Sometimes adulterants find their way by accident into our food. A good many years ago numbers of people were poisoned by drinking beer, in some cases with fatal results. Tests were made and the beer was found to contain arsenic but how it got there remained a mystery. At length the glucose, a kind of sugar used in making beer and added also to a good many of our foods, was found to contain the substance. Now in the making of glucose, sulphuric acid is used and in this particular case the impure or commercial acid had been taken. This impure acid frequently contains arsenic and, in the case we mention, the results of its use were disastrous.
CHAPTER XI
BACTERIA
There is probably no scientific work more wedded to the microscope than the study of bacteria. We may learn a great deal about birds or insects or rocks or minerals, without any instrument but we can learn little of the bacteria unless they are highly magnified.
There is such an extraordinary amount of misconception concerning bacteria that, it will be time well spent if we attempt to clear up all misunderstanding at the start. Bacteria, often called microbes or germs, are looked upon with considerable awe by most people, who associate them in some vague way with disease. There is no denying that many bacteria are responsible for certain diseases; many more are perfectly harmless and a goodly number are exceedingly useful.
To enumerate all the bacterial activities would require a large book but briefly, apart from the disease-causing bacteria, they enter into the manufacture of cheese and butter, of wine and vinegar; they are essential to brewing and tanning; they act as scavengers over the face of the earth, breaking up a mass of decaying animal and vegetable matter into simple chemical substances which can then be used again as food for plants; some of them also can take a gas called nitrogen, from the air, and pass it on to green plants.
What are these active little creatures? The question is a natural one. They are merely very minute, one-celled plants. Each one possesses a firm cell-wall, filled with living matter; in an earlier chapter, we described the one called protean animalcule and, although it was composed of but a single cell it had no definite wall. This is one of the essential differences between plants and animals, both of them are made up of one or more, maybe millions of cells, but each plant cell is surrounded by a well-defined wall, animal cells have no such walls. The exact position of the bacteria in the plant world is still open to doubt. Most scientists place them amongst the fungi; for, with very few exceptions they possess no chlorophyll. One or two of them, however, do possess a green colouring matter which, if not chlorophyll is very near to it; on this account other scientists are of the opinion that they are related to the seaweeds. It is a matter, however, that does not concern us very deeply, for our purpose it is sufficient to know that they are plants. When they were discovered, nearly three hundred and twenty-five years ago, they were looked upon as minute animals and it is curious that the belief has survived this long period of time in the popular mind. Long before the activities of bacteria were connected with various phenomena, such as infectious diseases, souring of milk, etc., it was thought that these changes were brought about by chemical action. Like many of the early theories, this one contained a half truth, for a great many of the changes brought about by bacteria are really due to chemical action initiated by the organisms. In other words, the bacteria set free certain substances which actually cause the changes to take place.
Let us make our statement clear by a simple experiment. To a little fresh milk we add a weak acid, the milk curdles at once and by dipping a piece of litmus paper (obtained at any chemist’s) into the mixture, it will turn red, showing the presence of acid. Litmus, by the way, is obtained from a lichen; in the presence of acid it is red, an alkali, the opposite of an acid, turns it blue. In a neutral solution, that is to say one that is neither acid nor alkaline, litmus is of a purplish hue.
To continue our experiment, we allow another sample of the same milk to stand for a day or two in a warm, dark place and again the milk will be curdled. A test with the litmus will show that the solution is acid. The bacteria themselves have not curdled the milk but they have liberated a substance, called a ferment, which has split up part of the milk into an acid, amongst other things and that acid has actually done the curdling. For this reason, weak alkalies are sometimes added to milk. Acids and alkalies, of equal strength form neutral solutions, so that, when the milk bacteria begin their activities which result in the formation of acid, it is at once made neutral by the alkali. By this means, curdling is postponed for a little while, though there comes a time, of course, when all the alkali is used up, then the acid gains the upper hand and curdling takes place. We could if we wished continue adding more and more alkali to keep pace with the formation of acid, but too much alkali would be as unpalatable as too much acid, so nothing would be gained.