N. MATERIA MEDICA, THERAPEUTICS, AND HYGIENE. 539 



one continued for many months or years almost inevitably 

 produces flital results. 



Dr. Richardson states that the principal conditions in a 

 mask, in order to secure perfection of action, are a free en- 

 trance of air and freedom of exit, with balance for respira- 

 tion and expiration so arranged that the expiration sliould 

 be at least one sixth freer than the inspiration ; second, the 

 removal of the water of expiration ; third, selection of a ma- 

 terial for filtration that shall absorb the smallest possible 

 amount of water, offer as little resistance as is possible, and 

 filter finely ; fourth, arrangement of the filter so that it 

 should, as far as possible, clear itself, in expiration, of the 

 dust it had received durinoj the time the air ^vas beino: drawn 

 through in the act of inspiration. 



For these purposes numerous devices have been presented ; 

 among them one by which the air is passed through cotton. 

 In this, however, the cotton soon becomes saturated with 

 water; the breathing is difficult even in pure air. A second 

 form, made of layers of crape spread over a light framework, 

 is easily put on and off, and it admits the passage of the air 

 in either direction freely, but it allows the water to accumu- 

 late in the meshes, and it filters badlv. It answers fairlv for 

 very coarse dusts, but the finer sorts, such as flour, are not 

 excluded. 



Other very complicated forms were mentioned ; but the 

 one to which the lecturer referred as havinsr all the eood 

 qualities and none of the disadvantages of the rest is made 

 of feathers arranged around the outside of a perforated breath- 

 ing tube of a convenient size, a line of feathers being fasten- 

 ed to the tube, and wrapped around it so closely as to cover 

 all the perforations. By breathing through the upper end of 

 this tube, the feathers come down to the perforations in in- 

 spiration, and expand in expiration so as to be lifted from 

 the openings, and all the intercepted dust is blown off. Very 

 little moisture is collected, and the apparatus is considered 

 almost perfect. 



The tube is fastened in a mask, which is tied around the 

 feathers, so as the more readily to keep them in the proper 

 position. It can be put on and taken off as easily as if it 

 were a pair of spectacles. The filter-tube is placed below, 

 so as to catch all the inspired air, and by means of the two 



