[401] SCIENTIFIC MANUAL. 115 



panying circumstances accurately observed, the most diffi- 

 cult task still remains to be performed, viz : The correct 

 interpretation of the answer contained in the results. 



Again, surrounding circumstances are often beyond the 

 control of the experimenter, and may vary so as to render 

 the interpretation of the results difficult: hence arises the 

 necessity for frequent repetitions of the same experiment 

 to establish a single truth. 



There arc two classes of agricultural experiments, dif- 

 fering in the surrounding circumstances. 



They may be properly designated as laboratory experi- 

 ments, and field experiments. 



Labofatory experiments embrace those surrounded by ar- 

 tificial circumstances, such as those conducted in pots con- 

 taining cither water or charred sand, to which are added 

 various fertilizing substances, either singly or in different 

 combinations, for the purpose of determining the require- 

 ments of plant nutrition. These experiments, every fac- 

 tor of which is known and under the control of the expe- 

 rimenter, are valuable, both in a scientific and practical 

 point of view, since their results are readily interpreted; 

 but the most fruhfui of practical results are 



FIELD EXPERIMENTS, 



which, having natural surroundings, are more valuable to 

 the practical farmer, since they may be locally employed 

 to ascertain the needs of particular soils and plants. 



Laboratory experiments are competent to determine gen- 

 eral principles of universal application, and thus serve as a 

 valuable guide to the conduct of field experiments. 



They ascertain definitely what conditions and substan- 

 ces are^necessary for plant nutrition; they add to our 

 knowledge of the general conditions of plant-life, and sup- 

 ply the information necessary as a basis for the conduct of 

 field experiments. They establish general principles; 

 field experiments determine local and definite facts. 



The range of field experiment is a very wide one, and 



