When it comes to personal hygiene, cats are nature's four-legged fanatics. They spend hours each day obsessively licking themselves clean while ritually burying their waste as a matter of instinct. The fastidious felines probably would be mortified to discover that their urine strikes humans as one of the most pungent and disagreeable odors in the animal kingdom.

The cat's constitution is believed to have evolved in desert climates, making their bodies extremely efficient in recycling water they drink. And that, alas, makes their urine highly concentrated.

Ever since ancient Egyptians first welcomed cats into their homes, caretakers have had to contend with the accompanying stench. Litter boxes filled with sand, sawdust or wood shavings were no match for the overwhelming aroma, so indoor cat ownership usually was limited to those most dearly devoted.

An accidental discovery in 1947 by a Michigan businessman named Edward Lowe, however, brought relief to cat owners whose fastidiousness rivaled that of their pets. He called it Kitty Litter.

Lowe, who died last month at 75, had been working for his father, delivering sawdust, sand and other absorbents of oil and grease to factories and other industrial settings. One January morning, his cat-loving neighbor, Kay Draper, called him to order sand for her litter box.

In a burst of inspiration, Lowe offered her an experimental alternative that his father was trying as an oil and grease absorbent: tiny chunks of clay. Draper loved the stuff, called for more, word spread and a business was born.

Clay remains the key. The natural material is porous, filled with countless microscopic cavities that can absorb large amounts of liquid. Raw clay consists of extremely fine particles of silicate-based mineral that measure, by definition, less than 1/256 of a millimeter across. That means about 6,500 particles must be aligned to stretch one inch. This is the same pastelike material used to make pottery and bricks.

When clay is taken from the ground, it usually contains about 30 percent moisture. To make cat litter, the large chunks are broken into smaller pieces and sent through a drying machine. Shaped like an elongated clothes dryer, the machine heats the pieces while tumbling them.

This reduces moisture content to about 5 or 6 percent. Then the clay chunks are milled into smaller fragments, less than one-fourth of an inch across. Pieces too large or too small are sifted out. After the treated clay is cut to litter size, various ingredients may be added to control dust, odor and bacteria. Litter companies use different ingredients in various combinations or none at all. Additives have little to do with the primary work of the clay.

Clumping cat litter, which binds when wetted, is made with smaller particles and a different variety of clay, usually one called bentonite, mixed with standard other clays in a family referred to as smectites. Wet bentonite particles naturally stick together.

At a 5 percent moisture level, cat litter typically has enough internal spaces to soak up almost its equivalent in weight.

In addition to absorbent qualities, clay has secondary features that make it ideal for litter boxes. It is inorganic, so bacteria cannot feed on it and grow, and also controls odor naturally.

This happens because the ammonia , NH3, in urine, which evaporates easily and creates the strongest part of cat urine's odor, binds to charged hydrogen atoms on the clay's surface, forming ammonium ions, NH4 . These ions do not readily evaporate and thus remain bound to the clay surface.

Lowe's bright idea may not have changed the world, but it has helped to make cats much more welcome. So much so, in fact, that the little felines have surpassed dogs as America's most popular pet. Meow!