This special form of absorbers presented many difficulties in construction, but the mechanical difficulties were overcome by the potter's skill and a number of such vessels were furnished by the Charles Graham Chemical Pottery Works. Here again these vessels served our purpose for several months, but unfortunately the glaze used did not suffice to cover them completely and there was a slight, though persistent, leakage of sulphuric acid through the porous walls. To overcome this difficulty the interior of the vessels was coated with hot paraffin after a long-continued washing to remove the acid and after they had been allowed to dry thoroughly. The paraffin-treated absorbers continued to give satisfaction, but it was soon seen that for permanent use something more satisfactory must be had. After innumerable trials with glazed vessels of different kinds of pottery and gla.s.s, arrangements were made with the Royal Berlin Porcelain Works to mold and make these absorbers out of their highly resistant porcelain. The result thus far leaves nothing to be desired as a vessel for this purpose. A number of such absorbers were made and have been constantly used for a year and are absolutely without criticism.

Fig. 28 shows the nature of the interior of the apparatus. The air enters through one opening at the top, pa.s.ses down through a bent pipe, and enters a series of roses, consisting of inverted circular saucers with holes in the rims. The position of the holes is such that when the vessel is one-fourth to one-third full of sulphuric acid the air must pa.s.s through the acid three times. To prevent spattering, a small cup-shaped arrangement, provided with holes, is attached to the opening through which the air pa.s.ses out of the absorber, and for filling the vessel with acid a small opening is made near one edge. The specifications required that the apparatus should be made absolutely air-tight to pressures of over 1 meter of water, and that there is no porosity in these vessels under these conditions is shown by the fact that such a pressure is held indefinitely. The inside and outside are both heavily glazed. There is no apparent action of sulphuric acid on the vessels and the slight increase in temperature resulting from the absorption of water-vapor as the air pa.s.ses through does not appear to have any deleterious effect.

[Ill.u.s.tration: FIG. 28.--Cross-section of sulphuric-acid absorber. The air enters at the top of the right-hand opening, descends to the bottom of the absorber, and then pa.s.ses through three concentric rings, which are covered with acid, and it finally pa.s.ses out at the left-hand opening. Beneath the left-hand opening is a cup arrangement for preventing the acid being carried mechanically out through the opening.

The opening for filling and emptying the absorber is shown midway between the two large openings.]

The vessels without filling and without rubber elbows weigh 11.5 kilograms; with the special elbows and couplings attached so as to enable them to be connected with the ventilating air-system, the empty absorbers weigh 13.4 kilograms; and filled with sulphuric acid they weigh 19 kilograms. Repeated tests have shown that 5.5 kilograms of sulphuric acid will remove the water-vapor from a current of air pa.s.sing through the absorbers at the rate of 75 liters of air per minute, without letting any appreciable amount pa.s.s by until 500 grams of water have been absorbed. At this degree of saturation a small persistent amount of moisture escapes absorption in the acid and consequently a second absorber will begin to gain in weight. Experiments demonstrate that the first vessel can gain 1,500 grams of water before the second gains 5 grams. As a matter of fact, it has been found more advantageous to use but one absorber and have it refilled as soon as it has gained 400 grams, thus allowing a liberal factor of safety and no danger of loss of water.

POTASH-LIME CANS.

The problem of absorbing the water-vapor from so rapid a current of air is second only to that of absorbing the carbon dioxide from such a current. All experiments with pota.s.sium hydroxide in the form of sticks or in solution failed to give the desired results and the use of soda-lime has supplemented all other forms of carbon dioxide absorption.

More recently we have been using potash-lime, subst.i.tuting caustic potash for caustic soda in the formula, and the results thus obtained are, if anything, more satisfactory than with the soda-lime.

The potash-lime is made as follows: 1 kilogram of commercial pota.s.sium hydroxide, pulverized, is dissolved in 550 to 650 cubic centimeters of water and 1 kilogram of pulverized quicklime added slowly. The amount of water to be used varies with the moisture content of the potash. There is a variation in the moisture content of different kegs of potash, so when a keg is opened we determine experimentally the amount of water to be used. After a batch is made up in this way it should be allowed to cool before testing whether it has the right amount of water, and this is determined by feeling of it and noting how it pulverizes in the hand.

It is not advisable to make a great quant.i.ty at once, because we have found that if a large quant.i.ty is made and broken into small particles and stored in a container it has a tendency to cake and thus interfere with its ready subsequent use.

