But now it is realized that supplies of some of then are limited, and it is even possible to give a reasonable estimate of their “expectation of life” the time it will take to exhaust all known sources and reserves of these materials.

• realized adj.实现的；已实现的 vt.实现
• supplies n.物资；供应品 v.供应；提供
• limited adj.有限的 n.高级快车
• reasonable adj.合理的；公道的；通情达理的
• estimate n.估计；估价 v.估计；估量
• expectation n.期待；预期
• exhaust v.使精疲力尽；耗尽 n.废气；排气管
• reserves n.储量；准备金；后备队 v.储备；保留；预订

该句为 and 连接的并列复合句。But now it is realized that supplies of some of then are limited 中 realized 为谓语，it 为形式主语，真正的主语为 that 引导的主语从句 that supplies of some of then are limited.此从句为 A of B of C 的形式，C 修饰 B，B 修饰 A。
在and it is even possible to give a reasonable estimate of their “expectation of life” the time it will take to exhaust all known sources and reserves of these materials.中 谓语为 is,it 为形式主语，真正的主语为 to give a reasonable estimate of their “expectation of life” the time it will take to exhaust all known sources and reserves of these materials.其中 their “expectation of life” 修饰 reasonable estimate；the time 为 their “expectation of life” 的同位语，it will take to exhaust all known sources and reserves of these materials.就是省略了引导词的定语从句，修饰 the time。

译：但现在人们意识到，当时的某些物质的供应是有限的，甚至有可能对它们的“预期寿命”作出合理估计，即耗尽所有已知的这些物质的来源和储量所需的时间。