. LIMITS AXD STANDARDS. 167 



herd, except perhaps in May and June ; such milk very rarely, 

 if ever, falls appreciably below this limit. It is far more frequent, 

 especially during July, August, and September, for milk to 

 contain less than 8"5 per cent, of solids not fat ; in the majority 

 of these cases, the author has found that at least 0'50 per cent. 

 of total nitrogen and 070 per cent, of ash was present, and 

 this experience has received much confirmation. Smetham and 

 some American observers have, however, found that even these 

 limits are somewhat too high for the milk of Dutch or Holstein- 

 Frisian cows, and the author has also found some samples which 

 do not conform to this rule. At the present time this breed 

 of cows does not form a majority of English milch-cattle ; on 

 farms where they are kept other breeds yielding milk of higher 

 quality are also milked. 



Multiple Standard. — For all practical purposes the multiple 

 standard of 8 '5 per cent, of solids not fat, 4' 5 per cent, of 

 milk-sugar, 0'50 per cent, of total nitrogen, and 070 per 

 cent, of ash may be adopted for the purpose of judging 

 whether a milk is of genuine composition or not. The 

 figure for the ash is, however, liable to be increased by the 

 addition of mineral substances added to the milk ; thus boric 

 acid and borax, used as preservatives, and salt, added to mask 

 the addition of water, would raise the ash ; estimation of the 

 boric acid, which is absent in genuine milk, or of the chlorine, 

 which does not often exceed O'lO per cent., will show additions 

 of this nature. The amount of ash insoluble in hot water is also 

 a useful figure ; it amounts in milk to at least 050 per cent., 

 and is very nearly equal to the total nitrogen. 



A milk should never be pronounced as watered on the evidence 

 of the solids not fat alone, unless this is well below 8"0 per cent. ; 

 a determination of the milk-sugar, total nitrogen, and ash should 

 be made in addition ; a judgment formed on the three determina- 

 tions will be in all probability correct, and if the figures for at 

 least two of them are above the limit, the milk is probably 

 1,'enuine. 



Variations of Fat in Milk on Standing. — The fat 

 globules of milk have a natural tendency to rise to the surface 

 and to thus cause an unequal distribution of fat in different 

 portions of the milk. 



Table XXXVII. will give an idea of the rate at which appre- 

 ciable change in the composition of the milk occurs ; 12 gallons 

 of well-mixed milk were placed in a churn with a tap at the 

 bottom at ir25 a.m., and a measure holding 7 quarts was drawn 

 out every half-hour till 2'"2.5 p.m. ; each of these quantities was 

 iinalysfd. as also the residue left in the churn (5 quarts) ; the 

 milk was undisturbed throughout. 



