THE MICROSCOPE IN .ETIOLOGY. 321 



CHAPTER XV. 



THE MICROSCOPE IN ETIOLOGY. 



THE study of aetiology, or the knowledge of the causes 

 of disease, although so important a branch of practical 

 medicine, needs the careful and united efforts of many 

 observers to be classified and recorded before approaching 

 perfection. Here, also, the microscope will be found an 

 important aid. The numerous external causes of disease, 

 such as physical or organic impurities in the atmosphere, 

 soil, water, and food, vegetable or animal parasites, and 

 "disease germs," with their relation to epidemic or en- 

 demic disorders, all require skilful use of the microscope. 



I. EXAMINATION OF THE AIR. 



The pressure, temperature, moisture, and electricity of 

 the air, all of which are important in considering causes 

 of disease, require other modes of investigation, but the 

 microscope may be reasonably expected to aid in inquiries 

 concerning mechanical, chemical, or organic impurities. 



Many methods have been proposed for collecting mat- 

 ters suspended in the atmosphere. A shallow dish con- 

 taining distilled water, or a clean glass vessel containing 

 ice, so as to condense the atmospheric moisture with its 

 impurities on the outside, and allow it to trickle into a 

 conical receiver, have been used. Glass plates moistened 

 with glycerin and exposed to the air are still better. The 

 aeroscope of Dr. Maddox is a funnel-shaped tube turned 

 to the wind by a vane. The narrow end of the tube is 

 opposite a slide moistened with glycerin. 



Many absurd " discoveries " have been paraded respect- 

 ing matters in air and water, yet careful observation will 



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