A record was kept of the gains in weight of a can filled with potash-lime during a series of experiments where there were three silver-plated cans used. This can was put at the head of the system and when it began to lose weight it was removed. The records of gains of weight when added together amount to 400 grams. From experience with other cans where the loss of moisture was determined, it is highly probable that at least 200 grams of water were vaporized from the reagent and thus the total amount of carbon dioxide absorbed must have been not far from 600 grams. At present our method is not to allow the cans to gain a certain weight, but during 4-hour or 5-hour experiments, in which each can may be used 2 or 3 hours, it is the practice to put a new can on each side of the absorber system (see page 66) at the beginning of every experiment. This insures the same power of absorption on each side of the absorption system so that the residual amount of carbon dioxide in the chamber from period to period does not undergo very marked changes. This has been found the best method, because if one can is left on a day longer than the other there is apt to be alternately a rise and fall in the amount of residual carbon dioxide in the apparatus, owing to the unequal efficiency of the absorbers.

These cans are each day taken to the bas.e.m.e.nt, where the first section[19] only is taken out and replaced with new potash-lime. Thus, three-quarters of the contents of the can is used over and over, while the first quarter is freshly renewed every day. Potash-lime has not been found practicable for the U-tubes because one can not, as in the case of soda-lime, see the whitening of the reagent where the carbon dioxide is absorbed.

The importance of having the soda-lime or potash-lime somewhat moist, to secure the highest efficiency for the absorption of the carbon dioxide, makes it necessary to absorb the moisture taken up by the dry air in pa.s.sing through the potash-lime can. Consequently a second vessel containing sulphuric acid is placed in the system to receive the air immediately after it leaves the potash-lime can. Obviously the amount of water absorbed here is very much less than in the first acid absorber and hence the same absorber can be used for a greater number of experiments.

BALANCE FOR WEIGHING ABSORBERS.

The complete removal of water-vapor and carbon dioxide from a current of air moving at the rate of 75 liters per minute calls for large and somewhat unwieldy vessels in which is placed the absorbing material.

This is particularly the case with the vessels containing the rather large amounts of sulphuric acid required to dry the air. In the course of an hour there is ordinarily removed from the chamber not far from 25 grams of water-vapor and 20 to 30 grams of carbon dioxide. This necessitates weighing the absorbers to within 0.25 gram if an accuracy of 1 per cent is desired. The sulphuric-acid absorbers weigh about 18 kilograms when filled with acid. In order to weigh this receptacle so as to measure accurately the increase in weight due to the absorption of water to within less than 1 per cent, we use the balance shown in fig.

29. This balance has been employed in a number of other manipulations in connection with the respiration calorimeter and accessory apparatus and the general type of balance leaves nothing to be desired as a balance capable of carrying a heavy load with remarkable sensitiveness.

The balance is rigidly mounted on a frame consisting of four upright structural-steel angle-irons, fastened at the top to a substantial wooden bed. Two heavy wooden pieces run the length of the table and furnish a substantial base to which the standard of the balance is bolted. The balance is surrounded by a gla.s.s case to prevent errors due to air-currents (see fig. 2). The pan of the balance is not large enough to permit the weighing of an absorber, hence provision is made for suspending it on a steel or bra.s.s rod from one of the hanger arms. This rod pa.s.ses through a hole in the bottom of the balance case, and its lower end is provided with a piece of pipe having hooks at either end.

Since the increase in weight rather than the absolute weight of the absorber is used, the greater part of the weight is taken up by lead counterpoises suspended above the pan on the right-hand arm of the balance. The remainder of the weight is made up with bra.s.s weights placed in the pan.

[Ill.u.s.tration: FIG. 29.--Balance for weighing absorbers, showing general type of balance and case surrounding it, with counterpoise and weights upon right-hand pan. A sulphuric-acid absorber is suspended in position ready for weighing. Elevator with compressed-air system is shown in lower part of case.]

