Fig. 17.—Soap bubbles in A (air), and B, carbon dioxide.
Before you finish experimenting you should know how to detect the presence of carbon dioxide. Take a little lime water, which may be obtained from any druggist, and pour it into a glass containing carbon dioxide. Shake the glass, and carefully observe the change which takes place. The lime water, which was previously colorless, has assumed a certain milkiness, and if allowed to stand the white powder causing this milkiness will settle at the bottom of the glass. This powder proves to be calcium carbonate, or chalk, which is always formed when lime water comes in contact with carbon dioxide, so that you have here a means of detecting the presence of carbon dioxide. Breathe into a little lime water and you will learn, from the milky appearance it at once assumes, that the air we exhale contains a certain quantity of this interesting gas.
CHAPTER XL
PHOTO PASTIMES
Camera Knights’ Experiments
It has been presumed in commencing these notes that most would-be experimenters already possess a camera, or will at least shortly do so. Thus the greater number of experiments are such as would interest a camera fiend more deeply than the ordinary reader, although the latter might still derive much enjoyment from conducting them so far as the lack of a “dark box” will allow him.
It will perhaps be as well to spend a paragraph at the outset in describing simply and noting a few peculiarities about the commonplace camera. Photography means drawing by the agency of light. Now light is reflected from an illuminated object in straight lines or rays, of which a proportion may be collected by a lens and thrown in points upon a surface behind. (See [Fig. 1], A, illuminated object; B, lens; C, surface behind lens; D, rays of light thrown upon surface C.)
Fig. 1.—Rays of light collected by lens and thrown upon surface behind.
The front of a camera contains the lens, and is provided with a movable shutter, so that light may be only allowed to enter the dark box when a picture is to be taken on one of the sensitive plates inside. According to [Fig. 2]—which represents a camera in position to photograph the object A—the light is reflected in rays, which are collected in myriads of groups and cast pointed upon the surface of the sensitized plate B. Such ray groups—being parallel when they leave the object and pointed after passing the lens—are termed pencils of light, a most applicable name when they are employed in “sketching” a portrait on the photographic plate.