12 VOELCKER on the Chemistry of Food. 



The ash which remains behind on burning any of these oil- 

 cakes contains a large proportion of phosphoric acid, lime, and 

 potash. Oil-cakes thus contain much of the constituents of which 

 the bony skeleton of the animal body is made up. 



In explanation of the preceding table, I would observe 

 that the differences in the proportion of oil and flesh forming 

 matters obtained in the analyses of different samples of cakes of 

 the same kind are quite as large, or sometimes even larger, than 

 the differences which are here stated in the average composition of 

 cakes of different kinds. Thus, for example, it is stated in the above 

 tabulated results that linseed-cake contains t on an average 12 '7 9 

 per cent, of oil, and rape-cake 11*10 per cent.; but it does not 

 follow from this that linseed-cake always contains more oil than 

 rape-cake. Generally it does contain somewhat more oil ; but, 

 on the other hand, there are some rape-cakes which contain more 

 oil than most linseed-cakes. 



This remark, indeed, applies to all feeding materials. We are 

 too much in the habit of speaking of the composition of cake, 

 turnips, mangolds, or hay, as if these complex mixtures of sub- 

 stances were simple chemical combinations, presenting us with a 

 fixed composition, whereas the differences in two samples of the 

 same produce are often very great. Any one who has tried 

 practically the nutritive effect of good and badly-made hay, or of 

 turnip grown on good turnip land and on peaty land, knows well 

 that there is a vast difference between hay and hay, or a turnip 

 and a turnip. In speaking of the nutritive value of any article 

 of food too precise a language is out of place ; and it is simply ab- 

 surd to draw nice general conclusions from small differences which 

 the analyses of different feeding-materials may have yielded. Unless 

 the differences are strongly-marked and constantly observed in a 

 great number of cases, it is unsafe and irrational to attach a pre- 

 cise nutritive value to different articles of food, especially if the 

 opinion is founded solely upon analytical data, and not corrobo- 

 rated by actual experimental trials : for, after all, the chemical 

 composition alone of an article of food is insufficient to determine 

 its practical value. 



The history of oil- cakes presents us with a striking example 

 illustrating the truth of these remarks. 



Good rape-cake contains nearly as much oil, and even more 

 flesh-forming principles, than the best linseed-cake. In a purely 

 chemical point of view, rape-cake ought to be, if not superior, at 

 least quite as nutritious as linseed-cake. Notwithstanding a 

 certain diversity of opinion entertained by practical men respect- 

 ing the merits of both cakes, I believe an extended experience of 

 the best stock- farmers has proved beyond dispute that, weight for 



