6 VETERINARY HYGIENE 



waters that do not readily form a lather. The description " soft " 

 is given to waters that form a soapy lather with ease, and which 

 have a soft feel. Since a hard water does not readily form a lather 

 with soap while a soft water does it follows that more soap will be 

 required to produce a lather in a given quantity of hard water 

 than in the same quantity of soft water; on the degree of hardness, 

 therefore, depends the soap-destroying power of a water and, as 

 will be shown later, this power of a water may be used to determine 

 its relative hardness. 



In order to understand clearly the soap-destroying power of 

 water it is necessary to know how soap is made and the changes 

 that take place when soap is rubbed up in normal pure water. 



The natural fats and oils (an oil is a liquid fat) are insoluble 

 in water, but if they are boiled with a solution of a caustic alkali 

 in water they are split up into glycerol and an alkali salt of the 

 fatty acid (soap). This change is spoken of as saponification, and 

 is expressed as follows : 



C,H B (C 18 H S5 8 )8 + 3NaOH = 3 NaC 18 H 35 O 2 + C 3 H B (OH) 3 

 Tristearin + Soda = Sodium + Glycerine 



stearate 



In hot water the sodium stearate or soap and the glycerine are 

 in solution, and the former is precipitated or " salted out " by the 

 addition of common salt ; it is then purified and forms the soap of 

 commerce. When soda is used as the base, a hard soap is produced ; 

 when potash, a soft soap. 



Both the hard and soft soaps are soluble in water, and they 

 dissociate by hydrolytic cleavage into free fatty acid and free alkali 

 (sodium or potassium hydroxide), forming an emulsion or lather 

 which possesses cleansing properties. 



Most natural waters contain in solution some of the earthy 

 or mineral salts such as the carbonates, or rather bicarbonates, 

 of calcium and magnesium; the sulphates and chlorides of calcium 

 and magnesium; or iron or zinc salts. As the cleavage of soap 

 into fatty acid and alkali occurs, the fatty acids combine with 

 such compounds of calcium and magnesium as may be present, 

 forming insoluble salts, which are precipitated as a " curd." 



So long as there are compounds of calcium and magnesium 

 available for combination with the dissociated fatty acids no 

 emulsion will take place, and consequently little or no cleansing. 

 The greater the amount of mineral matter held in solution in the 

 water the more soap will be destroyed before a lather occurs, the 

 soap-destroying power of a hard water, therefore, will be greater 

 than that of a soft water. 



