184 PRINCIPLES OF CHEMISTRY 



taste when dissolved in water (or saliva, for then only can they act on 

 the palate). Vinegar, for example, has an acid taste because it contains 

 acetic acid dissolved in water. Sulphuric acid, of which we have made 

 mention many times, because it is the acid of the greatest importance 

 both in practical chemistry and for its technical applications, is really 

 a hydrate formed by the combination of sulphuric anhydride with 

 water. Besides their acid taste, dissolved acids or acid hydrates have 

 the property of changing to red the blue colour of certain vegetable 

 dyes. Of these dyes litmus is particularly remarkable and much used. 

 It is the blue substance extracted from certain lichens, and is used for 

 dyeing tissues blue ; it gives a blue infusion with water. This 

 infusion, on the addition of an acid, changes from blue to red. 5n 



Basic oxides, in combining with water, form hydrates, of which, 

 however, very few are soluble in water. Those which are soluble in 

 water have an alkaline taste like that of soap or of water in which ashes 

 have been boiled, and are called alkalis. Further, alkalis have the 



50 Blotting or unsized paper, soaked in a solution of litmus, is usually employed for 

 detecting the presence of acids. This paper is cut into strips, and is called lest paper ; 

 when dipped into acid it immediately turns red. This is a most sensitive reaction, and 

 may be employed for testing for the least traces of acids. If 10000 parts by weight of water 

 be mixed with 1 part of sulphuric acid, the coloration is distinctly perceptible, and it is 

 quite distinguishable on the addition of ten times more water. Certain precautions 

 must, however, be taken in the preparation of such very sensitive litmus paper. Litmus 

 is sold in lumps. Take, say, 100 grams of it ; pound it, and add it to cold pure water in 

 a flask. Shake and decant the water. Kepeat this three times. This is done to wash 

 away easily-soluble impurities, especially alkalis. Transfer the washed litmus to a 

 flask, and pour in (!00 grams of water, heat, and allow the hot infusion to remain for 

 some hours in a warm place. Then filter, and divide the filtrate into two parts. Add a 

 few drops of nitric acid to one portion, so that a faint red tinge is obtained, and then 

 mix the two portions. Add spirit to the mixture, and keep it thus in a stoppered bottle 

 (it soon spoils if left open to the air). This infusion may be employed directly ; it reddens 

 in the presence of acids, and turns blue in the presence of alkalis. If evaporated, a 

 solid muss is obtained which is soluble in water, and may be kept unchanged for any 

 length of time. The test paper may be prepared as follows : Take a strong infusion of 

 litmus, and soak blotting-paper with it ; dry it, and cut it into strips, and use it as test- 

 paper for acids. For the detection of alkalis, the paper must be soaked in a solution 

 of litmus just reddened by a few drops of acid ; if too much acid be taken, the paper will 

 not be sensitive. Such acids as sulphuric acid colour litmus, and especially its infusion, 

 a brick-red colour, whilst more feeble acids, such as carbonic, give a faint red-wine tinge. 

 Test-paper of a yellow colour is also employed ; it is dyed by an infusion of turmeric roots 

 in spirit. In alkalis it turns brown, but regains its original hue in acids. Many blue 

 and other vegetable colouring matters may be used for the detection of acids and alkalis ; 

 for example, infusions of cochineal, violets, log-wood, &c. Certain artificially-prepared 

 substances and dyes may also be employed. Thus rosolic acid, C 2 oH 1(j O 3 , and 

 phenolphthale'm, C..> H 14 O 4 , are colourless in an acid, and red in an alkaline, solution. 

 Cyanine is also colourless in the presence of acids, and gives a blue coloration with 

 alkalis. These are very sensitive tests. Their behaviour in respect to various acids, 

 alkalis and salts sometimes gives the means of distinguishing substances from each, 

 other. 



