G. NATURAL HISTORY AND ZOOLOGY. 277 



G. NATURAL HISTORY AND ZOOLOGY. 



MICROSCOPICAL EXAMINATION OF AIR. 



A valuable document has lately been published by the 

 English government in Calcutta, consisting of a report upon 

 a microscopical examination of air, prepared by Dr. D. Doug- 

 lass Cunningham. This gives the result of examinations of 

 atmospheric dust from various localities, as deposited on 

 ledges, leaves of trees, etc., and upon moist glass slides. It 

 includes also observations on organisms contained in dew, in 

 rain-water, and in the air of sewers. The work is illustrated 

 by fourteen plates, giving magnified representations of the 

 objects as seen by the microscope. Among the general con- 

 clusions obtained by Dr. Cunningham, at least so far as ob- 

 servations in the vicinity of Calcutta are concerned, are the 

 following: (1.) Specimens of dust washed from exposed sur- 

 faces can not be regarded as fair indices of the constituents 



CD 



of atmospheric dust, as they are liable to contain bodies which 

 may have reached the surface otherwise than by means of 

 the air, and they do not, at any rate, indicate the relative pro- 

 portions of the different constituents. (2.) There is general- 

 ly but little trace of distinct infusorial animalcules in dry 

 dust; and although distinct bacteria are frequently to be 

 found among deposits from the moist air of the sewers, they 

 are almost entirely absent as constituents of dry dust. (3.) 

 The addition of dry dust, which has been exposed to tropical 

 heat, to putrescible fluids, is followed by a rapid development 

 of fungi and bacteria, although recognizable specimens of the 

 latter are very rarely to be found in it while dry. (4.) Spores 

 and other vegetable cells are constantly present in atmos- 

 pheric dust, and usually occur in considerable numbers. The 

 majority of them are living, and capable of growth and devel- 

 opment. The amount of them present in the air appears to 

 be independent of conditions of velocity and direction of wind, 

 and their numbers are not diminished by moisture. (5.) No 

 connection can be traced between the numbers of bacteria, 

 spores, etc., present in the air, and the occurrence of diarrhoea, 

 dysentery, cholera, ague, or dengue, nor between the presence . 



