The impurities likely to be present in water make such a formidable list that we can only mention a small number. There may be various mineral substances, such as lime, sand or clay; vegetable substances, starch grains, fragments of wood, pieces of decayed leaves and the like or there may be hair, scales of fish, etc. The impurities may be living plants, of which the most likely to be found are bacteria, diatoms, desmids and Volvox, amongst plants, and rotifers, Vorticella, Paramœcium, Amœba, also portions of tapeworms and their eggs amongst animals. These creatures are all described elsewhere so we need not dwell on their peculiarities here.
In addition to all of the above and many more not mentioned there are four metals commonly found in impure water, either in small solid particles, or in the form of one of their compounds soluble in water. The metals are iron, copper, zinc and lead. Very simple chemical tests will show whether they are present or not. To a drop of the water add a very minute quantity of hydrochloric acid and of potassium ferrocyanide solution. When iron is present the solution will turn blue; in the presence of copper it will become bronze coloured, whilst zinc turns it a milky white. To detect lead, take another drop of water and add a very small quantity of potassium chromate solution. If the suspected impurity is present the solution turns yellow. All the chemicals for these simple tests may be obtained at any chemists and there is this great advantage in testing under the microscope—only very small quantities are required.
Butter can hardly be described as an interesting object for the microscope, nevertheless, it may be of use to explain the methods of its examination. A small sample should be placed upon a clean slide, a drop of olive oil added and the whole covered with a cover slip which may be pressed firmly till the specimen forms but a thin layer. Of course the most important impurities likely to be present are bacteria but these we cannot see without special preparation and we are not dealing with bacteria in the present chapter.
If our specimen is in a film sufficiently thin to be transparent, and we should have made it so by pressure on the cover slip, we may first of all examine it carefully for starch grains which, by the way, should be absent. The amount and size of the drops of water, which every butter sample contains, are of importance in deciding its quality. In good butter there are a few scattered drops of various sizes; in milk-blended butter the water globules are all very small and all of the same size, or as nearly so as the eye can judge; in butter containing an excessive amount of water the drops appear large, much larger than in a normal sample.
If we examine various samples, we shall find that some are much more transparent than others, the transparent samples being fresh butter. The curd also in fresh samples is much more finely and evenly scattered in the field of view than in older samples. Renovated butter, that is to say rancid butter which has been melted and made palatable by forcing steam through it, should be examined by oblique light—easily arranged by tilting the mirror at an angle—when the curd appears as white patches on a dark background.
It is curious that one article of food, honey, is more likely to be pure when it contains impurities. This sounds like a bull but a great deal of honey is manufactured from various sugars but not by bees. This artificial honey contains no pollen grains, in fact any honey found to be free of pollen should be looked upon with suspicion. Starch often occurs in artificial honey, never in real bee-made honey.
To many foods adulterants are added as preservatives, the nature and quantity of such additions is settled by Act of Parliament. Many foods are preserved with small quantities of Borax or Boric Acid. The use of Formaldehyde, formerly sold under the German trade name of Formalin, is not unknown but it is very injurious. Salicylic Acid which was formerly much used is being supplanted by Benzoic Acid, for the reason that the latter is not so easily detected and therefore prosecution for excessive quantities is not so likely to follow. These preservatives are not easily detected by the microscopist unless he be a chemist also.
As we have already remarked adulterants are added for the sake of colour, either because the public demand certain colours, or to hide fraud. Milk for instance, when watered, assumes a characteristic blue colour; to hide the blue shade various dyes, anetto, turmeric or carrot juice are added or one of the aniline dyes, products of coal tar. This form of deception became so common that now the public demand yellow milk and butter. Jams made from inferior or over-ripe fruit are frequently coloured with coal tar dyes, so also are cheap sweets. Oxide of iron is added to potted meats, sauces, etc., to improve their appearance. Bottled peas, which if untreated would be of a yellowish-green colour, are made to appear bright green by the addition of the poisonous blue vitriol; fortunately this chemical unites with the chlorophyll of the peas to form a compound which is insoluble in the human body and so no great harm is done. We may compare the chlorophyll in a healthy, undoctored green pea with that in a pea which has been treated with blue vitriol; under the microscope we shall notice the striking difference between the two.
When we admire the beautiful crystals which go to the making of a piece of lump sugar we little dream that, if those crystals were pure they would be yellow. The housewife, however, demands her lump sugar white so ultramarine is added to mask the yellow colour and give the sugar its white appearance.
Most of these impurities are difficult or impossible of detection under the microscope; they are added to give the food a more pleasing appearance in the first place, for there are undoubtedly certain people who prefer to consume food which appeals to the eye, though of doubtful purity, rather than unadulterated though perfectly pure fare. When added solely for the sake of appearance it matters little, but the habit of making these additions is frequently cultivated to hide bad material and imperfections in manufacture.