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.
該句為 and 連線的並列複合句。But now it is realized that supplies of some of then are limited 中 realized 為謂語，it 為形式主語，真正的主語為 that 引導的主語從句 that supplies of some of then are limited.此從句為 A of B of C 的形式，C 修飾 B，B 修飾 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 謂語為 is,it 為形式主語，真正的主語為 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」 修飾 reasonable estimate；the time 為 their 「expectation of life」 的同位語，it will take to exhaust all known sources and reserves of these materials.就是省略了引導詞的定語從句，修飾 the time。
譯：但現在人們意識到，當時的某些物質的供應是有限的，甚至有可能對它們的「預期壽命」作出合理估計，即耗盡所有已知的這些物質的來源和儲量所需的時間。