bath will not therefore reach 100°. The interior of the oven should be coated with a non-detachable carbon paint to promote the radiation of the heat from its walls, as well as to protect the parts from oxidation where acid fumes are produced during desiccation. Instead of a reflux condenser a constant water level may be maintained in the bath by means of a mariotte bottle or other similar device.

When a bath of this kind is arranged for use with a partial vacuum, it should be made cylindrical in shape, with conical ends, as shown in fig. 5, in order to bear well the pressure to which it is subjected. Among the many forms of steam-baths offered, the analyst will have but little difficulty in selecting one suited to his work. To avoid radiation the exterior of the apparatus should be covered with a non-conducting material.

=14. Drying In a Closed Water Oven.=—When it is desired to keep the temperature of a drying oven exactly at 100° instead of at the heat of boiling water, a closed water oven with a thermostat is to be employed. The oven should be so constructed as to secure a free circulation of the water about the inner space. Since as a rule the water between the walls of the apparatus will be subjected to a slight pressure, these walls should be made strong, or the cylindrical form of apparatus should be used. The thermostat used by the Halle Station is shown in Fig. 6. A =⋃= shaped tube, with a bulb on one arm and a lateral smaller tube sealed on the other, is partly filled with mercury and connected by rubber tubes on the right with the gas supply, and on the left with the burner. The end carrying the bulb is connected directly by a rubber and metal tube with the water space of the oven. This device is provided with a valve which is left open until the temperature of the drying space reaches about 95°. The tube conducting the gas is held in the long arm of the =⋃= by means of a cork through which it passes air-tight and yet is loose enough to permit of its being moved. Its lower end is provided with a long ▲ shaped slit. When the valve leading to the water space is closed and the water reaches the boiling point, the pressure of the vapor depresses the mercury in the bulb arm of the =⋃= and raises it in the other. As the mercury rises it closes the wider opening of the ▲ shaped slit, thus diminishing the flow of gas to the burner. By moving the gas entry tube up or down a position is easily found in which the temperature of the drying space, as shown by the thermometer, is kept accurately and constantly at 100°.

In a bath arranged in this way a steam condenser is not necessary. Since, however, in laboratories which are not at a higher altitude than 1,000 feet the boiling-point of water is nearly 100°, it does not seem necessary to go to so much trouble to secure the exact temperature named. There could be no practical difference in the percentage of moisture determined at 100°, and at the boiling-point of water at a temperature not more than 1° lower.

=15. Drying in an Air-Bath.=—In drying a substance in a medium of hot air surrounded by steam, as has been described, the process is, in reality, one of drying in air. The apparatus usually meant by the term air-bath, however, has its drying space heated directly by a lamp, or indirectly by a stratum of hot air occupying the place of steam in the oven already described. The simplest form of the apparatus is a metal box, usually copper, heated from below by a lamp. In the jacketed forms the currents of hot air produced directly or indirectly by the lamp are conducted around the inner drying oven, thus securing a more even temperature. The bodies to be dried are held on perforated metal or asbestos shelves in appropriate dishes, and the temperature to which they are subjected is determined by a thermometer, the bulb of which is brought as near as possible to the contents of the dish. One advantage of the air-bath is in being able to secure almost any desired temperature from that of the room to one of 150° or even higher. Its chief disadvantage lies in the difficulty of securing and maintaining an even temperature throughout all parts of the apparatus. Radiation from the sides of the drying oven should be prevented by a covering of asbestos or other non-combustible and non-conducting substance. The burner employed should be a broad one and give as even a distribution of the heat as possible over the bottom of the apparatus.

=16. Spencer’s Air-Drying Oven.=—In order to secure an even distribution of the heat in the desiccating space of the oven, Spencer has devised an apparatus, shown in the figure, in which the temperature is maintained evenly throughout the apparatus by means of a fan. The oven has a double bottom, the space between the two bottoms being filled with air. The sides are also double, the space between being filled with plaster. The fan is driven by a toy engine connected with the compressed air service or other convenient method. Thermometers placed in different parts of the apparatus, while in use, show a rigidly even heat at all points so long as the fan is kept in motion. The actual temperature desired can be controlled by a gas regulator. This form of apparatus is well suited to drying a large number of samples at once. Portions of liquids and viscous masses may also be dried by enclosing them in bulbs and connecting with a vacuum.