In order to suspend this heavy absorber, a small elevator has been constructed, so that the vessel may be raised by a compressed-air piston. This piston is placed in an upright position at the right of the elevator and is connected with the compressed-air service of the building. The pressure is about 25 pounds per square inch and the diameter of the cylinder is 2.5 inches, thus giving ample service for raising and lowering the elevator and its load. By turning a 3-way valve at the end of the compressed-air supply-pipe, so that the air rushes into the cylinder above the piston, the piston is pushed to the base of the cylinder and the elevator thereby raised. The pressure of the compressed air holds the elevator in this position while the hooks are being adjusted on the absorber. By turning the 3-way valve so as to open the exhaust leading to the upper part of the cylinder to the air, the weight of the elevator expels the air, and it soon settles into the position shown in the figure. The weighing can then be made as the absorber is swinging freely in the air. After the weighing has been made, the elevator is again lifted, the hooks are released, and by turning the valve the elevator and load are safely lowered.

The size of the openings of the pipes into the cylinder is so adjusted that the movement of the elevator is regular and moderate whether it is being raised or lowered, thus avoiding any sudden jars that might cause an accident to the absorbers. With this system it is possible to weigh these absorbers to within 0.1 gram and, were it necessary, probably the error could be diminished so that the weight could be taken to 0.05 gram. On a balance of this type described elsewhere,[20] weighings could be obtained to within 0.02 gram. For all practical purposes, however, we do not use the balance for weighing the absorbers closer than to within 0.10 gram. In attempting to secure accuracy no greater than this, it is unnecessary to lower the gla.s.s door to the balance case or, indeed, to close the two doors to the compartment in which the elevator is closed, as the slight air-currents do not affect the accuracy of the weighing when only 0.1 gram sensitiveness is required.

PURIFICATION OF THE AIR-CURRENT WITH SODIUM BICARBONATE.

As is to be expected, the pa.s.sage of so large a volume of air through the sulphuric acid in such a relatively small s.p.a.ce results in a slight acid odor in the air-current leaving this absorber. The amount of material thus leaving the absorber is not weighable, as has been shown by repeated tests, but nevertheless there is a sufficiently irritating acid odor to make the air very uncomfortable for subsequent respiration.

It has been found that this odor can be wholly eliminated by pa.s.sing the air through a can containing cotton wool and dry sodium bicarbonate.

This can is not weighed, and indeed, after days of use, there is no appreciable change in its weight.

VALVES.

In order to subdivide experiments into periods as short as 1 or 2 hours, it is necessary to deflect the air-current at the end of each period from one set of purifiers to the other, in order to weigh the set used and to measure the quant.i.ty of carbon dioxide and water-vapor absorbed.

The conditions under which these changes from one system to another are made, and which call for an absolutely gas-tight closure, have been discussed in detail elsewhere.[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 subst.i.tuted 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 c.u.mbersome 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 whole apparatus is it necessary to be more careful than with the couplings which connect the various absorber systems with each other and with the valves; for these couplings are opened and closed once every hour or two and hence are subject to considerable strain at the different points. If they are not tight the experiment is a failure so far as the determination of oxygen is concerned. For the various parts of the absorber system we have relied upon the original type of couplings used in the earlier apparatus. A rubber gasket is placed between the male and female part of the coupling and the closure can be made very tight. In fact, after the absorbers are coupled in place they are invariably subjected to severe tests to prove tightness.

For connecting the piping between the calorimeter and the absorption system we use ordinary one-inch hose-couplings, firmly set up by means of a wrench and disturbed only when necessary to change from one calorimeter chamber to another.

ABSORBER TABLE.

The purifying apparatus for the air-current is compactly and conveniently placed on a solidly constructed table which can be moved about the laboratory at will. The special form of caster on the bottom of the posts of the table permits its movement about the laboratory at will and by s.c.r.e.w.i.n.g down the hand screws the table can be firmly fixed to the floor.

The details of the table are shown in fig. 30. (See also fig. 4, page 4.) The air coming from the calorimeter pa.s.ses in the direction of the downward arrow through a 3/4-inch pipe into the blower, which is immersed in oil in an iron box F. The blower is driven by an electric motor fastened to a small shelf at the left of the table. The air leaving the blower ascends in the direction of the arrow to the valve system H, where it can be directed into one of the two parallel sets of purifiers; after it pa.s.ses through these purifiers (sulphuric-acid vessel 2, potash-lime container K, and sulphuric-acid vessel 1) it goes through the sodium-bicarbonate can G to a duplicate valve system on top of the table. From there it pa.s.ses through a pipe along the top of the table and rises in the vertical pipe to the hose connection which is coupled with the calorimeter chamber.

