558 THE FATTY SUBSTANCES IN BUTTER- 



ceeding 180° F., and be then washed with repeated portions of warm 

 water, a nearly colourless fluid oil is obtained, which, if not transpar- 

 ent, becomes so when filtered through paper, and when cool congeals into 

 a more or less pure white solid fat. If this fat be put into a linen cloth 

 and be submitted to a strong pressure in a hydraulic or other press at the 

 temperature of 60° F., a slightly yellow, transparent oil will flow out, 

 and a solid white fat will remain behind in the linen cloth. The solid 

 fat is known to chemists by the name of margarine. The liquid oil is 

 peculiar to butter, at least it has not hitherto been found in any other sub- 

 stance ; it is therefore called the oleinc of butter, or simply butter-oil. 



The pure fat of butter consists almost entirely of these two substances, 

 there being generally^present in it only a small quantity of certain fatiy 

 acids, which I shall presently introduce to your notice. Thus a speci- 

 men of butter made in the month of May gave a fat which was found 

 by Bromeis to consist of about — 



Margarine * 68 per cent. 



Butter oil 30 " 



Butyric, caproic, and capric acids .... 2 '* 



100* 

 But the proportion of the solid and fluid fats in butter varies very much. 

 ft is familiar in every dairy that the butter is harder and firmer at 

 one time and with one mode of churning than with another, — and this 

 greater firmness depends mainly upon the presence of the solid fat {mar- 

 garine) in larger proportion. According to Braconnot, summer butter 

 contains much more of the butter-oil than winter butter does ; and he 

 states their relative proportions in these two seasons, in the butter of the 

 Vosges, which he examined, to be as follows : — 



Summer. Winter. 



Margarine 40 65 



Butter oil 60 35 



100 100 



Of course these proportions ire not to be considered as constant. In- 

 deed, the proportion of oil here given for summer butter is much greater 

 than in the butter examined by Bromeis. It is probable, therefore, that 

 the relative proportions of the two fats are affected by climate, by sea- 

 son, by the race, the food, and the constitution of the animal; by the way 

 in which the butter is made, by the manner in which it is kept, and by 

 other circumstances not hitherto investigated. 



2°. Margarine. — This solid fat, which exists so largely in butter, is 

 also the solid ingredient in olive oil, and in goose and human fat. But- 

 ter, therefore, appears to be a most natural food for the human race, since 

 it contains so large a proportion of one of those substances which enter 

 directly into the constitution of the human frame. 



Margarine is white, hard, and brittle, and melts at 118° F. In the 

 pure state it may be kept for a length of time without undergoing any 

 sensible change, but in the state of mixture in which it exists in milk and 

 butter it is apt to absorb oxygen from the atmosphere, and to be partially 



* Annal. der Chem. und Phar., xlii., p. 70. 



