824 PATHOGENIC MICROORGANISMS 



supporting timbers, as this would result in open spaces, permitting the 

 entrance of rodents. Groceries, stables, warehouses, markets, and food depots 

 in general are best rat-proofed by having a concrete floor in addition to 

 concrete walls. In these structures, untenanted as they are at night, rats 

 might well enter by a doorway or window carelessly left open, or be intro- 

 duced concealed in merchandise, and, gnawing through plank flooring, obtain 

 a well-protected hiding place. 



"In addition to concrete floor and walls, these food depots must have 

 tight-fitting doors, and all windows and other openings should be properly 

 screened. A 12-guage wire is preferable on account of its strength and 

 durability, and the mesh should not be larger than one-half inch. 



"Rat-proofing by elevation of the building is chiefly applicable to small 

 and medium sized frame dwellings. The purpose is to have a sufficient 

 elevation, about two feet, so that the ground area beneath will be as exposed 

 and free from covert as land unbuilt upon. Marginal rat-proofing will 

 suffice in more pretentious dwellings where sufficient care can be exercised 

 to prevent rats from gnawing through the plank floors. 



"Chicken pens can be protected by marginal concrete walls, sunk into 

 the ground two feet or more, and by covering the sides and top with ^-inch 

 mesh wire netting. Garbage cans should be made of serviceable metal and 

 should have properly fitting tops. 



"Plank sidewalks and plank coverings for yards should be avoided. 

 Cinders and concrete should be used instead. The latter should have marginal 

 protection to prevent rats from burrowing beneath it. 



"Double walls, with a dead space between, should be avoided, or, if used, 

 they should be rat-proofed at the top and bottom with heavy wooden timbers, 

 4 by 4-inch fillers, or by a concrete fill. Attics should be well opened and 

 kept free of rubbish or other refuge for rats. 



"These precautions against rat harborage and for the protection of food 

 supplies, in connection with careful trapping and poisoning, will be attended 

 with considerable success in the destruction of rats. 



"The appended model ordinance is applicable, with perhaps slight modifi- 

 cations, to any urban community. It should be examined by competent local 

 counsel for changes in form, or in substance if necessary, as dictated by 

 special constitutional, legislative, or charter considerations." 



Plague Vaccination. The immunization of animals with suspen- 

 sions of plague bacilli, killed by moderate heating, 50 for one 

 hour, was first attempted by Yersin, Calmette and Borrel in 1897. 

 Kolle, 25 Haffkine 26 and others studied plague vaccination particularly 

 in the subsequent years. A great many different vaccines have been 



x Kolle, loc. cit. 

 "Haffkine, loc. cit. 



