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As it ages white paper turns a distinctive¹ yellow. But why? To find out, scientists artificially aged² modern paper to reveal the changes on the molecular³ level. The research is in the Journal Physical Review Letters.

For 48 days, three unbleached paper samples aged rapidly in reactors⁴ that simulated to different environmental conditions. The researchers then compared the artificially aged samples to the real deal——three pieces of paper crafted in 15 century year Europe. This technique allowed them to gauge⁵ the types and amounts of changes going on. About 90 percent of the weight of all paper is cellulose, the sturdy material that makes up plant cell walls. But overtime⁶ cellulose fibers⁷ oxidized. The process modified parts of various molecules⁸ and turns them into what we called chrome*, which absorbs light. White paper is white because it reflects all colors of light. Aging paper filled with chrome* reflects wavelengths⁹ that make it look like yellow. The non-destructive technique used in this study could inform researchers to preserve and even whiten ancient textile and art. And give paper conservation a brighter outlook.

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1 distinctive
adj.特别的，有特色的，与众不同的
参考例句：
She has a very distinctive way of walking.她走路的样子与别人很不相同。 This bird has several distinctive features.这个鸟具有几种突出的特征。

2 aged
adj.年老的，陈年的
参考例句：
He had put on weight and aged a little.他胖了，也老点了。 He is aged,but his memory is still good.他已年老，然而记忆力还好。

3 molecular
adj.分子的；克分子的
参考例句：
The research will provide direct insight into molecular mechanisms.这项研究将使人能够直接地了解分子的机理。 For the pressure to become zero, molecular bombardment must cease.当压强趋近于零时，分子的碰撞就停止了。

4 reactors
起反应的人( reactor的名词复数 )；反应装置；原子炉；核反应堆
参考例句：
The TMI nuclear facility has two reactors. 三哩岛核设施有两个反应堆。来自英汉非文学 - 环境法 - 环境法 The earliest production reactors necessarily used normal uranium as fuel. 最早为生产用的反应堆，必须使用普通铀作为燃料。

5 gauge
v.精确计量；估计；n.标准度量；计量器
参考例句：
Can you gauge what her reaction is likely to be?你能揣测她的反应可能是什么吗？ It's difficult to gauge one's character.要判断一个人的品格是很困难的。

6 overtime
adj.超时的，加班的；adv.加班地
参考例句：
They are working overtime to finish the work.为了完成任务他们正在加班加点地工作。 He was paid for the overtime he worked.他领到了加班费。

7 fibers
光纤( fiber的名词复数 )；（织物的）质地；纤维，纤维物质
参考例句：
Thesolution of collagen-PVA was wet spined with the sodium sulfate as coagulant and collagen-PVA composite fibers were prepared. 在此基础上，以硫酸钠为凝固剂，对胶原-PVA共混溶液进行湿法纺丝，制备了胶原-PVA复合纤维。 Sympathetic fibers are distributed to all regions of the heart. 交感神经纤维分布于心脏的所有部分。

8 molecules
分子( molecule的名词复数 )
参考例句：
The structure of molecules can be seen under an electron microscope. 分子的结构可在电子显微镜下观察到。 Inside the reactor the large molecules are cracked into smaller molecules. 在反应堆里，大分子裂变为小分子。

9 wavelengths
n.波长( wavelength的名词复数 )；具有相同的/不同的思路；合拍；不合拍
参考例句：
I find him difficult to talk to—we're on completely different wavelengths. 我没法和他谈话，因为我们俩完全不对路。来自《简明英汉词典》 Sunlight consists of different wavelengths of radiation. 阳光由几种不同波长的射线组成。来自辞典例句