[14] The reader who desires further information on this and kindred subjects will find it clearly and soundly treated (without any of the noxious pedantry that too commonly prevails in such treatises) in Dr. Andrew Combe’s _Physiology of Digestion_, which, although written by a dying man nearly half a century ago, still remains, like his _Principles of Physiology_, the best popular work on the subject. Subsequent editions have been edited and brought up to date by his nephew, Sir James Coxe. CHAPTER X. FAT—MILK. WE all know that there is a considerable difference between raw fat and cooked fat; but what is the _rationale_ of this difference? Is it anything beyond the obvious fusion or semi-fusion of the solid? These are very natural and simple questions, but in no work on chemistry or technology can I find any answer to them, or even any attempt at an answer. I will therefore do the best I can towards solving the problem in my own way. All the cookable and eatable fats fall into the class of ‘fixed oils,’ so named by chemists to distinguish them from the ‘volatile oils,’ otherwise described as ‘essential oils.’ The distinction between these two classes is simple enough. The volatile oils (mostly of vegetable origin) may be distilled or simply evaporated away like water or alcohol, and leave no residue. The fixed oils similarly treated are dissociated more or less completely. This has been already explained in Chapter VII. Otherwise expressed, the boiling point of the volatile oils is below their dissociation point. The fixed oils are those which are dissociated at a temperature below their boiling point. My object in thus expressing this difference will be understood upon a little reflection. The volatile oils, when heated, being distilled without change are uncookable; while the fixed oils if similarly heated suffer various degrees of change as their temperature is raised, and may be completely decomposed by steady application of heat in a closed vessel without the aid of any other chemical agent than the heat itself. This ‘destructive distillation’ converts them into solid carbon and hydro-carbon gases, somewhat similar to those we obtain by the destructive distillation of coal. If we watch the changes occurring as the heat advances to this complete dissociation point we may observe a minor or partial dissociation proceeding gradually onward, resembling that which I have already described as occurring when sugar is similarly treated (Chapter VII. page 87). But in ordinary cooking we do not go so far as to carbonise the fat itself, though we do brown or partially carbonise the membrane which envelopes the fat. What then is the nature of this minor dissociation, if such occurs?