Mr. Faraday on Chemical Manipulation, 279 



quired for use, the acid of a small portion of it should be 

 neutralized by caustic potash, or soda, (not by ammonia,) 

 when it will assume an intensely deep blue colour, and will, 

 in most cases, require dilution with twelve or fourteen parts 

 of water. The red liquor of pickle cabbage will, occa- 

 sionally, answer the uses of the solution, and is, when re^ 

 quired for service, to be neutralized in a similar manner." 



For test-papers, litmus and turmeric are the most essential, 

 and several precautions in preparing and usin^ them are here 

 pointed out, which, though apparently trivial, are, in fact, 

 extremely important in insuring correct conclusions. We 

 transcribe a part of the account of the applications of these 

 coloured papers, as a specimen of the clear minuteness with 

 which the details of the work are given, and as a sample of 

 the author's general method and style, where subjects of 

 much greater intricacy are to be explained. 



"591. In using these test papers with a fluid suspected to 

 contain free acid or alkali, or knowing that one of these 

 substances is predominant, to ascertain which is so, all that 

 is necessary is to moisten them with the liquid, and observe 

 the change : if the fluid be acid, the blue colour of the litmus 

 will immediately become red ; if alkaline, the yellow colour 

 of the turmeric will be changed to a brown. The moisten- 

 ing may be effected by dipping the paper into the liquid ; 

 but a better method is to touch the edge of the slip with a 

 rod dipped in the fluid. In the latter case there is no risk 

 of contamination to the fluid from the paper, and. only a 

 very minute quantity of the liquid is used at once. 



" 592. These trials must be made by day-light ; artificial 

 light not permitting that just estimation of the changes by 

 which the presence of a small excess of acid or alkali is to be 

 determined. As the proportion of free acid or alkali di- 

 minishes, the intensity of the new tint produced upon the 

 paper is also diminished ; and when in very small quantity, it 

 requires considerable attention before a decision can bo 

 arrived at. The test paper should occasionally be touched 

 with pure water in the immediate neighbourhood of the part 

 where the solution has been applied, for any change in 

 appearance that may have occurred, not due to mere moist- 

 ening, is then readily perceived. 



** 593. Although acid is generally tested for by litmus 

 paper, and alkali by turmeric paper, yet the former is some- 

 times used advantageously for the hitter purpose, being first 

 slightly reddened, either by exposure to the air, or by 

 momentary contact with muriatic acid fumes. When the 



