Originally published in 1921. PREFACE TO THIRD EDITION FOR many years the sole use found for milk curd was in the manufacture of cheese to be used as a food- stuff. Skimmed or separated milk, produced in large quantities as a by-product in butter-making, was fed to pigs or even run into a stream. It is only through the scientific researches of the past few decades that the true value of albuminoid substances has been realised and that the manufacture of casein from skim milk has become a practicable proposition. In- cidentally to this, the preparation of milk sugar has become a possibility, and the easy digestibility of this product has resulted in its being foods and similar preparations. used in infants It is easy to understand that our knowledge of casein is somewhat imperfect, especially as regards its manifold uses, but as time goes on it will become more and more appreciated in industrial applications since it has valuable properties which are not easily found in any other produc