The loss or gain in body-weight has always been taken as indicating the
nature of body condition, a loss usually indicating that there is a loss
of body substance and a gain the reverse. In experiments in which a
delicate balance between the income and outgo is maintained, as in these
experiments, it is of special interest to compare the losses in weight
as determined by the balance with the calculated metabolism of material
and thus obtain a check on the computation of the whole process of
metabolism. Since the days of Sanctorius the loss of weight of the body
from period to period has been of special interest. The most recent
contribution to these investigations is that of the balance described by
Lombard,[31] in which the body-weight is recorded graphically from
moment to moment with an extraordinarily sensitive balance.
In connection with the experiments here described, however, the weighing
with the balance has a special significance, in that it is possible to
have an indirect determination of the oxygen consumption. As pointed out
by Pettenkofer and Voit, if the weight of the excretions and the loss in
body-weight are taken into consideration, the difference between the
weight of the excretions and the loss in body-weight should be the
weight of the oxygen absorbed. With this apparatus we are able to
determine the water-vapor, the carbon-dioxide excretion, and the weight
of the urine and feces when passed. If there is an accurate
determination of the body-weight from hour to hour, this should give the
data for computing exactly the oxygen consumption. Moreover, we have the
direct determination of oxygen with which the indirect method can be
compared.
In the earlier apparatus this comparison was by no means as satisfactory
as was desired. The balance there used was sensitive only to 2 grams,
the experiments were long (24 hours or more), and it seemed to be
absolutely impossible, even by exerting the utmost precaution, to secure
the body-weight of the subject each day with exactly the same clothing
and accessories. Furthermore, where there is a constant change in
body-weight amounting to 0.5 gram or more per minute, it is obvious that
the weighing should be done at exactly the same moment from day to day.
It is seen, therefore, that the comparison with the direct oxygen
determination is in reality an investigation by itself, involving the
most accurate measurements and the most painstaking development of
routine.
With the hope of contributing materially to our knowledge regarding the
indirect determination of oxygen, the special form of balance shown in
fig. 9 was installed above the chair calorimeter. This balance is
extremely sensitive. With a dead load of 100 kilograms in each pan it
has shown a sensitiveness of 0.1 gram, but in order to have the
apparatus absolutely air-tight for the oxygen and carbon-dioxide
determination, the rod on which the weighing-chair is suspended must
pass through an air-tight closure. For this closure we have used a thin
rubber membrane, weighing about 1.34 grams, one end of which is tied to
a hard-rubber tube ascending from the chair to the top of the
calorimeter, the other end being tied to the suspension rod. In playing
up and down this rod takes up a varying weight of the rubber diaphragm,
depending upon the position which it assumes, and therefore the
sensitiveness noted by the balance with a dead load and swinging freely
is greater than that under conditions of actual use. Preliminary tests
with the balance lead us to believe that with a slight improvement in
the technique a man can be weighed to within 0.3 gram by means of this
balance. A series of check-experiments to test the indirect with the
direct determination of oxygen are in progress at the moment of writing,
and it is hoped that this problem can be satisfactorily solved ere long.
During the process of weighing, the ventilating air-current is stopped
so as to prevent any slight tension on the rubber diaphragm and furnish
the best conditions for sensitive equilibrium. After the weighing has
been made and the time exactly recorded, the load is thrown off the
knife-edges of the balance, and then provision has been made to raise
the rod supporting the chair and simultaneously force a rubber stopper
tightly into the hard rubber tube at the top of the calorimeter, thus
making the closure absolutely tight. It is somewhat hazardous to rely
during the entire period of an experiment upon the thin rubber membrane
for the closure when the blower is moving the air-current.
To raise the chair and the man suspended on it in such a way as to draw
the cork into the hard-rubber tube, we formerly used a large hand-lever,
which was not particularly satisfactory. Thanks to the suggestion of Mr.
E. H. Metcalf, we have been able to attach a pneumatic lift (fig. 9) in
that the cross-bar above the calorimeter chamber, to which the
suspension rod is attached, rests on two oak uprights and can be raised
by admitting air into an air-cushion, through the central opening of
which passes the chair-suspending rod. As the air enters the air-cushion
it expands and lifts a large wooden disk which, in turn, lifts the iron
cross-bar, raising the chair and weight suspended upon it. At the proper
height and when the stopper has been thoroughly forced into place, two
movable blocks are slipped beneath the ends of the iron cross-bar and
thus the stopper is held firmly in place. The tension is then released
from the air-cushion. This apparatus functionates very satisfactorily,
raising the man or lowering him upon the knife-edges of the balance with
the greatest regularity and ease.