Spencer’s oven can also be used to advantage in drying viscous liquids in a partial vacuum. For this purpose the flask A, Fig. 7, containing the substance is made with a round bottom to resist the atmospheric pressure. Its capacity is conveniently from 150 to 200 cubic centimeters. It is closed with a rubber stopper carrying a trap, H Hʹ, to keep the evaporated water from falling back. The details of the construction of the trap H are shown at the right of the figure. The vapors enter at the lateral orifice, just above the bulb, while the condensed water falls back into the bulb instead of into the flask A. A series of flasks can be used at once connected through the stopcocks G with the circular tube E leading to the vacuum. A water pump easily exhausts the apparatus, maintaining a vacuum of about twenty-seven inches. The hot air in the oven is kept in motion by the fan B, thus ensuring an even temperature in every part. The flask A may be partly filled with sand or pumice stone before the addition of the samples to be dried, and the weight of water lost is determined by weighing A before and after desiccation. If it be desired to introduce a slow current of dry air or some inert gas into A, it is easily accomplished by passing a small tube, connected with the dry air or gas supply, through the rubber stopper and extending it into the flask as far as possible without coming into contact with the contents.

=17. Drying Under Diminished Air Pressure.=—The temperature at which any given body loses its volatile products is conditioned largely by the pressure to which it is subjected. At an air pressure of 760 millimeters of mercury, water boils at 100° but it is volatilized at all temperatures. As the pressure diminishes the temperature at which a body loses water at a given rate falls. This is a fact of importance to be considered in drying many agricultural products. This is especially true of those containing oils and sugars, nearly the whole number. Invert sugar especially is apt to suffer profound changes at a temperature of 100°, the levulose it contains undergoing partial decomposition. Oils are prone to oxidation and partial decomposition at high temperatures in the presence of oxygen.

In drying in a partial vacuum therefore a double advantage is secured, that of a lower temperature of desiccation and in presence of less oxygen. It is not necessary to have a complete vacuum. There are few organic products which cannot be completely deprived of their volatile matters at a temperature of from 70° to 80° in a partial vacuum in which the air pressure has been diminished to about one-quarter of its normal force.

=18. Electric Drying-Bath.=—The heat of an electric current can be conveniently used for drying in a partial vacuum by means of the simple device illustrated in Fig. 8. In ordering a heater of this kind the voltage of the current should be stated. The current in use in this laboratory has a voltage of about 120, and is installed on the three wire principle. It is well to use a rheostat with the heater in order to control the temperature within the bell jar. The ground rim of the bell jar rests on a rubber disk placed on a thick ground glass or a metal plate, making an air-tight connection. A disk of asbestos serves to separate the heater from the dish containing the sample, in order to avoid too high a temperature.

=19. Steam Coil Apparatus.=—For drying at the temperature of superheated steam, it is convenient to use an apparatus furnished with layers or coils of steam pipes. The drying may be accomplished either in the air or in a vacuum. In this laboratory a large drying oven, having three shelves of brass steam-tubes and sides of non-conducting material, is employed with great advantage. The series of heating pipes is so arranged as to be used one at a time or collectively. Each series is furnished with a separate steam valve, and is provided with a trap to control the escape of the condensed vapors. In the bottom of the apparatus are apertures through which air can enter, which after passing through the interior of the oven escapes through a ventilator at the top. With a pressure of forty pounds of steam to the square inch and a free circulation of air, the temperature on the first shelf of the apparatus is about 98°; on the second from 103° to 104°, and on the third about 100°. The vessels containing the bodies to be dried are not placed directly on the brass steam pipes, but the latter are first covered with thick perforated paper or asbestos. For drying large numbers of samples, or large quantities of one sample, such an apparatus is almost indispensable to an agricultural laboratory.

A smaller apparatus is shown in Fig. 9. The heating part G is made of a small brass tube arranged near the bottom in a horizontal coil and continued about the sides in a perpendicular coil. Bodies placed on the horizontal shelf are thus entirely surrounded by the heating surfaces except at the top. The steam pipe S is connected with the supply by the usual method, and the escape of the condensation is controlled either by a valve or trap in the usual way. The whole apparatus is covered by a bell jar B, resting on a heavy cast-iron plate P, through which also the ends of the brass coil pass. The upper surface of the iron plate may be planed, or a planed groove may be cut into it, to secure the edge of the bell jar. When the air is to be exhausted from the apparatus, a rubber washer should be placed under the rim of the bell jar. The latter piece of apparatus may either be closed, as shown in the figure, by a rubber stopper, or it is better, though not shown, to have a stopper with three holes. One tube passes just through the stopper and is connected with the vacuum; the second passes to the bottom of the apparatus and serves to introduce a slow stream of dry air or of an inert gas during the desiccation. The third hole is for a thermometer. When no movement of the residual gas in the apparatus is secured, a dish containing strong sulfuric acid S’ is placed on the iron plate and under the horizontal coil, as is shown in the figure. The sulfuric acid so placed does not reach the boiling-point of water, and serves to absorb the aqueous vapors from the residual air in the bell jar. By controlling the steam supply the desiccation of a sample can be secured in the apparatus at any desired temperature within the limit of the temperature of steam at the pressure used. Where no steam service is at hand a strong glass flask may be used as a boiler, in which case the trap end of the coil must be left open. The vacuum may be supplied by an air or bunsen pump. When a vacuum is not used an atmosphere of dry hydrogen may be supplied through H.

