CHEMISTRY OP SALIVA. 49 



stantly bear in mind that the real reaction of a fluid is the 

 ratio between free H- and OH-ions (see p. 30). By analysis 

 saliva has been found to contain phosphates and carbonates, both 

 of which are capable of existing either as acid or alkaline salts, 

 that is to say, as NaH 2 P0 4 and NaHC0 3 (acid salts) or Na 2 HP0 4 

 and Na 2 C0 3 (alkaline salts). Since the reaction given by solu- 

 tions which contain such mixtures of acid and alkaline salts 

 depends, first, on the relative proportions of these salts and, 

 secondly, on the exact indicator employed to test the reaction 

 (see p. 31), it is plain that the reaction of the saliva as ordi- 

 narily tested must be very haphazard. 



To determine the H-ion concentration of saliva, some of this 

 fluid is diluted about ten times with distilled water, which has 

 been boiled and cooled so as to free it of carbon dioxide, and 

 0.5 c. c. of paranitrophenol solution is added. The resulting tint 

 is then compared with that obtained by adding 0.5 c. c. of the 

 same indicator to each of a series of test tubes containing vary- 

 ing proportions of acid and alkaline phosphate solutions (y i5 

 normal). The tint of this series which matches with that of the 

 saliva indicates the H-ion concentration of the latter because 

 this is known in the standard from the proportion of the two 

 phosphates present. 



THE METHOD OP MEASURING THE NEUTRALIZING POWER OP SA- 

 LIVA. Interesting though H-ion results may be, they do not ap- 

 pear to be of any practical value in connection with the relation- 

 ship between the saliva and caries of the teeth. To study this 

 question it has been found to be of more value to determine the 

 neutralizing power of saliva ; that is, to find out how much stand- 

 ard acid or alkali we must add to a measured quantity of saliva 

 in order to get a change with one or more of the above indicators. 

 In doing this, however, we are immediately struck with the fact 

 that the reaction does not change in proportion to the amount 

 of acid or alkali added, but that the saliva under such conditions 

 possesses the property of changing very slowly in reaction. This 

 same property also exists in the blood, and it depends on a series 

 of changes which the phosphates and carbonates can undergo, 

 when acids or alkalies are added to solutions containing them, 