The electric motor is provided with a snap-switch on one of the posts of the table and a regulating rheostat which permits variations in the speed of the motor and consequently in the ventilation produced by the blower. The blower is well oiled, and as oil is gradually carried in with the air, a small pet-c.o.c.k at the bottom of the T following the blower allows any acc.u.mulated oil to be drawn away from time to time.

The air entering the valve system at H enters through a cross, two arms of which connect with two "white star" valves. The upper part of the cross is connected to a small rubber tubing and to the mercury manometer D, which also serves as a valve for pa.s.sing a given amount of air through a series of U-tubes for a.n.a.lysis of the air from time to time. It is a.s.sumed that the air drawn at the point H is of substantially the same composition as that inside the chamber, an a.s.sumption that may not be strictly true, but doubtless the sample thus obtained is constantly proportional to the average composition, which fluctuates but slowly. Ordinarily the piping leading from the left-hand arm of the tube D is left open to the air and consequently the difference in the level of the mercury in the two arms of D indicates the pressure on the system. This is ordinarily not far from 40 to 50 millimeters of mercury.

[Ill.u.s.tration: FIG. 30.--Diagram of absorber table. 1 and 2 contain sulphuric acid; K contains potash-lime; G, sodium bicarbonate can; F, rotary blower for maintaining air-current; H, valves for closing either side; and D, mercury manometer and valve for diverting air to U-tubes on table. Air leaves A, pa.s.ses through the meter, and then through drying tower B and through C to ingoing air-pipe. At the left is the regulating rheostat and motor and snap-switch. General direction of ventilation is indicated by arrows.]

The absorber table, with the U-tubes and meter for residual a.n.a.lyses, is shown in the foreground in fig. 2. The two white porcelain vessels with a silver-plated can between them are on the middle shelf. The sodium bicarbonate can, for removing traces of acid fumes, is connected in an upright position, while the motor, the controlling rheostat, and the blower are supported by the legs near the floor. The two rubber pipes leading from the table can be used to connect the apparatus either with the bed or chair calorimeter. In fig. 4 the apparatus is shown connected with the bed calorimeter, but just above the lowest point of the rubber tubing can be seen in the rear the coupling for one of the pipes leading from the chair calorimeter. The other is immediately below and to the left of it.

OXYGEN SUPPLY.

The residual air inside of the chamber amounts to some 1,300 liters and contains about 250 liters of oxygen. Consequently it can be seen that in an 8-hour experiment the subject could easily live during the entire time upon the amount of oxygen already present in the residual air. It has been repeatedly shown that until the per cent of oxygen falls to about 11, or about one-half normal, there is no disturbance in the respiratory exchange and therefore about 125 liters of oxygen would be available for respiration even if no oxygen were admitted. Inasmuch as the subject when at rest uses not far from 14 to 15 liters per hour, the amount originally present in the chamber would easily suffice for an 8-hour experiment. Moreover, the difficulties attending an accurate gas a.n.a.lysis and particularly the calculation of the total amount of oxygen are such that satisfactory determinations of oxygen consumption by this method would be impossible. Furthermore, from our previous experience with long-continued experiments of from 10 days to 2 weeks, it has been found that oxygen can be supplied to the system readily and the amount thus supplied determined accurately. Consequently, even in these short experiments, we adhere to the original practice of supplying oxygen to the air and noting the amount thus added.

The oxygen supply was formerly obtained from small steel cylinders of the highly compressed gas. This gas was made by the calcium-manganate method and represented a high degree of purity for commercial oxygen.

More recently we have been using oxygen of great purity made from liquid air. Inasmuch as this oxygen is very pure and much less expensive than the chemically-prepared oxygen, extensive provisions have been made for its continued use. Instead of using small cylinders containing 10 cubic feet and attaching thereto purifying devices in the shape of soda-lime U-tubes and a sulphuric-acid drying-tube, we now use large cylinders and we have found that the oxygen from liquid air is practically free from carbon dioxide and water-vapor, the quant.i.ties present being wholly negligible in experiments such as these. Consequently, no purifying attachments are considered necessary and the oxygen is delivered directly from the cylinder. The cylinders, containing 100 cubic feet (2,830 liters), under a pressure of 120 atmospheres, are provided with well-closing valves and weigh when fully charged 57 kilograms.

