PETER SPARKS reaches over and flips open a tawny Thomas Carlyle tome, circa 1890. As he leafs through the pages, pieces of the aged book flake off and fall in his lap.
''This is a very brittle book,'' says the director of preservation for the Library of Congress. ''We have millions like this here - and the potential for millions more to get like it.''
Deterioration of the printed word represents one of the biggest problems facing libraries, museums, and archives today. Library of Congress officials call it ''the greatest crisis facing scholarship.'' Dr. Carlton Rochell, dean of libraries at New York University, terms it a ''national crisis that threatens to irrevocably diminish our intellectual heritage.''
Science has helped halt the disintegration of books. Preservationists, for instance, can strengthen a binding as neatly as a carpenter can bolster a sagging porch. Holes in pages can be patched, worn covers revived. But much of this effort goes into saving rare books. Refurbishing the mass of volumes yellowing on shelves is beyond the financial and technical ability of most libraries.
Now, however, two new tools may help slow the demise in the stacks - and put preservation within reach of the average archivist. One is the advent of new electronic storage technologies. The other is coming out of chemistry. Neither, however, is coming any too soon. Consider:
* An estimated one-third of the books in US libraries are seriously decomposed.
* Nearly 3 million of the 12 million volumes in the Library of Congress general and law book collections are too brittle to be used.
* The average life expectancy of books published since 1850 is 50 to 100 years. Some turn to dust in as little as 25 years.
The villain in all of this isn't age, but acid - in the very chemicals used in making paper. Before the early 1800s, paper was made from rags of linen, cotton, or other plant fibers and could last for centuries. The Library of Congress has some 900-year-old Chinese manuscripts.
But modern machinery and the growing demand for reading material changed all that. There wasn't enough rag to produce enough books. So papermakers turned to plentiful wood pulp.
This works fine, except that chemicals have to be added to break down the wood into fibers and then to whiten and stiffen them. The bleaching and sizing agents leave residues in the paper that, together with elements in the environment, create acids that ''burn'' books. Some acid-free paper is being produced, but not in large quantities or at small prices.
Scientists are trying to perfect quick, low-cost ways to neutralize acids by painting or spraying chemicals on by hand - but not just page by page but book by book or even stack by stack.
Enter a mass ''deacidification'' process being developed by the Library of Congress. The key ingredient is diethyl zinc (DEZ), used in a gaseous state. It permeates a book and neutralizes the acids. But if exposed to air or water, DEZ will ignite. Thus it requires a special chamber.
In their most recent tests, library chemists gassed books in a vacuum chamber supplied by the National Aeronautics and Space Administration. The books were first packed in milk crates and put in the chamber to dry, so no water would be left in the pages. Then they were enveloped in the gas for four to five days.
The technique, according to Sparks, should boost the life expectancy of a book from 100 years to ''five or six hundred.'' This fall - if congressional funding permits - the library will begin building a gas treatment center capable of handling 500,000 volumes a year.
Another batch-treatment process, this one a liquid technique, is operating at the Canadian Public Archives in Ottawa. ''The results are unbelievable,'' says Richard Smith, developer of the process and president of Wei T'o Associates, an Illinois firm named after the ancient Chinese god believed to protect books.
At full tilt, the Canadian plant will handle 5,000 books a week. Smith puts the treatment cost at $3 to $3.50 per volume. The Library of Congress process is expected cost $3 to $4 a volume.
The other promising technology that may help slow untimely book ends doesn't physically preserve books. It preserves the information they contain. The process involves lasers. The tool: the digital optical disk - a laser-read storage record that can hold up to 20,000 pages of text. The idea is to store those volumes that don't need to be kept in book form - scientific journals, for instance - on a disk that readers can call up by computer.
Microfilm serves a similar purpose, but it can be costly and cumbersome.
The Library of Congress plans a $1.7 million pilot project, which will put 1 million pages of book, newspaper, map, and photographic material on digital disks.
Still, these new techniques represent just a prologue in the volume of work that remains to be done in order to preserve the nation's written heritage. Cooperative library networks and regional preservation activities are helping. But, as Rochell sums it up, ''We are just beginning to nibble away at it.''