194 THE ANALYSIS OF MILK PRODUCTS. 



N 

 30 c.c. alkali, taken for fractional distillation ; to this is added 



a quantity of N sulphuric acid, very slightly greater than is 

 necessary to neutralise the soda used, and the whole made up 

 to 100 c.c. Nine successive fractions, each about one-tenth of 

 the total volume, are distilled, best in the steam-jacketed 

 apparatus described under Butter Analysis, and titrated separat ly 



N 

 with strontia ; 25 c.c. of water is added to the residue, and a 



further 25 c.c. distilled and titrated, and this treatment may be 

 repeated till no more volatile acid comes over. From the last 

 titration the total quantity of volatile acid is obtained, this is 

 always slightly less than the total acidity of the first distillate, 

 due, no doubt, to the presence of a little lactic acid. As much as 

 possible of the first distillate is made up to a convenient bulk, 

 after adding a quantity of N sulphuric acid in slight excess of 

 that required to neutralise the soda, and exactly one-third is 

 distilled ; this is made up to a convenient bulk, and exactly 

 one-third again is distilled. The last distillate is made up to 

 100 c.c., and nine fractions of 10 c.c. each are distilled and 

 titrated separately. The residue in the flask and condenser is 

 washed out and titrated to obtain the total quantity. 



The author has shown that the rate of distillation of the lower 

 fatty acidr varies slightly with the concentration, and has calcu- 

 lated tables to allow for these ; they have been deduced from the 

 mean of a number of distillations, and apply to strengths of acid 



N 

 such that 100 c.c. of solution take about 10 to 50 c.c. of - acid. 



The column marked x is the percentage of solution or c.c. 

 per 100 c.c. distilled ; the columns marked y give the molecular 



percentage of acid passing over, and that marked ^ the factor 



to be used to multiply a small excess or deficiency in the volume 

 distilled to correct for corresponding percentage of acid. Thus. 



if 30-15 c.c. out of 100 c.c. were distilled and -^ = 0-77, 0-12 



Ax 



(= 0*15 X 0-77) must be added to the percentage of acid distilled 

 given in the table (XXXIV.) for 30 c.c. to find the theoretical 

 amount distilled for 30-15 c.c. 



For each fraction calculate from the table the proportion of 

 each of the acids which would distil, and calculate the ratios 

 of butyric to acetic, and butyric to propionic acid, which corre- 

 spond to the proportion actually distilled. The ratios from 

 first and last fractions are liable to be slightly erroneous, the 

 first because a small amount of a very volatile acid, as carbonic, 



