APPENDIX. 235 



aspirating apparatus. This apparatus consists of a couple 

 of litre-flasks, suspended by hooks from the tripod stand 

 which supports the whole apparatus. The cylinder, caps, 

 and plug, are washed with solution of corrosive sub- 

 limate, and then with alcohol. After being thus cleansed, 

 50 ccm. of nutrient gelatine are introduced, and the whole 

 sterilised by steaming for half an hour for three successive 

 days. After the final sterilisation the cylinder is rotated 

 on its long axis, so that the nutrient medium solidifies 

 in the form of a coating over the whole of the interior. 

 When required for use, the cotton-wool plug is removed 

 from the small glass tube, and the latter connected with 

 the upper flask by means of the india-rubber tubing. 



The apparatus is placed in the air which is to be 

 examined, the outer india-rubber cap removed from 

 the glass cylinder, and the upper flask tilted until the 

 water begins to flow into the lower one. The emptying 

 continues by syphon action, and air is drawn in along 

 the cylinder to replace the water. When the upper flask 

 is empty, the position of the two is reversed, and the flow 

 again started. When a sufficient volume has been drawn 

 through the cylinder the outer cap and the cotton-wool 

 plug are replaced, and it is set aside for the colonies to 

 develop. As an example, twenty-five litres of air from 

 an open square in Berlin gave rise to three colonies of 

 bacteria and sixteen moulds ; on the other hand, two 

 litres from a school-room just vacated by the scholars 

 gave thirty-seven colonies of bacteria and thirty-three 

 moulds. 



Various forms of " aeroscopes " and " aeroniscopes " have 

 from time to time been employed. Pouchet's aeroscope 

 consists of a small funnel, drawn out to a point below 

 which is a glass slip coated with glycerine. The end. of 

 the funnel and the glass slip are enclosed in an air-tight 

 chamber, from which a small glass tube passes out con- 

 nected by india-rubber tubing with an aspirator. The 

 air passing down the funnel strikes upon the glycerine, 



