Milk. 
317 
studied how far the specific gravity of milk can be relied upon 
as an indication of its quality. 
A good many recent experiments have led me to the conclu- 
sion that within certain limits the specific gravity is a trustworth}' 
indicator of quality. It is true that the cream globules are lighter 
than milk, and thus milk containing much cream has a lower 
specific gravity than skim-milk ; but surely no instrument is 
required to tell us whether milk is extra rich, or, like skim-milk, 
poor in cream. The lactometer was never intended to indicate 
the relative richness of extra good samples of milk, but it was 
designed to be a simple instrument which should unmistakably 
point out whether samples of a fair or doubtful appearance had 
been watered, or were of a naturally defective composition ; and 
this purpose it satisfactorily fulfils. 
Some of the objections to the use of hydrometers for testing 
milk, are based on the mistaken opinion tUat cream is lighter 
than water. This is not the case ; it is lighter than milk, but 
denser than water, in the proportion of 1012, or even 1019 to 
1000. The addition of cream, therefore, cannot depress the 
specific gravity of the milk in the same degree as the addition of 
water. A low specific gravity, therefore, always indicates a large 
quantity of water ; at all events I find milk rich in butter, of a 
gravity that is a good deal higher than milk adulterated with 
even little water. 
I will conclude with a few experiments, showing the quantity 
of cream which is thrown on the surface by pure milk of known 
composition, and milk purposely adulterated with fixed quantities 
of water, and also the specific gravity of milk adulterated with 
variable portions of water. 
The milk used on the 4th of March had the following compo- 
sition : — 
Water 86-80 
Pure fat (butter) 3-71 
*Casein and albumen 3'37 
Milk-sugar 5-31 
Mineral matters '81 
100-00 
*Contalning nitrogen -54 
Percentage of solid matter 13' 20 
This milk had a specific gravity of 1-0320 at 62^ Fahr. After 
standing for 15 hours it threw up 11-5 per cent, of cream by 
volume, having a specific gravity of 1'0183 at 62' Fahr. 
Portions of milk were mixed with 10, 20, 30, 40, and 50 per 
cent, of water respectively, and the specific gravity of each sample 
thus diluted with water compared with that of pure milk, when 
the following results were obtained : — 
