GARDENING probably claims the interest of more enthusiasts than J all other avocations combined. Whether the plot comprises acres of winding paths and formal beds or only a bit of potted soil on the ledge of an apartment window, those who do the tilling share a common love of things that grow. Whether the gardener senses it or not he is a confirmed experimentalist, everlastingly testing nutrient, location, lighting and variety. To the extent that his efforts are guided by observation, analysis and test, the gardener is also an amateur scientist.

Nat E. Mankin of Chicago, a railroad freight-expediter for a nationwide transportation concern, is an extreme example of the casual gardener turned amateur botanist. His work in the field of soilless gardening, or hydroponics, during the past 20 years has attracted wide attention in both professional and lay circles. His experimental techniques rank with those of the best professionals. Despite the impressiveness of his accomplishments Mankin's two decades of off-hour fun have cost him little in terms of time and money-no more than other city dwellers spend maintaining a few potted plants. "A few seeds and $10 worth of chemicals," he says, "will keep you going a lifetime."

Experiments with growing plants in liquid solutions, according to Mankin, date back to the days of the Roman Empire, when plants were grown in jars and vessels to which fertilizer was added from time to time. History does not indicate what objective prompted the experiments of the Romans, nor how their scientific studies, if any, came out. But in view of the complex chemistry of organic fertilizers and the wide variation of their quality, it is safe to assume this early work lacked any form of experimental control. The first recorded attempt at a controlled study appears to have been made in 1699 by an English botanist named Woodward. He grew spearmint in water containing an extract of soil. Although he observed that without aeration plants continuously immersed in a solution soon turn yellow and die, Woodward did not achieve true hydroponic culture because he, and everyone else, had insufficient knowledge of chemistry.

The credit for pioneering hydroponics, in the modern meaning of the term, goes to the French chemist Jean Boussingault, whose work in South America during the early decades of the 19th century proved that plants could be grown in sand, charcoal, quartz and other inert materials to which inorganic solutions were added. His studies proved that plants cannot assimilate free nitrogen from the atmosphere, and he first evaluated the role of manures. In recognition of these and related contributions, he was invited to occupy the chair of chemistry at Lyons, moving later to that of agricultural chemistry in Paris.

Boussingault's methods were quickly taken up independently by two German workers, A. Knop and Julius von Sachs, the latter a botanist at the University of Wurzburg. They first focused attention on the fact that growing plants operate the world's largest chemical industry, and that with the proper control plants are powerful tools for determining how nature converts the simple chemicals of the atmosphere into complex food substances. The controlled techniques of Knop and Sachs for studying the irregularities affecting soil-grown crops have survived nearly a century of use; their basic formulas for nutrient solutions still serve as the starting point for most hydroponic research.

Despite its long history hydroponics did not become a popular branch of amateur science until 1929. In that year William F. Gericke, a plant physiologist at the University of California, developed a special technique for applying hydroponics to commercial crop production. The transition from laboratory tool to business enterprise was attended by widespread publicity. Soon seed stores from coast to coast were sending customers from their counters laden with packages of mineral salts.

"The successful growth of a robust crop," says Mankin, "is based on the balanced supply of two classes of nutrients: the major fertilizing element which supply most of the plant's food requirements, nitrogen, potassium, calcium, phosphorus and sulfur, and the minor trace elements such as iron, manganese, boron, zinc and copper. Minute amounts of these trace elements, along with vitamin B1, play a decisive role in maintaining the health of plants.

"The precise function of all the trace elements is not known-one of the things that attracts both professional and amateur botanists to the hydroponic technique. Without iron no green coloring forms in the plant. In the absence of boron no seeds develop. Deficiencies in other trace elements cause plants to be stunted, lacking in color or malformed. In contrast, a well-balanced diet of -major and minor elements makes it possible to grow plants more luxuriant than almost any that can be grown in soil. By soilless culture a single tomato plant of the Marglobe variety, for example, can be grown to a height of 25 feet and made to bear 20 pounds of perfect tomatoes. So effective is the technique that the Army Air Force maintained soilless gardens on Ascension Island, in British Guiana and on Iwo Jima during g; World War II for the large-scale production of food crops. A number of commercial enterprises with personnel quartered in the Tropics and arid parts of the world operate similar installations."

Soilless culture takes one of three forms, according to the means of mechanically supporting the plants: sand, gravel or water. Each of the three techniques has certain advantages and disadvantages, but all share the common distinction of giving the experimenter more control of the plant's nutrition than is possible with soil.

