Etiology : Causes of Disease. 1 1 



and of tuberculosis carried with the dry dust and inhaled. Of 

 toxic agents borne on the atmosphere we see the compounds of 

 arsenic, mercury and lead. Moisture and dryness of the air in- 

 duce respectively a lymphatic constitution and low tone of health, 

 and a nervous constitution and a tendency to neurosis, ophthalmias, 

 and skin diseases. The pressure of the atmosphere has a pro- 

 found effect on animal health as seen in the extreme troubles 

 of the heart and circulation in the diving bell, and the respiratory, 

 hasmorrhagic and brain affections of high altitudes. A low bar- 

 ometer is attended by nervous disorders (neuralgia) (S. Weir 

 Mitchell). Surgical operations do best with a high or rising bar- 

 ometer (Adinell, Hewson). The electrical tension of the atmos- 

 phere shows familiarly, in man, in the feeling of heaviness, dull- 

 ness and malaise that precedes the bursting of a thunderstorm 

 and the relief that follows its termination. To this influence 

 many of the domestic animals are incomparably more sensitive 

 than man, as witnessed in the disposition of swine to hide in their 

 pens or under litter on the approach of the storm, the nervous 

 disturbance even to abortion of certain ewes which are heavy in 

 lamb, and the great discomfort and even piteous cries of some 

 domestic felines on such occasions. The greatest electric tension 

 is seen in the drier climates, where the air, robbed of its moisture, 

 proves a poor conductor and equalizer, and the positive and nega- 

 tive electricity get stored up separately in air, cloud and soil. The 

 presence of ozone in the air, as a habitual concomitant of electric 

 discharges, has been supposed to be a disturbing influence, since 

 it is distinctly irritating to the mucous membrane when present 

 in excess, but such excess apart from its artificial production is 

 highly improbable. As habitually met with it is antiseptic and 

 health giving. Darkness always deteriorates the general health, 

 producing bloodlessness and pallor. Light is usually invigorat- 

 ing, yet bright sunshine falling upon the eyes from a window in 

 front of the stall, or in the open air when the face is turned up by 

 an overdraw check rein, or reflected from white dust and, above, 

 all, from snow, will often induce inflammation and blindness. 

 Soils are often potent 'etiological factors. Dense, damp, cold, 

 undrained soils, are habitually covered by a stratum of cold air, 

 saturated with moisture, which greatly lowers the vital stamina. 

 Damp clays, and waterlogged soils of various kinds, rich in 



