CLIMATOLOGY 



1421 



especially in houses, where it increases, relatively to the other, up 

 to the third floor, but above that level it alone infects the house. It 

 is not so common in gullies, compounds, stables, go-downs, and food 

 and tea shops as E. norvegicus. The common meeting-ground of 

 the two species appears to be the lower floors of houses, gullies, and 

 go-downs. Though a domesticated rat, it can climb and burrow. 

 It forms its nest in cupboards, heaps of firewood, etc., and breeds 

 all the year round, but especially from June to October, the average 

 family being five. 



The spread of the plague from E. norvegicus to E. ratUis, accord- 

 ing to the experiments of the Commission, is neither by direct con- 

 tact nor by air, soil, or food, but solely by the flea. Contact was 

 excluded by placing healthy rats in the same room with plague- 

 infected animals from which the fleas had been removed, when it 

 was found that none of them developed plague. The experiment 

 was kept up for a long time, replacing dead infected rats with 

 freshly infected rats, and, further, the room was never cleaned out, so 

 that the healthy animals lived in contact with the infected urine and 

 faeces, and even ate food polluted therewith, and yet not one con- 

 tracted plague, thus excluding transmission by contact, soil, and food. 



Again, when healthy animals were suspended in cages 2 feet 

 from the ground, so that the fleas could not get to them, or placed 

 on the ground, and surrounded by 6 inches of tangle-foot, over 

 which a flea cannot pass, as it is said to be incapable of jumping 

 more than 4 inches, or surrounded by a curtain of wire gauze so 

 fine that a flea could not penetrate it, and exposed to infection, 

 they escaped, though o+hers not so protected became infected, thus 

 disproving aerial infection. Further, the transmission by the rat- 

 flea was proved by constructing a glass box, inside which two wire 

 cages were placed at a little distance, but side by side, each standing 

 in a tray filled with sand. Each cage had a lid, through which rats, 

 food, water, etc., could be introduced, and the whole apparatus 

 was covered in with fine muslin to prevent the escape of the fleas. 

 A plague-infected rat and a number of rat -fleas were placed in one 

 cage. When this rat died, a healthy rat was placed in the other 

 cage, and after some time the dead body of the infected rat was 

 removed, when it was found that the new rat became infected with 

 plague, and fleas containing bacilli were found upon it. This 

 experiment was repeated many times, 45 per cent, of the exposed 

 rats taking the disease. Further, fleas infected by biting plague 

 rats, when placed upon healthy rats, produced the disease in 55 per 

 cent, of the experiments. 



The Commission calculated that the blood of an ordinary plague 

 rat in two-thirds of the cases contains more than 100,000,000 bacilli 

 per cubic centimetre, and that a flea's stomach could hold 0-5 cubic 

 millimetre of blood. Therefore, when the flea gorged itself on the 

 average plague-stricken rat it received at least 5,000 bacilli. These 

 bacilli are found only in the stomach and in the alimentary canal 

 posterior to that viscus, especially the rectum , and escape from the 



