Silk constructed his heater from electroconductive glass of the kind used by automobile manufacturers for defrostable windows. The glass has a thin, transparent layer of either tin oxide or indium-tin oxide deposited on one side. An electric current flowing through this layer can heat the glass to temperatures greater than the boiling point of water. (Although Silk independently developed the idea of using conductive glass, Stephen A. Skirius described a similar project in a 1984 issue of the Microscope magazine.) The main drawback is that I have been unable to find an inexpensive source of the material. So I've purchased and cut up a large sheet of it and will make the pieces available through the Society for Amateur Scientists; details are at the end of this article.

Once you have secured the glass, you must prepare it to accept current. Silk used two thin strips of copper about 3 millimeters (about 0.1 inch) wide to form the electrodes. You can cut these strips from a sheet of copper foil, which is stocked by most sheet-metal dealers and art supply shops. Place the glass so that the oxide layer is up and affix the electrodes onto the left and right sides [see illustration above] using a metallized epoxy, which unlike most adhesives will let the electricity through. Resins filled with silver or aluminum powder are available at most hardware stores. If you can't find any, call Epoxies, Etc., in Greenville, R.I. (401-231-2930).

Next you'll need to protect the oxide layer from scratching while exposing specimens to the heat. Silk placed a drop of glycerin at the center of the oxide side of the glass stage and covered it with a large but thin coverslip. If you don't have any glycerin, any fluid with a high boiling point will work, such as radiator fluid, brake fluid or motor oil. For the coverslip, Silk recommended a No. 0 grade (the thinnest grade) that is 22 by 30 millimeters (0.9 by 1.2 inches), but just about any size will work. This arrangement lets your specimens respond quickly to any adjustments you may make in the amount of heating.

The object of study goes on top of the coverslip. Blanket it with an 18-millimeter square glass cover. For ease of handling, this topmost cover should be of a thicker grade, either No. 1 or No. 2. Fisher Science Education in Burr Ridge, Ill., sells a collection of various coverslips in boxes of 100 for $3 (call 800-955-1177; item number CQS17525A).

Finally, attach the electrodes to a power source that can deliver enough current. Silk used a variable 15-volt, 1-amp AC or DC supply. You can probably find such a source at most electronics surplus stores for about $20. Parts suppliers--such as Jameco (800-831-4242 or www.jameco.com)--can also provide lower-power sources, but you may need to wrap the stage partially in insulating material, such as newspaper. Or try picking through the wares of those small electronics suppliers who sell at local swap meets or flea markets. I once came across an entire bin of power supplies for $4 each. Silk reported that his easily heats the stage to 100 degrees Celsius (212 degrees Fahrenheit). Naturally, never leave the stage unattended with the power on.

It is easy to get beautiful multicolored images using a set of polarizing filters. These filters act on the electromagnetic nature of light by passing only that energy whose electric field lies along a particular direction, known as the polarization axis of the filter. A double filter made by rotating two polarizers so that their axes are perpendicular should block all light. Odd as this may sound, adding a third polarizer between them can cause light to be transmitted again. Some light will pass through the added filter because its polarization axis is not at right angles to the axis of the first filter. Whatever light does get through must be aligned with the intermediate filter's axis. In effect, this filter rotates the electric field. Therefore, when the rotated light reaches the final filter, some can pass through.

Most crystals also polarize light because of the highly regular arrangement of their atoms. A crystal generally polarizes different wavelengths with different efficiencies, which causes some colors to come through more vividly than others. To capture this effect on film, position one polarizing filter above the light source and attach another to the objective lens of your microscope. Most large scientific supply houses carry such filters. Fisher Science Education sells a kit for $23 that allows you to construct both filters and even contains some crystalline material to study (item number CQS19709-3). Or you can experiment with making your own filters. Edmund Scientific in Barrington, N.J., has available high-quality polarizing, thin plastic sheets for $6 (call 609-573-6250; catalogue number H43781).

To study crystallization, place on the stage a material with a melting point between room temperature and 100 degrees C. By tuning the power supply to keep the stage temperature near the melting point of that material, you can alternately create and destroy the crystals by making small adjustments to the power. This experiment is much more satisfying than, say, watching salt crystals form by evaporation, during which you get to see such crystals only once before you must clean and reset the stage.

There are a number of materials that are safe to melt on the stage. Silk recommended thymol, camphor, menthol, stearic acid, trimyristin or myristic acid. You can obtain a mixture of trimyristin and myristic acid by soaking nutmeg in methanol and then filtering and evaporating the solvent. Most of the other substances you should be able to find in drugstores and chemistry sets. Consult the Merck Index for other ideas.

Of course, be careful. Some materials, such as naphthalene (once the active ingredient in mothballs), emit harmful gases when heated. (Modern mothballs often use paradichlorobenzene, which, though classified as an irritant, is not considered toxic to humans. And paradichlorobenzene crystals are a delight to study.) Never experiment with something unless you know it is safe or you are sure that nobody could be exposed to potentially poisonous fumes. Dedicated experimenters may want to invent a way to seal the cell hermetically or vent the vapors. A sealed cell could be used to incubate thermophilic organisms. I invite you to share your ideas on the relevant discussion area at the Society for Amateur Scientists Web page.

As a service to the amateur community, the Society for Amateur Scientists can provide the conductive glass for this project for $12 plus $3 shipping until May 2000. To place an order, call 1-401-823-7800 or send a check to 5600, Post Road, #114-341, East Greenwich, RI 02818. For more information about this and other projects from this department, check out the Society for Amateur Scientists Web page at http://www.sas.org/.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.