Among the most widely used and most effective poisons is arsenous acid boiled with rice, or mixed with cheese or cornmeal in the form of a paste, or placed upon sweets and fruits.
Crude phosphorus is chiefly used in similar pastes. When mixed with glucose its inflammable properties are said to be lost. Its inflammability is, of course, a serious obstacle to its general use.
Strychnine, owing to its bitter taste, is of little value in poisoning rats, and when used is best combined with glucose and one per cent. of cyanide of potassium. Soaked wheat, bread or similar food is then treated with this mixture and placed where rats may eat it. It is said to be eaten readily by ground squirrels with fatal effect. It is, however, expensive and apt to be taken by domestic fowls. Most rat poisons have the disadvantage of being dangerous to human life and must be used with caution wherever children and ignorant native persons are about.
Trapping.—Trapping has been found to be a very effective means of rat destruction in cities. (See later pages for relative efficiency of traps.) Rat traps are of several varieties and are constructed upon various principles. It is sometimes desirable to catch the rats alive and uninjured, and for this purpose barrel traps, wire cage traps and similar devices are placed in the rat highways. These highways are readily discovered in the cities. Considerable care must be taken to overcome the natural caution of the rat, and this includes judgment in the use of attractive bait, the concealing and smoking of traps after handling and perhaps the use of some scent, such as the oil of anise, of which rats seem to be fond. As a general rule bait should differ from the food naturally supplied by the locality. For example, about granaries and stables fresh animal food should be used for bait, while about slaughter houses, meat-markets, fish-markets and similar places, where animal offal is abundant, the rat should be tempted with vegetable bait.
Where the circumstances will permit, and this is apt to be so for ground-squirrel destruction, the burrows may be filled with some asphyxiating or poisonous gas. In this manner whole families of rodents, and their fleas as well, are destroyed.
The system is not often applicable in houses, but aboard ships it is found most effective, the holds of ships being flooded with sulphur dioxide, developed by burning sulphur in a special furnace provided with a pumping and piping system for delivering the gas at distant parts of the ship. In empty ships' holds and elsewhere the simple burning of sulphur in open vessels effects the same results, provided sufficient sulphur and a sufficient number of vessels be used and further provided that the generation and confining of gas be sufficiently prolonged. In San Francisco harbor, where for more than a year nine vessels were disinfected per day, this method was adopted as more effective, speedy and economical than any other system. It has the disadvantage, in the case of laden ships, of affording some danger of fire.
Carbon bisulphide has been extensively used in California in the burrows of ground squirrels. Its fumes, being heavier than air, penetrate the burrows and promptly poison or asphyxiate all living animals and fleas. Absorbent material of some kind is saturated with the liquid and placed in the entrance of the burrow, which is then quickly sealed to confine the gas.
It will be seen that, in common with other methods of rat destruction, fumigation has a limited application and a number of serious objections. It is particularly useful aboard ships.
The method should never be employed by unskilled persons or those unacquainted with the dangers to human life from noxious or asphyxiating gases.
Starving Rats.—The subjects of the starvation of rats and rat-proof construction may be considered together.