Other rare elements, like neodymium, exist in more than one isotopic form, the ratios of which can provide a measure of the depth from which the flow material may ultimately have been derived.

Further to the west are several new mines associated with recent railway builind and the drive to develop industrial centres in the interior. At Shih-kuai-kou (Shihkwaikow), convenient to supply the new steel-works at Pao-t'ou (Paotow), the first three shafts were sunk in 1958. Reserves there are put at 700 million tons.

Both during the doctorate and in subsequent revisions along the path to bookdom, the manuscript benefitted from discussions with many experts, colleagues and friends.

Throughout this paper, the term physiological dead space will be used to refer to the volume of the conducting airway down to the location at which a large change in gas composition occurs.