THE PREDICTION OF NATURAL PHENOMENA. 543 



well-observed instance. An experienced photographer knows that his 

 work will be successful, provided he carries out with care certain di- 

 rections 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 pre- 

 viously 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 ex- 

 pected. 



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 dur- 

 ing 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 conse- 

 quences. 



The examples cited will suffice to show that considerable impor- 

 tance 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 believ- 

 ing in depends, of course, on the amount of care with which the con- 

 ditions 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- 



