well-observed instance. An experienced photographer knows that his work will be successful, provided he carries out with care certain directions that have proved efficient.
If new gas-works or telephone-stations are to be set up in a town, it is desirable that one proceed in the same manner as has been done in the erecting of the best of similar establishments elsewhere. If this be done, then good gas-light, etc., may be guaranteed.
At this place should also be mentioned the repetition of laboratory experiments on a large scale. In such cases the results attained may astound the lookers-on, especially if nothing of the kind has been previously known, but the originator may calmly await developments after he has once made sure of the result on a small scale. It was but a short time ago that the reefs at Hell-Gate were blasted. This grand act was brought about ultimately by the pressure of a child's finger on an electric knob, and the event took place precisely as had been expected.
Of late the correct application of the law of causation has become of great importance in agricultural chemistry. It is a well-known fact that plants need for their nourishment not only water, warmth, and light, but also a quantity of certain salts contained in the soil. When wood or other vegetable fiber is burned, ashes remain; these represent the salts that the plant has abstracted from the ground during its life. Bearing this in mind, the ashes of the cereals, of clover, and other plants used for feeding purposes have been examined—the ashes of the seeds as well as of the leaves and stalks.
In connection with these investigations the so-called water-cultures of some plants were undertaken. These consist in raising plants in flasks with water, adding to this, in some cases, certain salts found in the ashes of the plant, and in other cases withholding some of these salts, in order to study their respective influence. In this manner the effect of the different constituents of the ash has been traced, and in this way the means have been found, not only of securing the proper nutriment for the products of the field and the flowers of our gardens, but of raising crops of a desired quality—in fact, of causing crops to grow on soil that would previously not bear at all. An effectual guard has thus been found against exhaustion of the soil and all its consequences.
The examples cited will suffice to show that considerable importance attaches to this class of predictions and generalizations, based on the law of causation. Generally speaking, these are the most reliable predictions that can be made. To what extent these are worth believing in depends, of course, on the amount of care with which the conditions that affect them have been observed, also on the extent to which they may be varied, and on the more or less accurate knowledge possessed as to the effects which are produced by these conditions.
Other predictions and generalizations are based on theories and hy-
