384 BACTERIA. 



gives the result of a number of similar observations, in which 

 the above statements are in the main confirmed. 



Of the methods of examination of the air for micro-organisms the 

 simplest and most convenient, but perhaps the least reliable, is that of 

 allowing the germs to settle on plates of sterilized gelatine or potatoes, that 

 are left uncovered for a. definite length of time. Currents of air may 

 completely destroy the accuracy of these results, but in rooms that 

 have been left undisturbed for some time, in which the doors are 

 closed, and in which no currents are set up by heat coming through the 

 windows or from fireplaces,. moderately accurate average "countings" 

 may be obtained. In place of these glass plates covered with gelatine, 

 shallow glass trays with glass lids soon came to be used to contain the 

 gelatine. Those used by Koch for this purpose are shallow glass capsules 

 about half an inch deep and a couple of inches in diameter, in which the 

 sterilized gelatine is placed. These are placed inside tall glass cylinders 

 about five or six inches high, the mouths of which are closed with large 

 cotton wadding jjlugs. The glass capsule is lowered into the cylinder and 

 again removed from it by means of a piece of soft metal bent at right 

 angles. After the whole has been sterilized the cotton wadding plug is 

 removed, the gelatine is left exposed, say, for ten minutes, the plug is 

 re-inserted, and organisms that have settled on it are allowed to develop at 

 the temperature of the room. These soon make their appearance as small, 

 white, yellow, or pink points, according to the nature of the germs that are 

 present in the air. In addition to these, however, a number of fluffy 

 white, green, or black, forms make their appearance. The former consist of 

 bacteria, sarcina, or yeasts ; the latter of penicillia, mucors, and aspergilli. 



Another method of examining the dust and micro-organisms contained 

 in the air is one in which an apparatus somewhat like a chemical " wash 

 bottle " with the bottom knocked out is used. In the neck of the bottle 

 is an india-rubber cork with two holes ; through one of these holes the long 

 limb of a tube bent into a U shape, with a long limb and a short limb is 

 passed ; the long limb, which is drawn out into a pretty fine point, projects 

 about two-thirds down into the bottle ; in the other hole of the stopper is a 

 short glass tube, to which is attached a piece of india-rubber tubing ; the 

 bottom of the bottle carefully ground, is luted with vaseline on to a 

 glass plate on which has been placed a microscope slide, so supported as 

 to rest with its upper surface immediately under the drawn out opening of 

 the longer tube. This slide has previously been coated (as recommended 

 by Miquel) with a mixture of one part of grape sugar and two parts of 

 glycerine. A given quantity of air is then drawn into the bottle by means 

 of an aspirating apparatus, and all particles of dust, spores, or moulds, 

 and organic and inorganic fragitients of all descriptions are made to 

 impinge on the sticky surface, where they are retained and may afterwards 

 be examined under the microscope, or, if necessary, the glycerine and 

 sugar mixture may be washed into a nutrient fluid such as peptonized meat 

 gelatine, and a regular biological examination of the organisms may be 

 made. A modification of this seroscope used by Miquel is a flask con- 

 taining a small quantity of fluid, into which the air is drawn. This apparatus 

 is somewhat complicated ; it is like a Pasteur flask, with three openings, 

 through one of which the air enters, the neck of the flask being continued 

 as a kind of tube down into the fluid in which the organisms are to be 



