138 PUBLIC HEALTH CHEMISTRY 



sodium thiosulphate solution, and number of c.c. of same 

 required subtracted from 30 gives number of c.c. of 

 N/10 bromine solution absorbed. From the quantities 

 taken, this number, multiplied by four, gives percentage 

 of phenol in sample. 



The chief impurities in carbolic acid are the light and 

 heavy coal-tar oils, which are largely composed of hydro- 

 carbons of the benzene series. The light oils float on water, 

 and the heavy sink; hence the terminology. In " carbolic 

 powders " there is sometimes very little phenol, it being 

 replaced by cresols or sulphites. Even when present the 

 phenol may be rendered inert by having lime as a basis 

 instead of silica. Pure phenol crystallizes in long colourless 

 prisms, melts at 42 C, boils at i83C, is soluble in water 

 (1 part in 15 at 20 C), and is very soluble in alcohol, ether, 

 benzene, chloroform, carbon disulphide, and glacial acetic 

 acid. Commercial phenol is a colourless crystalline mass, 

 which gradually acquires a reddish colour, and deliquesces 

 on exposure to the air. 



Cresols are found in coal tar, give a blue colour with 

 ferric chloride, and are otherwise similar to phenol. They 

 are hydroxy derivatives of toluene or methyl-benzene, and 

 are known as ortho-, meta-, and para-cresol, all of them 

 being present in coal tar and in the tar from pine and 

 beech woods. Ortho-cresol melts at 31 C. and boils at 

 188 C. ; meta-cresol at 4 to 5 C. and 201 C. ; and 

 para-cresol at 36 C. and 198 C, respectively. Creosote is 

 obtained by distillation of wood tar, and contains among 

 other things cresols and guaiacol. 



Carbolic Powders. A good powder should contain 

 not less than 15 per cent of tar acids (crude carbolic acid), 

 of which 62-5 per cent is crystallizable at 15 to 20 C. 

 when examined by C. Low's test. The base should 

 contain no lime or chalk. Test reaction of powder with 

 litmus. If alkaline, it contains free lime. If otherwise, 

 the base is probably siliceous. 



Examination. 



I. If the base is silica. Mix the powder well and take 

 50 grm. Extract with 150 c.c. of methylated spirit, 

 which dissolves all the tar acids not in combination with 

 lime. Now separate, and mix extract with 50 c.c. of 10 per 



