Milk being the most perfect emulsion obtainable, such a mixture of fat which simulates this compound most closely must likewise be regarded as superior in the degree that these qualities are intensified. To be sure, an artificial emulsion always represents a greater percentage of fat than milk, and its preservation is therefore relatively easier than in that obtained from nature, but this fact merely modifies the result, and does not involve the principle. The greater proportion of water in milk also favours decomposition, but, on the other hand, the minute, perhaps even molecular division of the fat globules, renders it possible to withstand decomposition longer than an equally dilute artificial emulsion, wherein the oil globules are not so thoroughly disseminated. We, of course, recognise the fact that milk contains different animal bodies not present in ordinary artificial emulsions, which are prone to decomposition, so that the similarity drawn between the two is based more upon physical characteristics than their presenting any features in common chemically. But it is this attempt at compromising its principal physical feature - fluidity-with permanency, which makes the preparation of an emulsion so difficult.
To so change a fat as to render it miscible with water is a matter of easy execution, but when we attempt to embody the desirable feature of fluidity, then we are thwarted by physical laws, and resort to chemical means as a compromise.
Condensed milk is a striking illustration wherein by a change of its physical condition, complete preservation has been attained much more satisfactorily than milk in its natural form could be preserved, even with chemical means. It is for this reason that consistence is the most desirable feature to ensure the permanence and preservation of any emulsion, natural or artificial.
It is well known that a perfect and permanent emulsion can be made with cod liver oil and malt extract, owing to the consistence of the preparation solely, as we have attempted to use the same agents represented in malt extract, namely - dextrine and glucose, and discovered that as soon as the consistence was abandoned these agents did not possess any advantage over those usually employed for emulsifying fats. To the albumen in milk has been ascribed the high degree of and most permanent emulsification, and therefore gelatine is employed in artificial emulsions, with not much better success however than other agents, when semi-fluid consistence is abandoned.
We will now consider what should be used as emulsifying agents and also such as, while largely used, are not desirable for obvious reasons.
Unfortunately the well-worn maxim, so justly applied to most classes of pharmaceutical preparations, "The sacrifice of medicinal value for elegance," has not been lost sight of in the preparation of emulsions. Periodically, different substances from all the different kingdoms of nature have been proposed, enjoyed a short, fashionable stay, and then been relegated to their well-merited oblivion.
The vegetable gums, acacia and tra-gacanth, have been the longest in use, and the first mentioned of these has probably answered the purpose of a reliable, convenient, and at least innocuous emulsifying agent better than the majority of latter-day substitutes.
The late Prof. Wm. Proctor announced the proportion to be used of gum acacia to produce a perfect temporary emulsion. His directions were as follows: " Mix intimately in a perfectly dry mortar the oil with one-half its weight of powdered acacia; to this add at once one-half as much water as the combined weight of oil and gum, and triturate briskly until the mixture has assumed the colour and consistence of a thick cream, which produces a crackling noise when the pestle is moved rapidly around the sides of the mortar." This is the emulsion proper, and to this can be added any amount more of water or other desirable vehicle or medicament to bring the finished preparation up to the quantity prescribed.
If perfectly made, this emultion will stand any degree of dilution with watery mixtures; in fact, its quality is proved when, by a large addition of water, the oil globules will not separate or aggregate at the top of the liquid.
Practice has demonstrated that the proportion of gum can be varied according to the nature of the oil employed, but the constant relation between the water used for the emulsion proper, and the mixture of oil and gum must be scrupulously adhered to as ensuring infallible results.
Fixed oils, rich in gum, per se, as copaiba, castor oil, etc, do not require as large an amount of gum as cod liver oil, while in the case of ethereal oils, for instance, oil of turpentine, an equal amount of gum, or weight for weight, is necessary. To prepare an emulsion from turpentine not unfrequently presents difficulties, and so much the more is this to be guarded against, as it is a powerful remedy, and, if presented in a merely mechanical mixture, will prove irritating, and perhaps engender serious consequences.
But then, if by careful observance of this method we can obtain a perfect emulsion, what more is desired? Although this emulsion is perfect it is not permanent, and to circumvent this negative feature is the problem for solution.
While we have not discovered any means or process whereby this problem can be solved, yet we have found agents capable of preventing this separation in a great degree, being guided in their selection by a knowledge of the constituents which are most favourable to this separation, and those that are not.
An emulsion should be palatable, and for this reason it is always sought to make it sweet by the introduction of cane sugar or glycerine. These two agents are the cause of the most dissatisfaction with emulsions. Sugar, owing to its affinity for water, and density, favours separation very rapidly, precipitating while the emulsified oil forms a compact, creamy and gradually diminishing stratum at the top of the vessel. Glycerine, probably from the same causes and its incompatibility with fixed oils, behaves in a similar manner, and for these reasons these otherwise desirable vehicles cannot be represented in an emulsion when permanence is to be obtained.