220 TESTS FOR SUGAR. 



[Effect of Tea. Tea has an intensely inhibitory effect on salivary digestion, which is due to 

 the large quantity of tannin contained in the tea-leaf. Coffee and cocoa have only a slight 

 effect on salivary digestion. The only way to mitigate the inhibitory effect of tea on salivary 

 digestion is "not to sip the beverage with the meal, but to eat first and drink afterwards" 

 (Boberts).] 



II. Saliva dissolves those substances which are soluble in water ; while the alka- 

 line reaction enables it to dissolve some substances which are not soluble in water 

 alone, but require the presence of an alkali. 



III. Saliva moistens dry food and aids the formation of the " bolus," while by 

 its mucin it helps the act of swallowing, the mucin being given off unchanged in the 

 faeces. The ultimate fate of ptyalin is unknown. 



[IV. Saliva also aids articulation, while according to Liebig it carries down into 

 the stomach small quantities of O.] 



[V. It is necessary to the sense of taste to dissolve sapid substances, and bring 

 them into relation with the end-organs of the nerves of taste.] 



Saliva has no action on proteids or on fats. 



The presence of a peptone-forming ferment has recently been detected in saliva (Hiifner, 

 Munk, Kiihne). [Perfectly healthy human saliva has no poisonous properties. J 



149. TESTS FOE SUGAR. 1. Trommer's test depends upon the fact that, in 

 alkaline solutions, sugar acts as a reducing agent ; in this case a metallic oxide is 

 changed into a suboxide. To the fluid to be investigated, add J of its volume of 

 a solution of caustic potash (soda), specific gravity 1*25, and a few drops of a weak 

 solution of cupric sulphate, which causes at first a bluish precipitate, consisting of 

 hydrated cupric oxide, but it is redissolved, giving a clear blue fluid, if sugar be 

 present. Heat the upper stratum of the fluid, and a yellow or red ring of cuprous 

 oxide is obtained, which indicates the presence of sugar ; 2CuO - O = Cu 2 0. 



The solution of hydrated cupric oxide is caused by other organic substances ; but the final 

 stage, or the production of cuprous oxide, is obtained only with certain sugars grape-, fruit-, 

 and milk- (but not cane-) sugar. Fluids which are turbid must be previously filtered, and if 

 they are highly coloured, they must be treated with basic lead acetate ; the lead acetate is after- 

 wards removed by the addition of sodium phosphate and subsequent filtration. If very small 

 quantities of sugar are present along with compounds of ammonia, a yellow colour instead of a 

 yellow precipitate may be obtained. In doing the test, care must be taken not to add too much 

 cupric sulphate. 



[2. Fehling's Solution is an alkaline solution of potassio-tartarate of copper. 

 Boil a small quantity of the deep-blue-coloured Fehling's solution in a test-tube, 

 and add to the boiling test a few drops of the fluid supposed to contain the sugar. 

 If sugar be present, the copper solution is reduced, giving a yellow or reddish pre- 

 cipitate. The reason for boiling the test itself is, that the solution is apt to 

 decompose when kept for some time, when it is precipitated by heat alone. This 

 is one of the best and most reliable tests for the presence of sugar. In Pavy's 

 modification of this test, ammonia is used instead of a caustic alkali ( 267).] 



(3) Bbttger's Test. Alkaline bismuth oxide solution is best prepared, according to Nylander, 

 as follows : 2 grms. bismuth subnitrate, 4 grms. potassic and sodic tartarate, 100 grms. 

 caustic soda of 8 per cent. Add 1 c.c. to every 10 c.c. of the fluid to be investigated, when 

 boiled for several minutes, the sugar causes the reduction and deposits a black precipitate of 

 metallic bismuth. [According to Salkowski the urine of a person taking rhubarb gives the 

 same reaction with this test.] 



(4) Moore and Heller's Test. Caustic potash or soda is added until tho mixture is strongly 

 alkaline ; it is afterwards boiled. If sugar be present, a yellow, brown, or brownish-black colora- 

 tion is obtained. If nitric acid be added, the odour of burned sugar (caramel) and formic acid 

 is obtained. 



(5) Mulder and Neubauer's Test. A solution of indigo-carmine, rendered alkaline with sodic 

 carbonate, is added to the sugar solution until a slight bluish colour is obtained. When the 

 mixture is heated, the colour passes into purple, red, and yellow. When shaken with atmo- 

 spheric air, the fluid again becomes blue. 



Molisch's Test. To 5 c.cm. of the fluid add 2 drops of a 17 per cent, alcoholic solution 

 of a-naphthol, or a solution of thymol. Add 1 to 2 c.cm. of concentrated sulphuric acid, 

 and shake the mixture. The presence of sugar colours the a - naphthol mixture deep violet, 



