FORMULA. 73 



(1) If equal volumes of difierent milks be mixed, the specific 

 gravity of the mixture will be the mean of the specific gravities 

 of the milks. 



(2) If equal weights of different milks be mixed, the specific 

 volume of the mixture will be the mean of the specific volumes 

 of the milks. 



Instead of taking the solids not fat as one substance, we niav 

 consider its constituents — lactose, protein, and mineral matter 

 — separately. 



Calling the percentage of fat F, lactose L, protein P, and 

 mineral matter A, we may write 



'- = lOfc F + lOc L + lOd P + lOt A 



by the same reasoning employed in deducing formula (5). 



The author has deduced from the mean of many iinalysos the 

 formula 



^ = - 0-761 F + 4 L + 2o714 V -[ .s--li; A. 



From these factors the following specific gravities are deduced, 

 as previously shown : — 



Specific gravity of fat, . il-'Xi 



lactii.sr, 1-lllii; 



protein, l-34li 



., . mineral matter, 5-5 



Usiii;; these specillc gravities, togethei- with \ ieth's estimate 

 of the proportions of lactose, protein, and mineral matter — 

 13 : 9 : '2 — we can calculate what the specific gravity of solids 

 not fat should be. As Vieth's proportions are by weight, we 

 must transform specific gravities into specific \olumes. 



SiK'cilie volume of lactose = , . . . = Ut). 



1-bbt) 



„ protein = ^_^^^ = „-li3. 



mineral matter = --_ = 0-lS:?, 



Then specific gravity of 



S 11 F = ^ = --- = l-61ii. 



13 X (];0 + 9 X 0-743 4^2_x 0-18i: 0-61SS 



'■2-1 



which agrees with that given above, 1-616. 



It is a useful check on an analysis to calculate the specific 

 gravity from the percentage of milk-sugar, protein, fat, and 



