lsewhere.[21] It is sufficient to state here that
the very large majority of mechanical valves will not serve the purpose,
since it is necessary to have a pressure of some 40 millimeters of
mercury on one side of the valve at the entrance to the absorber system
and on the other side atmospheric pressure. A valve with an internal
diameter of not less than 25 millimeters must be used, and to secure a
tight closure of this large area and permit frequent opening and
shutting is difficult. After experimenting with a large number of
valves, a valve of special construction employing a mechanical seal
ultimately bathed in mercury was used for the earlier apparatus. The
possibility of contamination of the air-current by mercury vapor was
duly considered and pointed out in a description of this apparatus. It
was not until two years later that difficulties began to be experienced
and a number of men were severely poisoned while inside the chamber. A
discussion of this point has been presented elsewhere.[22] At that time
mercury valves were used both at the entrance and exit ends of the
absorber system, although as a matter of fact, when the air leaves the
last absorber and returns to the respiration chamber, the pressure is
but a little above that of the atmosphere. Consequently, mechanical
valves were substituted for mercurial valves at the exit and the toxic
symptoms disappeared. In constructing the new calorimeters it seemed to
be desirable to avoid all use of mercury, if possible. We were fortunate
in finding a mechanical valve which suited this condition perfectly.
These valves, which are very well constructed, have never failed to show
complete tightness under all possible tests and are used at the exit and
entrance end of the absorber system. Their workmanship is of the first
order, and the valve is somewhat higher in price than ordinary
mechanical valves. They have been in use on the apparatus for a year now
and have invariably proved to be absolutely tight. They are easy to
obtain and are much easier to manipulate and much less cumbersome than
the mercury valves formerly used.

COUPLINGS.

Throughout the construction of the respiration apparatus and its various
parts, it was constantly borne in mind that the slightest leak would be
very disastrous for accurate oxygen determinations. At any point where
there is a pressure greater or less than that of the atmosphere, special
precaution must be taken. At no point in the w