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XLI. 



THE COMPARATIVE VARIATION OF THE CONSTITUENT 

 SUBSTANCES OF COWS' MILK. 



By E. J. SHEEHY, F.R.C.Sc.I. ; 

 Demonstrator in Zoology, Royal College of Science, Dublin. 



(COMMUNICATED BY PROFESSOR J. .WILSON, M.A., B.SC.) 

 [Read November 19, 191S ; published May 26, 1919.] 



The milk yield of cows varies in both quantity and quality. The variation 

 in quality, however, as judged by any single test, is not similar to that in 

 quantity ; that is to say, the relative proportions of total milk, sugar, fat, 

 protein, and ash are not absolutely constant. They differ in the different 

 breeds of cattle, in the individuals within a breed, and also in the successive 

 days' yields from an individual cow. It is generally recognized that the 

 percentage of solids other than fat in milk is more or less constant, 

 varying only with the period of lactation, and Richmond has shown that 

 a definite relationship exists between the percentage of total solids and 

 a combination of fat percentage and specific gravity. To what relative degree 

 do these different constituents vary, and how much is contributed by each 

 towards the variation shown in the total milk yield ? The answering of this 

 question constituted one of the objects of an experiment conducted at the 

 Albert Agricultural College in 1916. 



The milk from twelve cows was weighed and sampled twice daily for a 

 period extending from March 28 to June 17 (inclusive). From the sample, 

 which consisted of about half a pint, the percentage of fat was determined by 

 the Gerber method, and the percentage of total solids by Richmond's formula 

 (T = 1*2F + '25D + •14), the specific gravity being obtained by means of a 

 lactometer. The cows were milked at 5 a.m. and 2 p.m., and the following 

 table gives relevant information concerning the individuals; — 



