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close the lips perfectly with a tube placed in the mouth. When it is 

 desired to measure ordinary inspiration, it is advisable to use a mask 

 which encloses the chin, nose, and mouth, so that inspiration may 

 proceed easily and naturally through both openings. The sides 

 of the mask must be made of lead, so thick that when pressed upon 

 the features it will retain its position ; and it should be lined with 

 sheet caoutchouc, the better to adhere to the skin. The face must 

 be well introduced into the mask, and the thumbs placed under the 

 chin whilst the forefingers cover the free edge of the mask so as to 

 press the lead to the face and prevent any ingress or egress of the 

 air either at the bridge of the nose or on the sides. Persons with 

 large beards, and those with very thin and sunken cheeks, cannot 

 use the mask effectually. After the mask has been worn for some 

 time the vapour condenses within it, and the fluid trickles beneath 

 the chin, when it will be very difficult to prevent a little air entering 

 until the mask shall have been removed and wiped. The mask may 

 be held upon the features by bands which cross the head transversely 

 and longitudinally. Care must be taken that the valves close well 

 and act easily. If the mask has been laid aside for some time, the 

 valves will have curled up ; and it will be necessary to place it in luke- 

 warm water for half an hour, or to pour water upon the dry valves. 

 The measuring-instrument should offer but a very small amount of 

 resistance to the expired current of air ; so that, if it be a gas-holder, 

 as in Hutchinson's and Davy's spirometer, it should be accurately 

 counterpoised at every part of its progress ; or if it measure and 

 register ordinary inspiration, the adverse pressure should not exceed 

 ^ths of an inch of a column of water. The person must breathe 

 normally, by the aid of previous training and by abstracting the at- 

 tention. If there should be any sense of constriction about the chest, 

 it may be inferred that the respiration is not normal, and that the 

 chest is either too much collapsed, so that the act of expiration is too 

 prolonged, or it remains expanded above the ordinary degree, and the 

 act of expiration is shortened. The sense of ease and satisfactory re- 

 spiration must be at all times present. Hence practice and intelligence 

 are necessary. The experiments must embrace several minutes at a 

 time ; for a respiration rarely ends at a complete minute, and must 

 therefore be recorded as a fraction, and the attention cannot be 

 abstracted in the short period of one minute. Results obtained from 



