32 BACTERIA. 



whilst the special bacteria associated with other diseases 

 have also been demonstrated in the air of wards specially 

 set apart for the treatment of these diseases. Then, 

 again, it must be remembered that there are always 

 more micro-organisms in the air in those regions where 

 decomposing organic matter of any kind is allowed to ac- 

 cumulate, this being especially the case in positions in 

 which the air cannot be continuously renewed by currents 

 setting in from other and purer regions. Consequently, 

 it is found that in valleys and in low-lying country generally, 

 especially where there is any accumulation of decayed vege- 

 table matter, or where people are massed together in towns 

 or villages, many bacteria are usually present in the air. 

 Some of the organic growths obtained from such air 

 belong to the mould fungi, the spores of which, as is 

 well known, have considerable resisting power, and may 

 be readily carried from point to point by gentle currents 

 of air. As the low-lying lands are left, and the hill 

 country is reached, micro-organisms are comparatively 

 few in number, and at certain elevations, especially where 

 the temperature is low, the number of these germs of 

 moulds and bacteria may sink almost to zero. Again, 

 if the air brought by breezes coming from the sea on 

 the one hand and from the land on the other, be examined, 

 it will be found that in the former case it is im- 

 possible to demonstrate the presence of organic life of any 

 kind ; whilst in the latter, collected it may be at the same 

 point, an enormous number of micro-organisms of different 

 kinds may be met with. It is, however, impossible to give 

 any general rule as to the number of organisms that should 

 be found under these various conditions, though we may 

 take it as a result of Aitken's experiments on the presence 

 of solid particles in the air and their relation to fogs, that 

 the more solid particles there are in the air, the more micro- 

 organisms of various kinds are to be found. Some idea of 

 the number of organisms present at different seasons of the 

 year may be gathered from P. F. Frankland's investiga- 

 tions, the results of which were presented to the Royal 

 Society in 1886. He determined the number of colonies 

 found in two gallons of air (examined by Hesse's method), 

 collected on the roofs of the Science Schools at South 

 Kensington. He found fewest present during the month of 