Sand culture, developed for commercial purposes by New Jersey and Rhode Island agricultural experiment stations during the 1920s, is perhaps the simplest of the three methods. Mankin recommends it highly for the beginner. "Seeds planted in a sand-filled flower pot," he explains, "are kept moistened with nutrient solution until they mature as full-grown plants. Like the soil of the conventional garden, the sand must have drainage, and it should be flushed with pure water about once a week to remove excess mineral salts. This technique is also known as 'slop' culture. Its advantage lies in the fact that fresh air is brought into contact with the root system daily when the application of the nutrient solution drives stale air from the sand. The beginner may expect gratifying results from sand culture, at least equal to those he can achieve with a good quality of topsoil. The principal disadvantage of sand culture lies in the fact that the nutrients in a solution tend to crystallize on the grains of sand. This obviously alters the concentration of the nutrients, and deprives the experimenter of precise chemical control over his culture. The variation in nutrients may seem slight when considered in absolute terms of grams or ounces, yet it is sufficient to induce astonishing changes in a plant's metabolism. The presence or absence of one part of vitamin B in a billion can make the difference between a healthy plant and one that is seriously ill. Another disadvantage of the method is that many common sands contain soluble minerals that contaminate the nutrient solution or radically alter the concentration of its trace elements. Sand need not contain much ferrous sulfate to alter the concentration of a nutrient solution calling for one part of iron per million."

For commercial soilless culture gravel has several practical advantages over sand or water. Chief among these is the relative ease and speed with which solutions may be pumped into and out of gravel-filled growing-tanks. The medium may consist of any coarse-grained, chemically inert solid, ranging from stream gravel to crushed granite and coal cinders. As with sand culture the medium tends to introduce variations in the nutrient solution. Gravel culture is less exacting than water culture, but if the experimenter fully exploits its conveniences it requires almost as much equipment. Mankin urges the beginner to master the sand technique, and then to shift directly to water.

"If the amateur is reasonably handy with tools," writes Mankin, "he can build his own growing-tank for experimenting with water culture or 'pure' hydroponics. A common five-gallon can of the type found at filling stations can with little labor be converted into a serviceable tank. The side of the can is removed, and its four corners are cut to a depth of about half an inch. Then, with the aid of pliers, the metal strips are bent outward along an even line and folded back against the sides of the can. It is then easy to flatten them with a - mallet or hammer, producing a rounded edge for the tank. After tightening its screw-cap the tank is painted inside and out with an asphalt paint or emulsion. The paint must be of petroleum origin; others are toxic. Allow the asphalt a few days to dry.

"The completed tank is then fitted with a growing-tray. This is made of conventional one- by four-inch stock of pine, fir, spruce or white cedar. Redwood is toxic to most plants. The tray should be fitted to drop easily into the tank. The corners may be joined with corrugated fasteners, but for necessary rigidity and strength they should be reinforced with wood screws.

"The bottom of the tray is covered with either a fine-mesh chicken wire or a hardware cloth of heavy gauge: 1/2 to 3/4-inch mesh. The mesh is attached to the tray by staples, and is supported by two or three narrow widths of metal strip spaced evenly across the bottom. The netting must also receive a protective coat of asphalt. The tray is suspended in the tank by means of four metal angle-braces extending from its upper edge at each corner. Finally the netting is covered with an inch or so of shredded excelsior and, over this, enough sphagnum moss, glass wool, dried hay or even coarse sawdust to fill the tray to its top. Make sure that no shreds of this litter extend through the mesh or over the sides. Toxic materials such as redwood sawdust should be avoided.

"For introductory work it is advisable -that the amateur use young transplants whose roots have been gently washed free of soil. Tomatoes take readily to pure hydroponic culture, and hence they are good plants with which to gain experience. A hole is made through the litter to receive the plant; a sharpened stick will serve as a suitable tool. The plant is inserted through the litter so that about an inch of stalk separates the root system from the mesh. The plant is then secured in position by packing the hole with moss or other litter. Be careful not to injure the plant by applying heavy pressure, but close the hole completely. The solution must be kept in the dark to avoid the growth of algae.

"The root system must be supplied with oxygen; thus some means of aerating the solution must be provided. A piece of glass tubing closed at one end with a porous ceramic plug may be inserted through the litter so the plug rests against the bottom of the tank. This is coupled to an air pump through a length of rubber hose. Complete aeration rigs, including an electrically driven air pump, can be purchased for a few dollars in stores specializing in the needs of tropical fish, or the experimenter can do the job with a bicycle tire-pump. The aerator must run for at least 15 minutes twice a day. Another scheme that works well uses the so-called 'continuous flow' method of aeration. Solution drips continually from an elevated container into the center of a narrow-necked funnel. A length of tubing leads from the spout of the funnel to the bottom of the tank. Bubbles trapped between successive drops are carried into the tank, which must also be equipped with an overflow siphon.

