Considering the well-established facts that rats are specially susceptible to plague, that in many epidemics they have been attacked weeks before, the disease has shown itself in man, that their habits bring them into intimate association with man, there can be no doubt that these rodents play a very important part in the diffusion of the introduced virus. This being the case, it is reasonable to believe that if the rats were destroyed the risk from the introduction of a case of plague, whether by man, rat, or fomites, would be very much reduced in any community in which this measure had been carried out. The responsible authorities of all towns and villages in active communication with plague centres should, in anticipation of the possible advent of the infection, see to the destruction of all rats in their districts, and this in addition to instituting the ordinary measures for dealing with overcrowding, food and water supply, domestic and municipal cleanliness. The destruction of rats is a cheap measure in comparison with those adopted in India and elsewhere, which have so signally failed to arrest epidemic plague. The campaign against rats is usually carried on by the employment of rat-traps and rat-catchers, and the laying down, under precautionary conditions, of poisons such as arsenic and phosphorus. As no one method is satisfactory, it is usual to employ several at the same time. The pumping of SO 2 gas under pressure is useful for ships full of cargo and for warehouses with goods in them. So long as the sulphurous acid gas is dry, and is not used on damp articles, no damage is done to merchandise. Care has, however, to be taken with damp things, as they may get slightly discoloured.
Where possible houses and warehouses should be made rat-proof, not an easy measure considering the burrowing and climbing habits of the rat. Mus decumanus can penetrate ordinary lime mortar or soft brick, but is stopped by cement and concrete. Its burrows may attain a depth of 18 in. Mus rattusis not so active in this respect. Simpson re-commends that walls should be at least 6 in. thick when made of hard brick or concrete, and that they should extend
