454 SANITATION 



of mercuric chloride to 1,000 parts of water. Common salt is mixed 

 with the mercuric chloride before the water is added. The solution is 

 very poisonous if taken internally, but it is very efficient in destroying 

 germs that are on the hands, on washable clothing, and on floors. 

 Mercuric iodide, a compound of mercury and iodine, is an even better 

 disinfectant than mercuric chloride, especially for surgical instru- 

 ments. It is mixed with potassium iodide to make it soluble. Its 

 ordinary name is biniodide of mercury. Strong acids and alkalies are 

 powerful disinfectants, but they are not used as much as they might 

 be because they act upon skin, clothing, etc. The most common acid 

 used in natural disinfection is the hydrochloric acid of the stomach 

 (cf. 364). In artificial disinfection carbolic acid is used. In winter, 

 carbolic acid is usually a solid; in summer it is a thick liquid. The 

 solid melts readily if it is set in a pan of warm water. The carbolic 

 acid disinfecting solution contains 2.5 to 5 grams of the acid in 100 

 grams of water. This would be 2 to 3 tablespoons full of acid to a pint 

 of water. The most common base, or alkali, used in disinfection is 

 "milk of lime" (cf. 132). Lye is also used. The disinfectant known 

 as chloride of lime, or "bleaching powder" (cf. 110), is made by 

 passing chlorine into slaked lime. 



To disinfect a house by fumigation, gaseous germicides 

 are used. The two most common ones are sulphur 

 dioxide and formaldehyde. The sulphur dioxide is com- 

 monly made by the burning of sulphur " can dies. " The 

 candles used do not as a rule give enough sulphur dioxide 

 to destroy the germs ; they are also a source of danger, for 

 they may set the house on fire. Sulphur dioxide gas can 

 be obtained more safely if liquid sulphur dioxide, or the 

 solution of the gas, is poured out in saucers, and allowed 

 to evaporate. Sulphur dioxide is used to destroy vermin, 

 as well as germs. Formaldehyde is a compound of car- 

 bon, hydrogen, and oxygen (cf. 123) ; it is a much better 

 germicide than sulphur dioxide. One of the easiest ways 