"Two alternatives are available with respect to preparing the nutrient solutions. Seed stores market a number of packaged plant foods under such brand names as Hyponex, Gilbert's Nutrient Formulas, New Plant Life and a number of others. Simple directions on the package explain how to make up the solution. The use of these preparations obviously limits the extent to which the amateur can manipulate the growth process. In contrast, solutions prepared from basic formulas encourage experiment and enable the advanced worker to vary the plant's nutritional intake at will. Unlike conventional gardening, water culture enables the experimenter to manufacture any variety of 'soil' he desires.

"Enough solution should be prepared to fill the tank to within an inch and a half of the bottom of the tray. This air space between the top of the solution and the tray is important. From it the hairs near the top of the root system take up oxygen.

"All ingredients for a basic plant food are available through most drugstores. Just as no universal diet exists for all animals, no one formula meets the nutritional requirements of all plants. But plant physiologists in a number of universities and agricultural experiment stations have developed suitable nutrients for groups of common plants.

"The following formula will give good results with most common garden vegetables and household flowers:

"All solutions must be adjusted for pH, or acidity. Most common vegetables and flowers prefer a slightly acid solution, their tolerance extending from pH 5 to 6 on a scale calibrated from 0 (extremely acid) to 14 (extremely alkaline). On this scale 7 represents a neutral solution. The experimenter must procure a kit for making pH tests. These are available through most seed stores. Although these kits take a variety of forms, most employ chemically treated paper, the color of which changes when it is dipped into a nutrient solution. The test paper is then matched against a varicolored chart calibrated in pH. The pH of the nutrient solution may then be adjusted to the correct range by adding minute amounts of sulfuric acid or potassium hydroxide. If the solution is too acid, potassium hydroxide is added if it is too alkaline, acid is used."

Similar tests, described in handbooks on chemistry, enable the advanced amateur to maintain a close check on the concentration of all the minerals in the nutrient solution. Although these tests are useful, the most powerful indicator is the plant itself. By continuously observing and recording changes in the size color, rate of growth and general health of the plant, its body, root system and leaves, the amateur learns to detect hunger signs that lead to refined growing techniques. By varying the proportion and amounts of the minerals, ideal foods can be developed for each species of plant in relation to its local ecology. The role of each element can be observed, and thus the art as well as the science of hydroponics can be mastered.

EVERY amateur astronomer who has polished a disk of glass on a pitch lap to make a telescope mirror knows that pitch slowly flows. Though pitch is a solid at ordinary temperatures, it is classed by science as a liquid with a viscosity many billions of times greater than that of water. The term viscosity has several meanings: to most people it suggests stickiness; to the physicist it means a resistance to flow arising from internal fluid friction.

It is a strange fact that the writers of instruction books on telescope making have not explained why a pitch lap does its work. They only tell how. Thus it has remained for a ringside observer to state it. In a recent article the experimental physicist John Strong writes: "The viscous nature of the pitch prevents quick changes in the polishing surface so that the polishing pressures are greatest and the polishing action is greatest on areas of the work where the glass surface is relatively high."

After a single reading this sentence may seem to be no more than a simple statement of the obvious, yet it is an almost unique expression of a rather subtle process. Amateur telescope makers are often dissatisfied with the empirical; then they seek the underlying reasons for the phenomena of their art. The following discussion should not be confused with others that treat with the nature of the process that polishes glass; it has solely to do with the behavior of the pitch.

The sentence by Strong deals with the viscosity of pitch, but pitch is also elastic. Some optical workers would challenge this statement. Lively arguments have occurred between those who say, like John M. Pierce, "Pitch is certainly elastic, otherwise it would be no good for non-spherical surfaces such as paraboloids," and others who answer, "Liquid yes, and viscous yes, but elastic no- not so long as the indentation of my thumbnail remains in a lap." One advanced amateur optical worker comments: "The elastic recovery of pitch is new to me." When asked whether the same substance could be both viscous and elastic, a professional optical worker replied simply, "No." The question was submitted to Strong. "Is there a restoring force (elasticity) in pitch?" He answered, "Yes." He was also asked: "How can pitch be viscous and elastic too?" His answer was, "See the bouncing putty" True enough, silicone putty is viscous elastic and visco-elastic. A ball of it will bounce, but when left on a table for a few hours the same ball will flow under the force of gravity into a pancake.

