will act as obstructions to the passage of light, and but little light will pass through that part of the negative to the photographic paper, and consequently but little of the silver salt on the paper will be changed.  On the other hand, the light portion of the negative will allow free and easy passage of the light rays, which will fall upon the photographic paper and will change much more of the silver.  Thus it is that dark places in the negative produce light places in the positive or real photograph (Fig. 84), and that light places in the negative produce dark places in the positive; all intermediate grades are likewise represented with their proper gradations of intensity.

[Illustration:  FIG. 84.—­A positive or true photograph.]

If properly treated, a negative remains good for years, and will serve for an indefinite number of positives or true photographs.

125.  Light and Disease.  The far-reaching effect which light has upon some inanimate objects, such as photographic films and clothes, leads us to inquire into the relation which exists between light and living things.  We know from daily observation that plants must have light in order to thrive and grow.  A healthy plant brought into a dark room soon loses its vigor and freshness, and becomes yellow and drooping.  Plants do not all agree as to the amount of light they require, for some, like the violet and the arbutus, grow best in moderate light, while others, like the willows, need the strong, full beams of the sun.  But nearly all common plants, whatever they are, sicken and die if deprived of sunlight for a long time.  This is likewise true in the animal world.  During long transportation, animals are sometimes necessarily confined in dark cars, with the result that many deaths occur, even though the car is well aired and ventilated and the food supply good.  Light and fresh air put color into pale cheeks, just as light and air transform sickly, yellowish plants into hardy green ones.  Plenty of fresh air, light, and pure water are the watchwords against disease.

[Illustration:  FIG. 85—­Stems and leaves of oxalis growing toward the light.]

In addition to the plants and animals which we see, there are many strange unseen ones floating in the atmosphere around us, lying in the dust of corner and closet, growing in the water we drink, and thronging decayed vegetable and animal matter.  Everyone knows that mildew and vermin do damage in the home and in the field, but very few understand that, in addition to these visible enemies of man, there are swarms of invisible plants and animals some of which do far more damage, both directly and indirectly, than the seen and familiar enemies.  All such very small plants and animals are known as microorganisms.

Not all microoerganisms are harmful; some are our friends and are as helpful to us as are cultivated plants and domesticated animals.  Among the most important of the microoerganisms are bacteria, which include among their number both friend and foe.  In the household, bacteria are a fruitful source of trouble, but some of them are distinctly friends.  The delicate flavor of butter and the sharp but pleasing taste of cheese are produced by bacteria.  On the other hand, bacteria are the cause of many of the most dangerous diseases, such as typhoid fever, tuberculosis, influenza, and la grippe.