Photos by Flatters & Garnett
The Stinging Hairs of a Nettle
These hairs are much longer than ordinary plant hairs. Sharply pointed at one end, there are sacs at their bases containing acid.
Butterfly Wing Scales
Scales from the wing of a butterfly. Each scale is a hollow sac, affixed by its notched end to a pit in the insect’s wing.
The examination of cocoa for impurities is a matter rather for the chemist than for the microscopist. It contains a vast number of starch grains, not unlike those of rice, except that they are rounded. Coffee often contains a number of impurities, the chief being chicory, various starches, ground acorns and date stones. Chicory is really an impurity, though it is one often asked for by coffee-drinkers. It is easy to detect the amount of chicory present in a sample of ground coffee, by throwing a little of the mixture on to water. The chicory sinks at once, whereas the coffee floats for a while because it is oily. In pure coffee there should be no starch and the iodine test will readily show whether we are dealing with a sample free from starch or not.
Mustard is very rarely purposely mixed with any impurities, in fact it is probably the least likely to be adulterated of any article of food. Under the microscope a large number of small objects, very similar to starch grains, can be seen. They are the cells containing mustard oil and they are not stained blue by iodine. A specimen of pure mustard contains no starch. Pepper is by no means easy to test for impurities. It contains minute starch grains, which can be recognised under the microscope after staining. It is mentioned here because of a very interesting and easily performed experiment that will appeal to every microscopist. Place a little pepper on a clean slide and moisten it with a drop of alcohol, allow it to stand for a minute or so then add a little dilute glycerine, cover the specimen with a cover glass and examine it under the microscope after the lapse of about five minutes. The sight of wonderful prismatic crystals forming one by one in rapid succession will be ample reward for the trouble taken. A drop of strong nitric acid, which must not be allowed to come in contact with any part of the microscope or with one’s hands or clothes, will turn the crystals a rich orange colour. The crystals are composed of a substance called piperine.
Everyone knows of the importance of pure water for drinking purposes but the word pure in this case is used in a very wide sense, for the only really pure water is distilled water and it would not form a very good beverage. Although ordinary tap water may contain a number of impurities it is not easy to see them without taking a little trouble. If our tap water is so contaminated that a drop, examined at random, shows us all manner of solid matter floating in the water there must be something seriously wrong. Those who are engaged in testing water under the microscope, first take a big jug full of the water and allow it to stand for twenty-four hours, covering it the while to keep out dust. At the end of this time most of the water is carefully drawn off, from the top, with a siphon and the remainder, after stirring is poured into a conical glass and allowed to stand for a further twelve hours. Then the upper portion of the water is again siphoned off and a little of the remainder, which is left in the point of the conical glass is drawn by suction into a special kind of glass tube, called a pipette. This final sample contains all the solid matter which settles to the bottom of the water after standing for thirty-six hours.