[Ill.u.s.tration: FIG. 31.--Diagram of oxygen balance and cylinder. At the top is the balance arrangement, and at the center its support. At the left is the oxygen cylinder, with reducing valve A, rubber tube D leading from it, F the electro-magnet which opens and closes D, K the hanger of the cylinder and support for the magnet, R the lever which operates the supports for the cylinder and its counterpoise S, T' a box which is raised and lowered by R, and T its surrounding box.]

It is highly desirable to determine the oxygen to within 0.1 gram, and we are fortunate in having a balance of the type used frequently in this laboratory which will enable us to weigh this cylinder accurately with a sensitiveness of less than 0.1 gram. Since 1 liter of oxygen weighs 1.43 grams, it can be seen that the amount of oxygen introduced into the chamber can be measured by this method within 70 cubic centimeters.

Even in experiments of but an hour's duration, where the amount of oxygen admitted from the cylinder is but 25 to 30 grams, it can be seen that the error in the weighing of the oxygen is much less than 1 per cent.

The earlier forms of cylinders used were provided with valves which required some special control and a rubber bag was attached to provide for any sudden rush of gas. The construction of the valve and valve-stem was unfortunately such that the well-known reduction valves could not be attached without leakage under the high pressure of 120 atmospheres.

With the type of cylinder at present in use, such leakage does not occur and therefore we simply attach to the oxygen cylinder a reduction-valve which reduces the pressure from 120 atmospheres to about 2 or 3 pounds to the square inch. The cylinder, together with the reduction valve, is suspended on one arm of the balance. The equipment of the arrangement is shown in fig. 31. (See also fig. 5, page 4.) The cylinder is supported by a clamp K hung from the balance arm, and the reduction-valve A is shown at the top. The counterpoise S consists of a piece of 7-inch pipe, with caps at each end. At a convenient height a wooden shelf with slightly raised rim is attached.

In spite of the rigid construction of this balance, it would be detrimental to allow this enormous weight to remain on the knife-edges permanently, so provision is made for raising the cylinders on a small elevator arrangement which consists of small boxes of wood, T, into which telescope other boxes, T'. A lever handle, R, when pressed forward, raises T' by means of a roller bearing U, and when the handle is raised the total weight of the cylinders is supported on the platforms.

The balance is attached to an upright I-beam which is anch.o.r.ed to the floor and ceiling of the calorimeter laboratory. Two large turnbuckle eye-bolts give still greater rigidity at the bottom. The whole apparatus is inclosed in a gla.s.s case, shown in fig. 5.

AUTOMATIC CONTROL OF OXYGEN SUPPLY.

The use of the reduction-valve has made the automatic control of the oxygen supply much simpler than in the apparatus formerly used. The details of the connections somewhat schematically outlined are given in fig. 32, in which D is the oxygen cylinder, K the supporting band, A the reduction-valve, and J the tension-equalizer attached to one of the calorimeters. Having reduced the pressure to about 2 pounds by means of the reduction-valve, the supply of oxygen can be shut off by putting a pinch-c.o.c.k on a rubber pipe leading from the reduction-valve to the calorimeters. Instead of using the ordinary screw pinch-c.o.c.k, this connection is closed by a spring clamp. The spring E draws on the rod which is connected at L and pinches the rubber tube tightly. The tension at E can be released by an electro-magnet F, which when magnetized exercises a pull on the iron rod, extends the spring E, and simultaneously releases the pressure on the rubber tube at L. To make the control perfectly automatic, the apparatus shown on the top of the tension-equalizer J is employed. A wire ring, with a wire support, is caused to pa.s.s up through a bearing fastened to the clamp above J. As the air inside of the whole system becomes diminished in volume and the rubber cap J sinks, there is a point at which a metal loop dips into two mercury cups C and C', thus closing the circuit, which causes a current of electricity to pa.s.s through F. This releases the pressure at L, oxygen rushes in, and the rubber bag J becomes distended. As it is distended, it lifts the metal loop out of the cups, C and C', and the circuit is broken. There is, therefore, an alternate opening and closing of this circuit with a corresponding admission of oxygen. The exact position of the rubber diaphragm can be read when desired from a pointer on a graduated scale attached to a support holding the terminals of the electric wires. More frequently, however, when the volume is required, instead of filling the bag to a definite point, as shown by the pointer, a delicate manometer is attached to the can by means of a pet-c.o.c.k and the oxygen is admitted by operating the switch B until the desired tension is reached.