Possibly amateur optical workers, including the writer, should keep up to date with the sciences that touch upon their hobby. To attempt a remedy, the discipline of rheology was frontally attacked in the hope of finding an unequivocal answer about pitch. Rheology is the science of the deformation and flow of matter: gaseous, liquid and solid. The prefix "rheo" means flow, as in rheostat or diarrhea. Rheology deals with elasticity, viscosity, plastic flow, creep and other phenomena in liquids and resinous materials, of which pitch is an example, and in metals and other forms of matter, including crystals.

There is no elementary book on rheology, no "Rheology for Everybody." Its books, such as R. Houwink's Elasticity, Plasticity and the Structure of Matter, are elementary only to the physicist. To others they seem to begin in the middle. My investigation of the subject resembled a dog's attack on a porcupine. I did, however, find one simple demonstration that should convince anyone. Twist a length of pitch into a spiral. Release one end and the pitch untwists a little. The addition of a pointer may be needed to reveal this elastic effect. A curve was also found in E. N. da C. Andrade's Viscosity and Plasticity that describes the flow and recovery of "a pitch-like substance." Would it be safe to assume that this was also the curve for the actual pitch that is used by the optical worker? Even if so, would not the attempt of a non-rheologist to write an interpretation of it be brash, considering that rheology is slippery even for the rheologist? A brief inquiry sent to the Hercules Powder Company of Wilmington, Del., the largest manufacturer of naval-stores products, brought a fortunate answer. The following letter was written for the amateur telescope maker with pitch in his hair by W. H. Markwood, a research supervisor at the Hercules Experiment Station and a vice president of The Society of Rheology.

"As an interested reader of 'The Amateur Scientist' I am pleased at the opportunity to tell you about the rheological behavior of pine pitch during the polishing of optical surfaces. To begin with, materials of this sort are both elastic and viscous. However, one may not say that they have only one elasticity and/or one viscosity even at only one temperature. They are also influenced by how fast one makes them flow, that is, how hard they are pushed. Let us confine ourselves to the region between room temperature and the softening point (both wood and gum rosins 'melt' at about 180 degrees Fahrenheit). In this range, if a resinous material is pressed very gently it flows in the manner of a very thick oil, and stays put when the force is removed. It acts as though it were viscous only. On the other hand, if a great force is suddenly applied, it appears to react like an entirely elastic crystal. The applied force may bounce back or the 'crystal' may shatter.

"For intermediate forces and intermediate rates of force-application it seems to behave in three ways. There appear to be (1) a purely elastic component like a time-independent ideal spring, (2) an elastic component that will slowly recover after force removal and (3) a non-recoverable, viscous component. The middle one is sometimes called creep or visco-elasticity or elastoviscosity or recoverable elasticity or retarded elasticity. The ideal spring part deforms instantly in direct proportion to the force applied, while the viscous part deforms, that is, flows, at a rate that is proportional to the force. However, the visco-elastic deformation is dependent not only on how much force but on how fast it is applied. This idea of 'parts' can be confusing. A thing is a thing and not three things. This 'thing' has what might be called a multi-behavioristic attitude.

"Let's examine the curve you traced from Andrade's Viscosity and Plasticity. The curve was intended to illustrate the above. If the ordinate represents deformation under a constant load, if the abscissa is time and if the maximum on the plot is the time at which the load was suddenly removed, all the 'parts' described above are illustrated. At first a sudden, elastic jump occurs, deformation then slows down and becomes a combination of visco-elastic and viscous distortion for a while (curved part), then straightens out into 'pure' viscous flow. When the load is removed, an elastic rebound takes place that is equal to the initial jump; then a slow, time-dependent, retarded-elastic recovery appears, finally the material stops recovering and it is seen that a 'set' has taken place that is permanent and represents the viscous deformation only.

"One can go further and visualize models to describe phenomenologically this 'multi-behavior.' Let the purely elastic part be represented by an ideal spring, the purely viscous part by an oil-filled dashpot, and the visco-elastic part by a spring whose action is damped by a dashpot connected rigidly parallel to it. If these are added together the situation shown in the enclosed drawing obtains.

"Suppose one suspends this model vertically and hangs a weight on it. At the instant the weight is released the top spring stretches until the elastic modulus times the deformation of the spring just balances the weight. At this point (still zero time) the second spring begins to stretch, but its motion is damped by the dashpot, which first carries all the load, gradually transferring it to the spring until its elastic modulus times its strains also balances the effect of the weight, when its motion stops. However, the piston in the viscosity dashpot has also been moving and will continue to move linearly with time so long as the weight is applied. If the weight is now removed the elasticity spring will snap back to its original position in zero time; the viscoelastic spring will try to do the same but will again be retarded by the 'oil flow' in its dashpot and will arrive back at its initial position in exactly the time it took to stretch. In the meantime the whole viscosity dashpot will go along for the ride; that is, its piston will not change position relative to its cup. This use of a model is a pretty crude but somewhat useful conception in a physical sense. It is often employed by the rheologist. Furthermore, the concept is easily handled mathematically.

