is at rest, than it is in the performance of work. However, for the purposes of ordinary estimate, it may be assumed that a man, at rest, will inhale from 25 to 30 cu. in. of air at a single breath and this may be increased to 150 or possibly 200 cu. in. when making violent exertion. Practically, one-fifth of this volume of air is oxygen; but, in the act of breathing, only onethird or one-half of this oxygen is consumed.

The standard supply of oxygen, in mine breathing apparatus, has been fixed, therefore, at 2 liters per min. (122 cu. in.). Compressed to 120 atmospheres, this rate of supply of oxygen, for a 2-hr. period, will require a cylinder capacity of 2(122 × 60) 120 122 cu. in. Again, assuming that the average amount of carbon dioxide produced in breathing is equal to the volume of oxygen consumed, it appears that the quantity of the former gas required to be absorbed by the caustic soda in the regenerator, in a 2-hr. period, is 2(2 × 60) = 240 liters, or 8.47 cu. ft.

The following table gives carefully compiled data and the results of actual tests regarding the oxygen consumed, carbon dioxide produced, quantity of air breathed and number of respirations per minute, under different conditions of rest and exertion. These data were compiled by James M. Stewart, Instructor at the Brazeau Rescue Station, Alberta, Canada.*

* Bulletin, November, 1916, Rocky Mountain Branch of the Canadian

Mining Institute.

It is evident from the table that more than the standard supply of oxygen allowed in the design of breathing apparatus may be consumed by a person under great physical exertion. Mr. Stewart suggests, therefore, that it is of the utmost importance that the captain of a rescue team observe carefully that his men do not overexert themselves while in the performance of their duties in the mine. He also suggests that, in the use of the nose-clip, greater comfort and security is obtained by inserting a cotton-wool plug in each nostril, before adjusting the clip.

Development of Breathing Apparatus.-The development of breathing apparatus, during the past few years, since the Government took up the work of improving mining conditions (1907) has been rapid. In the earlier types of apparatus, a helmet was employed to cover the head and oxygen was supplied through rubber tubes that connected the helmet with a gas cylinder or bag containing the gas. Owing to the danger of these connecting tubes being broken in the rough service to which they are subjected in the mine, the first attempt to improve the apparatus resulted in the adoption of a form that was self-contained, so as to eliminate, as far as practicable, the tube connections.

Mining practice quickly demonstrated that the substitution of a simple mouthpiece, and noseclip to close the nostrils, gave better service underground than the clumsy helmet, although the latter afforded more comfort in breathing and enabled the wearer to talk to his comrades with greater facility than when the mouthpiece was used and a noseclip closed the nostrils. However, these disadvantages were largely outweighed by the greater facility offered for work by this form of apparatus.

Design of Breathing Apparatus.-Breathing apparatus is designed to supply the wearer with a perfectly respirable air independent of the atmosphere in which he may be placed. The design of the apparatus is to enable the wearer to work in an irrespirable atmosphere for a limited period of two hours. The principal features of the device consist in maintaining a sufficient supply of oxygen to replace that consumed by the

wearer of the apparatus, and absorbing the carbon dioxide he exhales.

Oxygen, compressed to 120 atmospheres, is contained in a strong steel cylinder. The quantity is sufficient to afford a supply of 2 liters of this gas (122 cu. in., normal temperature and pressure) per minute. A pressure of 120 atmospheres, at sea level, corresponds to about 1800 lb. per sq. in. A reducing valve is employed to control this pressure and reduce it to the normal pressure of the atmosphere, for breathing. An air-tight breathing bag filled with pure air and equipped with a release valve, forms part of the apparatus and is connected directly with the oxygen supply cylinder and the helmet or mouthpiece.

Another important feature of breathing apparatus is the regenerator, holding a supply of 4 or 5 lb. of caustic soda or caustic potash. This minimum weight of caustic soda (4 lb.) will absorb, if fully utilized, 532 liters of carbon dioxide and is ample for all contingencies. By the absorption of the carbon dioxide, the caustic soda is converted into sodium carbonate and some water is produced according to the equation

=

=

Na2CO3 + H2O

The molecular weight of the caustic soda or sodium hydroxide is 2(23 + 16 + 1) = 80, while the molecular weight of the carbon dioxide is 12 + 2 × 16 44. The ratio of the weight of carbon dioxide absorbed to that of the caustic used is, therefore, 4480 1120; and the 4 lb. of caustic soda, if completely utilized, would absorb 4(11/20) 2.2 lb., or 18.78 cu. ft. of carbon dioxide (532 liters), at normal temperature and pressure. In the absorption of carbon dioxide, however, the caustic soda becomes encrusted with the sodium carbonate formed, which prevents or at least impedes the action of absorption. The shaking of the regenerator helps to break up this crust and restore the absorptive power of the caustic.

Testing Breathing Apparatus. All breathing apparatus should be regularly tested to insure its perfect condition. Especially should this be done by the wearer before he enters an irrespirable atmosphere. The apparatus may be defective

from any one of a number of such causes as negative pressure; leaks in joints, tubes, breathing bag or other container; obstructed valves or tubes, imperfect regeneration, owing to insufficient absorption of carbon dioxide or inadequate supply of oxygen; etc

Before putting on the apparatus, the wearer should examine and test its various parts to ascertain that it is tight, the valves and tubes free from obstruction and the supply of oxygen and caustic soda adequate. Each tube, the bag and the assembled apparatus should be tested for leaks, by means of the pressure gage and observing the constant water level in the U tube kept for that purpose. The old habit of immersing apparatus in water to show leakage is harmful.

TYPES OF BREATHING APPARATUS

The principal types of breathing apparatus now in use in this country are the Draeger breathing apparatus, the Fleuss Proto apparatus, the Paul type of apparatus and the more recent and highly improved Gibbs apparatus, which combines all of the best features of other types and many improvements. Draeger Breathing Apparatus.-There are two general types of this apparatus, one employing the helmet and the other the noseclip and mouthpiece. These two types are shown in Fig. 14 together with side and rear views of the apparatus as worn by the rescuer. Owing to its bulkiness the helmet type is not so well adapted to mine work as that equipped with the noseclip and mouthpiece.

Since its introduction in 1903 the Draeger apparatus has undergone various marked improvements and is at present one of the standard types of rescue appliances in use. The canvas breathing bags, one for inhalation and the other for exhalation, are rubber-lined. The oxygen cylinder is supplied with a perfected high-pressure valve that enables the wearer to shut off the pressure at any moment desired, by a simple thumb pressure. These together with the safety locked couplings securing all tube connections, and the time recorder and pressure gage, always ready for inspection by the wearer, insure both safety and comfort.