HISTORICAL INTRODUCTION 



by gravity. Hesse found that moulds penetrated much farther into his 

 tubes than did the bacteria, and made the important deduction that 

 mould-germs as found in the atmosphere are on the average lighter than 

 the bacterial germs. This led him to conclude that, whereas fungus spores 

 were usually present in the air as single particles, the aerial bacteria 

 mostly occur in the atmosphere either as large aggregates, or attached to 

 relatively large carrier particles of dust, soil, or debris (Hesse 1884, 1888). 

 He also observed that most colonies consisted of a single species — bacteria 

 usually in small colonies of pure culture, and fungi as isolated spores — 

 and deduced that the airborne germs are not in the form of aggregates of 

 different t\'pes. 



Hesse's method was also used in London by Frankland (1886, 1887) 

 and Frankland & Hart (1887) on the roof of what is now known as the 

 Old Huxley Building of the Imperial College of Science and Technology, 

 and elsewhere. Simultaneous comparisons were made between the number 

 of micro-organisms per 10 litres (as indicated by colonies growing on 

 Hesse's tubes of peptone gelatine) and the number deposited on horizon- 

 tal dishes of the same medium, expressed as the number deposited per 

 unit area per minute. Tests were made both outdoors and inside crowded 

 or empt}' buildings. Frankland noted that the number of colonies was 

 greater when the mouth of the tube faced the wind rather than in other 

 directions, so he standardized his method by always turning it at an angle 

 of 135° to the wind. A control tube facing the wind but not aspirated was 

 always used, and sometimes it had a substantial number of colonics. 

 Frankland seems to have been the first to realize that aerodynamic effects 

 are of major importance in techniques for trapping the air-spora. 



These methods for studying the air-spora were continued into the 

 present century, notably by Saito (1904, 1908, 1922) in Japan, and by 

 Buller & Lowe (191 1) in the Canadian Prairies. 



The Allergists 



The idea that men, other animals, and plants, could become infected 

 by microbes which set up pathological changes, had been made acceptable 

 by the analog}'' of sterile organic infusions that become infected with 

 putrefying microbes. The idea became widely accepted during the latter 

 half of the nineteenth century and, when once the cause of the common 

 epidemic diseases had been established, advances in hygiene and therapy 

 began to transform the social scene. Yet there remained some diseases 

 for which no pathogenic or parasitic invader could be found. Some of 

 these, such as pellagra and beri-beri, have now been traced to a variety 

 of nutritional deficiencies. Another group, the so-called allergies, were at 

 first difficult to grasp because a peculiar condition of the patient was a 

 complicating factor. Allergic diseases, unlike those caused by invasion of 

 the body by a pathogenic micro-organism, are due to a changed condition 



II 



