THE AIR OP TOWNS. 377 



of beautifying our buildings and streets and squares, and of realizing 

 tlie ideal town described in my first lecture. 



Lecture 4. — The Germs of the Air. 



Until the beginning of the present century, physical science directed 

 the minds of philosophers mainly toward the study of the infinitely 

 great — the discovery of new worlds in space, the study of universal 

 gravitation, and the measurement of the velocity of light. The present 

 century has illumined a new path in the Dark Unknown. The science 

 of to day is essentially the science of the infinitely small. Dalton's 

 atomic theory, a theory of the invisible atomic structure of matter, is 

 the foundation of modern chemistry and physics. The germ theory of 

 disease, a theory which involves the existence of the microscopic living 

 matter dwelling within and around us, is the basis of modern pathology 

 and surgery. It is to these minute organisms that I have now to direct 

 your attention. 



The discovery of these living particles, particles so small that it is 

 probable that many of them defy the scrutiny of the most perfect 

 microscope, originated in the study of a very ancient process, the proc- 

 ess of fermentation. 



Boyle, in the seventeenth century, in his " Essay on the pathological 

 part of physik," with that almost prophetic clearness of vision which 

 marked his conclusions, wrote as follows: "And let me add that he 

 that thoroughly understands the nature of ferments and fermentations 

 shall probably be much better able than he that ignores them to give 

 a fair account of divers phenomena of several diseases (as well fevers 

 as others) which will, perhaps, be never properly understood without 

 an insight into the doctrine of fermentations." 



The making of wine and the brewing of beer have been practiced in 

 historic and prehistoric times. Theophrastes, who lived in Egypt B. C. 

 400, described beer as the "wine of barley." Noah, we read, "planted 

 a vineyard and drank of the wine which maketh glad the heart of man," 

 and one or both of these processes is practiced by nearly every nation, 

 civilized and uncivilized, at the present day. 



If the grape is crushed and left to itself at a moderate temperature 

 it begins to froth. After a few days its sweetness, which was due to 

 sugar, has gone and the juice has acquired a slightly burning taste. 

 It now contains no sugar, but alcohol. If barley is moistened and 

 allowed to germinate and the germination suddenly stopped by roast- 

 ing the grain, the barley has a sweetish taste. It is now called malt. 

 The constituents of the barley have been changed ; a new substance 

 has been formed, viz, diastase, a substance which has the peculiar 

 property of converting the starch of the grain into sugar as soon as the 

 grain is steeped in water. A little of this sugar has already appeared 



