lOO EXAMINATION OF AIR AND WATER 



were isolated by him. Staphylococci, sarcinae, streptothriceae, 

 and moulds were the most common species present. He con- 

 firmed the view of Chick as to the rarity of B. coli in air. 



We have ourselves carried out a number of investigations into 

 the bacteriology of air of byres, dairies, milk-shops, bakehouses, 

 workshops, and the open air in town and country. We found no 

 exact relationship to exist between percentage of COg in the air 

 and number of organisms, although in a general way it may be 

 said that where COg was most largely present organisms were 

 most numerous. In addition to general investigations of this 

 character, we have made several series of examinations of air in 

 relation to the milk supply, both in town and country. The 

 accompanying Plates are reproduced from photographs of the 

 colonies on nutrient agar in Petri dishes. These were incubated 

 for four days at ordinary room temperature, and the technique 

 adopted was that described below. Plates 5, 6, and 4 also illus- 

 trate similar examinations of byre and dairy air, farm water, 

 and the air of a certain spot in a London street, where milk is 

 delivered daily. 



These Plates illustrate as words cannot do {a) the differences 

 in the bacterial pollution of air in town and country, and {b) the 

 degree of contamination likely to occur in milk exposed in such 

 atmospheres. It should be added that the plates have been selected 

 from a large number as representative ; they are not the result of 

 accidental or haphazard examinations. As to the kind of organisms 

 present, we have found them chiefly to be the following : — staphylo- 

 cocci, diplococci, a few chromogenic organisms, sarcinae, saccharo- 

 myces, moulds, and a few representatives of the streptothrix group 

 of organisms. Occasionally, but by no means uniformly, we have 

 found lactic acid organisms. We have met with no B. coli, although 

 a constant outlook has been made for it. 



Bacteriolog'ical Methods i 



For the bacteriological examination of air many methods 

 have been devised, and we propose to summarise below the 

 various systems employed in this country and abroad, giving in | 

 greater detail those we ourselves have found most useful for the 

 purpose in our investigations in relation to milk pollution, , 



I. Koch's plate method— 



The simplest means of obtaining a purely qualitative bacteria-; 

 logical analysis of the air in any given locality or place is that 



