22 SALIVA. 



If the saliva has been filtered, no interest attaches to any 

 investigation of the residue left on the filter, at least in so far as 

 the nature of the saliva is concerned, seeing that true saliva con- 

 tains only soluble substances. 



Wright finds his ptyalin in this residue ; he cannot, however, 

 possibly have treated this residue with sufficient water, since, in 

 that case, it could not have contained so large a quantity of a 

 matter soluble in water as his numbers indicate. 



If carbonate of lime be mixed with this residue insoluble in 

 water, it may easily be extracted by very dilute acetic acid, and its 

 quantity subsequently determined. 



In reference to the filtered solution, it is generally of interest to 

 determine volumetrically the amount of acid which is saturated by 

 a certain quantity of saliva, in order to form an opinion regarding 

 the alkalinity of the saliva, or, in other words, regarding the 

 quantity of the weakly-combined alkali. In every case, however, 

 the filtered saliva must be neutralized with acetic acid, and then 

 heated ; if this gives rise to a turbidity, the albuminous substance 

 which is precipitated must be collected on a filter, and determined 

 quantitatively. The residue left on the evaporation of the filtered 

 saliva is then to be treated in the same manner as we treat the 

 residue in the case of most of the other animal fluids. 



It only remains for us to make a few remarks on the quantita- 

 tive determination of the sulphocyanogen. The following are at 

 present regarded as the two best methods of effecting this object. 

 One method consists in dissolving the alcoholic extract of the 

 saliva in water, and filtering the fluid, which is generally turbid 

 from the presence of fat ; the filtrate, after being somewhat con- 

 centrated by evaporation, is heated with phosphoric acid, and 

 distilled ; the distillate is saturated with baryta, and the filtered 

 fluid evaporated ; the residue is then boiled for a long time with 

 fuming nitric acid or aqua regia, and the quantity of sulphocyanide 

 of potassium is calculated from the amount of sulphate of baryta 

 which is separated. (Van Setten, Jacubowitsch, Tilanus.) In 

 following the other method, we first precipitate the aqueous 

 solution of the alcoholic extract with nitrate of silver, and treat 

 the well-washed deposit with water containing nitric acid, which 

 leaves the chloride of silver undissolved ; we then precipitate the 

 silver from the acid solution with hydrochloric acid, add a little 

 chloride of barium, and evaporate slowly, adding from time to time 

 some nitric acid : in this way also we obtain sulphate of baryta, 

 from which the sulphocyanogen must be calculated. Before the 

 addition of the baryta, we should also be able to precipitate the 



