What’s the story be­hind the glass panels in the floor by the U.S. Capi­tol’s Se­nate cham­ber?

The Washington Post Sunday - - METRO - Do you want An­swer Man to shed some light on a Wash­ing­ton mys­tery? Write an­swer­man@wash­post.com. Twit­ter: @johnkelly

What’s the story be­hind the glass panels in the floor out­side the Se­nate cham­ber in the U.S. Capi­tol?

— El­liot Carter, Wash­ing­ton

You may be fa­mil­iar with a line in the Bi­ble: “And God said, ‘Let there be light.’ And there was light.”

Easy for you, God. Hu­mans have it a lit­tle tougher.

Th­ese days, it’s easy to take light for granted. Il­lu­mi­na­tion is avail­able at the flick of a switch. This was not al­ways the case, es­pe­cially when nat­u­ral sun­light was all we had at our dis­posal.

Light­ing a small struc­ture — a one-room, sin­gle-level house, say — is fairly straight­for­ward. Just put a win­dow in each wall. Want to ad­mit a lot of light? Leave a hole in the roof and cover it with trans­par­ent glass. Voilà: sky­light!

But what if the build­ing has many rooms and sev­eral floors? What if the build­ing is the U.S. Capi­tol?

Imag­ine, if you will, that it is the year 1893 and you are a Li­brary of Congress clerk sent to fetch a tome from a shelf in the far­thest reaches of the great book de­pos­i­tory. You do not work in the hand­some Jef­fer­son Build­ing — that won’t open for four years — but in the afore­men­tioned U.S. Capi­tol.

When it was first built, its var­i­ous rooms were lit with can­dles and whale oil lamps. In the 1830s, kerosene — a byprod­uct of petroleum also known as paraf­fin — il­lu­mi­nated the Capi­tol. A decade later, nat­u­ral gas pro­duced light.

But nat­u­ral gas is not por­ta­ble. And so, as you hunt for that book, you use some old tech­nol­ogy. As the Wash­ing­ton Evening Star re­ported in 1893:

“The searchers for the books are com­pelled to use com­mon brass kerosene lamps when­ever they go into the al­coves or dark cor­ners for books that are wanted by read­ers. Th­ese lamps are of the char­ac­ter that might set fire to the li­brary at any mo­ment. The vaults be­low in the crypt and the gal­leries un­der the dome are filled with the most valu­able records . . . and at no time does a ray of light reach them save from matches or gas jets.”

To wring ev­ery avail­able lu­men from what­ever the light source — be it the sun or a glow­ing gas jet — ar­chi­tects can place a hole in the floor and cover it with glass, as was done on the third floor of the Capi­tol, be­tween the el­e­va­tor banks near the Se­nate Daily Press Gallery.

“Those are lay­lights, which al­lowed the trans­mis­sion of light from up­per floors to lower ones be­fore the in­tro­duc­tion of elec­tri­cal light­ing,” wrote Erin Court­ney of the Ar­chi­tect of the Capi­tol’s of­fice in an email to An­swer Man.

A lay­light can be in the ceil­ing — the ceil­ings in the Se­nate and House cham­bers orig­i­nally had glass lay­lights — or the floor. Of course, one room’s ceil­ing can be an­other room’s floor.

Lay­lights can dif­fuse light as well as spread it. The Na­tional Gallery of Art has lay­lights to pre­vent glare and shad­ows on the art­work. Me­mo­rial Con­ti­nen­tal Hall on 17th Street NW, home to the li­brary of the Daugh­ters of the Amer­i­can Revo­lu­tion, fea­tures an im­pres­sive lay­light: 25 glass frames, each 8-by-9 feet, set in dec­o­ra­tive met­al­work.

The DAR’s lay­light un­der­went an ex­ten­sive nine-month ren­o­va­tion com­pleted in 2013. The Christ­man Com­pany’s Greta Wil­helm worked on that project. An­swer Man won­dered: How is a lay­light dif­fer­ent from a sky­light?

“Ba­si­cally, the sky­light sep­a­rates the ex­te­rior of the build­ing and the in­te­rior of the build­ing,” Greta said. “Sky­lights typ­i­cally will have a pitch to them so they’ll shed wa­ter, whereas the lay­light is hor­i­zon­tal and in­te­rior to the build­ing. It could be be­neath a sky­light or just sep­a­rat­ing two in­te­rior spa­ces of the build­ing and al­low­ing light to trans­mit be­tween them.”

The Capi­tol started tran­si­tion­ing away from gas light in the 1880s and by 1898 had con­verted al­most com­pletely to elec­tric lights. The Star pre­dicted that bright elec­tric light would “pre­vent the acts of mis­cre­ants who are fond of con­gre­gat­ing in the gal­leries dur­ing the night ses­sions and are shielded by the half gloom that is caused by the con­cen­tra­tion of light in the cen­ter.”

Never for­get: Democ­racy dies in dark­ness. For pre­vi­ous col­umns, visit wash­ing­ton­post.com/johnkelly.

John Kelly's Wash­ing­ton

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