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    One day, Mr. Arnold was teaching a lesson, and things were going as normally as ever. He was explaining the story of human being to his pupils. He told them that, in the beginning, men were nomads (游牧); they never stayed in the same place for very long. Instead, they would travel about, here and there, in search of food, wherever it was to be found. And when the food ran out, they would move off somewhere else.

    He taught them about the invention of farming and keeping animals. This was an important discovery, because by learning to cultivate (耕作) the land, and care for animals, mankind would always have food steadily. It also meant that people could remain living in one place, and this made it easier to set about tasks that would take a long while to finish, like building towns, cities, and all that were in them. All the children were listening attracted by this story, until Lucy jumped up:

    “And if that was so important and improved everything so much, why are we nomads all over again, Mr. Arnold?”

    Mr. Arnold didn't know what to say. Lucy was a very clever girl. He knew that she lived with her parents in a house, so she must know that her family were not nomads; so what did she mean?

    “We have all become nomads again,” continued Lucy, “The other day, outside the city, they were cutting the forest down. A while ago a fisherman told me how they fish. It's the same with everyone: when there's no more forest left the foresters go elsewhere, and when the fish run out the fishermen move on. That's what the nomads did, isn't it?

    The teacher nodded, thoughtfully. Really, Lucy was right. Mankind had turned into nomads. Instead of looking after the land in a way that we could be sure it would keep supplying our needs, we kept developing it until the land was bare. And then off we would go to the next place! The class spent the rest of the afternoon talking about what they could do to show how to be more civilized (文明的).

    The next day everyone attended class wearing a green T-shirt, with a message that said “I am not a nomad!”

    And, from then on, they set about showing that indeed they were not. Every time they knew they needed something, they made sure that they would get it using care and control. If they needed wood or paper, they would make sure that they got the recycled kind. They ordered their fish from fish farms, making sure that the fish they received were not too young and too small. They only used animals that were well cared for, and brought up on farms.

    And so, from their little town, those children managed to give up being nomads again, just as prehistoric men had done, so many thousands of years ago.

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冷光奇观

    在6月的晨昏蒙影里,萤火虫以其独特的方式闪烁光芒。萤火虫发光之谜也未逃过科学家的慧眼。他们亲手捕捉了后惊讶地发现,萤火虫发出的耀眼光芒竟一点也不热,“冷光”由此驰名并后来居上,成为科学家梦寐以求想要研发的新颖照明技术。

    世界冷光生物五光十色,比比皆是。南极洋中的磷虾就是最为壮观的生物发光奇迹之一。富有经验的捕鲸者常利用夜间磷虾群在水面浮游时所发出的荧光来判明鲸的来往去向。美国在南极大象岛以北海域发现一个至少有1000万吨之巨的磷虾群,堪称迄今最宏伟壮观的海洋发光生物群。

    生物自动发光的“秘密”何在呢?据研究,发光生物体内含有荧光素与荧光酶,荧光素一旦吸收了氧和糖分子,就会在发光酶的催化下,发生微妙的化学反应,于是冷光奇迹活灵活现。但有些生物靠发光细菌或菌体内的荧光素也照样能发出冷光来。

    1887年,法国生理学家杜波依斯研究石蛤的发光机理。他发现,一旦停止发光,在石蛤的冷水提取液中再加入已冷却的新鲜石蛤热水提取液,类似的发光现象就又死灰复燃了。于是他断定,在停止发光的冷水液中存在荧光酶,而热水液则存在荧光素,二者珠联璧合才产生奇妙的冷光。他还在15~20 ℃的温水中,放入死鱼和乌贼予以培养,不料,一夜光阴,竟发现闪光的细菌菌落分布在它们的遗体上。可见,死鱼之类的生物也可以繁殖发光细菌。

    生物冷光奇迹举世瞩目。1900年,在德国柏林的万国博览会上,曾展出了轰动一时的“细菌灯”。时至今日,人们已进而模拟生物发光型式,不遗余力地开发化学冷光源,广泛地为工农业与生活照明服务。20世纪70年代,美国化学家发现,若将荧光素和激活剂相混,便能立竿见影,激发出冷光来。于是他们如法炮制,把上述两种化学物质集装于塑料管中,其间用玻璃相隔,只需折弯塑料管,即能产生照明冷光。

    科学家还发现,电子、电场及其基本粒子也可激活很多物质,如人造冷光。现代“日光灯”即冷光源之一。一旦通电,在交流电场下,灯管里的汞蒸气会辐射紫外线,激活硫化锌和硫化钙之类的管壁荧光涂料,发出耀眼的可见光。

    1936年,法国物理学家德斯特劳发现,强大的交流电场无需以不可见紫外辐照为前提,就可以直接把电能转化为光能,激活荧光物质发出可见光,“电致发光”由此蜂起。现代科学正在这项课题上继往开来,力图完全揭开其中的内幕。有的科学家独具慧眼,把荧光料掺入塑料或玻璃之类的有机或无机材料里,“电致发光板”应运而生。据乐观预言,不久电灯照明技术将出现一场别开生面的革命。当人们住进电致发光墙壁和天花板的房间,可直接按钮调控,顷刻间,便可沉浸在柔和悦目的光明世界中。

    现代物理学家认为,可见光是电磁波之一,只要将电磁波控制在可见光波长范围内,也能产生照明可见光。美国曾向250千米的高空发射了一枚探测火箭,利用火箭爆发出的钡蒸气云与太阳光作用后,竟出人意料地创造了堪与北极光奇迹媲美的斑斓夺目的光幕。依此类推,有朝一日,灿烂辉煌但却不像阳光和火光那般灼热灸人的超巨型照明冷光取代现在的照明技术,并不是不可能的。