=20. Carr’s Vacuum Oven.=—A convenient drying oven has been devised in this laboratory by Carr. It is made of a large tube, preferably of brass. The tube may be from six to nine inches in diameter and from twelve to fifteen inches long. One end is closed air-tight by a brass end-piece attached by a screw, or brazed. The other end is detachable and is made air-tight by ground surfaces and a soft washer. In the figure this movable end-piece is shown attached by screw-nuts, but experience has shown that these are not necessary. On the upper longitudinal surfaces are apertures for the insertion of a vacuum gauge and for attachment to a vacuum apparatus.

In the figure the thermometer and aperture for introducing dry air or an inert gas are shown in the movable end disk, but they would be more conveniently placed in the fixed end. The oven is heated below by a gas burner, which conveniently should be as long as the oven. The heat is not allowed to strike the brass cylinder directly, but the latter is protected by a piece of asbestos paper.

The temperature inside of the oven can be easily kept practically constant by means of a gas regulator, not shown in the figure, or by a little attention to the lamp. For a vacuum of twenty inches a temperature of about 80° should be maintained. When the vacuum is more complete a lower temperature can be employed. This apparatus is simple in construction, strong, cheap, and highly satisfactory in use.

=21. Drying in Hydrogen.=—In some of the processes of agricultural analysis it becomes important to dry the sample in hydrogen or other inert gas. This may be accomplished by introducing the dry gas desired into some form of the apparatus already described. The drying may either be accomplished in an atmosphere of hydrogen practically at rest or in a more limited quantity of the gas in motion. The latter method is to be preferred by reason of its greater rapidity. The analyst has at his command many forms of apparatus designed for the purpose mentioned above. It will be sufficient here to describe only two, devised particularly for agricultural purposes.

The first one of these, designed by the author, was intended especially for drying the samples of fodders for analysis according to the methods of the Association of Agricultural Chemists.

For the purpose of drying materials contained in flasks and tubes in a current of hydrogen the apparatus shown in Fig. 11 is used. This apparatus consists of a circular box, B, conveniently made of galvanized iron, having a movable cover, S, fitted for the introduction of steam into the interior of the apparatus. Condensed steam escapes at W. A stream of perfectly pure and dry hydrogen enters at H, passes up through the material to be dried, down through the bulb V, containing sulfuric acid, and follows the direction of the arrows through the rest of the apparatus. The stream of hydrogen is thus completely dried by passing through bulbs containing sulfuric acid, on the way from one piece of the apparatus to the other. A, represents a flask such as is used, with the extraction apparatus described. The apparatus which we have used will hold eight tubes or flasks at a time, and thus a single stream of hydrogen is made to do duty eight times in drying eight separate samples. The great advantage of the apparatus is in the fact that the stream of hydrogen must pass over and through the substance to be dried. In order to prevent any sulfuric acid from being carried forward into the next tube the bulb K, above the sulfuric acid, may be filled with solid pieces of soda or potash.

This apparatus has been in use for a long time and no accidents from sulfuric acid being carried forward have occurred, and there is no danger, provided the stream of hydrogen is kept running at a slow rate. If, however, by any accident the stream of hydrogen should be admitted with great rapidity, particles of the sulfuric acid might be carried forward and spoil the next sample. To avoid any such accident as this the proposal to introduce the potash bulb has been made. The apparatus works with perfect satisfaction, and it is believed that when properly adjusted check weighings can be made by weighing the bulbs, showing their increase in weight, which will give the volatile matter, and weighing the flasks or tubes, which will show the loss of weight. The only chance for error in weighing the bulbs is that some of the volatile matter may be material which is not dissolved in sulfuric acid, and is thus carried on and out of the apparatus. The blackening of the sulfuric acid in the bulbs, in the drying of all forms of organic matter, shows that the loss in weight of such bodies is not due to water alone, but also to organic volatile substances, which are capable of being decomposed by the sulfuric acid, thus blackening it.