"Looking again at the model, it is easy to visualize that if you pull on it fast enough and let it go fast enough only the elastic spring will stretch and recover. If you pull on it very, very slowly the viscosity dashpot will move before either spring can stretch without recovering in an infinitely short time as deformation proceeds-which brings us back to paragraph four.

"If the constant load we picked had been a different one, the behavior pattern would have been the same, but the relaxation time would have been different. There is not just one but a whole spectrum of them, corresponding to spectra of elasticities and viscosities.

"I believe that if you'll warm some pitch, press on it hard, remove the load and watch, you'll see some small, slow recovery, indicating visco-elasticity. If you strike it rapidly and lightly, no change in shape will be seen. If you press on it very slowly and remove the load, no recovery will occur. Of course its own mass will make it flow also which can be seen if it is either warm enough or if you wait long enough."

Now that Markwood has assured us that pitch really is elastic as well as viscous, it is interesting to note that the great 19th-century English astronomer Sir John Herschel suspected the same thing from his optical shop experience. It is probable that he heard it from his famous telescope-making father William Herschel. John Herschel also believed that if pitch were not elastic, a mirror could not be parabolized on it. The following quotation from the younger Herschel's book The Telescope, published in 1861, concerning the polishing and parabolization of speculum-metal mirrors on a pitch lap, contains crystal-clear proof that both facts were known to him a century ago. For some reason they have not been restated in recent books on the art. Herschel wrote:

"When the metal is reduced by grinding to a perfectly true and even surface, free from the smallest perceptible scratch, it will be found reflective enough to afford an image of a star, or of a distant white object, sufficiently distinct to try whether its focal length is correct; and if it be so, the process of polishing may be commenced, the object of which is not merely to communicate a brilliantly reflective surface, but at the same time a truly parabolic form. If the material of-the tool on which this operation is performed were perfectly hard and non-elastic, it is evident that this would be impracticable, since none but a spherical form could arise from any amount of friction on such a material once supposed spherical; and even if parabolic it could not communicate that form to a more yielding body worked upon it.... Happily, however, there exists a material which, with sufficient hardness to offer a considerable resistance to momentary pressure, is yet yielding enough to accommodate its form to that pressure when prolonged, and at the same time sufficiently elastic to recover it if quickly relieved; that substance is pitch, whose properties, in this respect, were at once taken advantage of by Newton, with that sagacity which distinguished all his proceedings, as the fitting material for a polisher."

Herschel, who so clearly understood the rheological behavior of pitch, was mistaken about Newton, who did not understand it. As is well known, Newton was unable to parabolize the two-inch spherical mirrors he made for the first reflecting telescopes in 1668 and 1671, and his description of their polishing, in Opticks, shows why. He used a pitch lap "as thin as a groat," so thin that its elasticity was too insignificant to be effective, a groat was thinner than a well-worn dime. The probable explanation is that Herschel wrote without referring to Opticks.

Most optical workers buy their pitch retail and cannot trace it through trade channels to its origin. Some use mineral pitch but many dislike it. Most of the pitch used in optical work is pine pitch. Asked about pine pitch Markwood replies: "Pitch is a general term applied to many natural and proprietary mixtures. There are two sources of such materials which, with other products, are commonly called naval stores. The first is from the living pine tree. When the tree is wounded by scoring it near the base, an oleoresin exudes which is a mixture of turpentine and rosin. These are separated by distilling out the turpentine.

"The second source is the stumps and heartwood of large virgin pine trees. By far the largest method of processing this wood is to grind it, extract it with a hydrocarbon solvent and distill out the oils to give a residue called wood rosin. Much of the rosin produced in this manner is refined by the use of selective solvents or adsorbents to give pale-colored rosin.

"Another method of processing pine-stump wood is to destructively distill it in retorts, using the technique employed for producing charcoal from hardwood. Pine-tar oil is volatilized from the wood. Most of the volatile material is removed from the product, leaving pine pitch. It softens at a lower temperature than rosin and contains less acidic material. Many of the commercial pitches sold under various names are obtained by this retort process. The compounded pitches are probably made either by modification of these materials or from rosin or related materials by plasticizing them with pine oils or other softeners to obtain the softening point desired."

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