1886.] The Distribution of Micro-organisms in Air. 511 



through which several of these tubes can protrude, their ends being 

 then covered by a glass shade externally coated with cotton- wodl or 

 any other non-conducting material. By contriving this simple ar- 

 rangement I was able to keep every portion of the tubes in an atmo- 

 sphere of steam, whilst no condensed water could find its way into 

 the tubes. 



Fig. 1. 



I have found it very necessary not to overdo the steaming, as the 

 melting point of the gelatine is considerably reduced by its being 

 prolonged, and in these experiments it is of the greatest importance 

 that the gelatine should be capable of resisting the temperatures 

 which are incidental to the experiments, and which are encountered 

 in travelling with the apparatus. If the gelatine be too sensitive to 

 heat many experiments may be entirely lost, and the time and labour 

 spent upon their careful execution wasted. I would therefore recom- 

 mend that both in the preparation and in the sterilisation of the gela- 

 tine-peptone used in these tubes, the steaming should be reduced to a 

 minimum consistent with sterility. 



When finally sterilised, and whilst the gelatine is still fluid, the 

 tube is held horizontally under a cold-water tap, being simultaneously 

 rotated and moved backwards and forwards, so that uniform coolino- 

 is effected. As the gelatine approaches its solidifying point it be- 

 comes more. and more viscid, and gradually adheres to the surface of 

 the tube, so that with a little management the whole interior can be 

 uniformly coated. In this operation contact between the gelatine 

 and the cotton- wool plug in the small tube at the end must be care- 

 fully avoided, otherwise when cold the tube may become blocked up. 



The tube after being preserved for some time to insure sterility, is 

 then fit for use. 



