44 THE PRINCIPLES OF SCIENCE. 



that A is the cause of X, I must maintain invariable all 

 surrounding objects and conditions, and I must then show 

 that where A is X is, and where A is not X is not. Now 

 this cannot really be accomplished in a single trial. If, 

 for instance, a chemist places a certain suspected substance 

 in the Marsh s test apparatus, and finds that it gives a 

 small deposit of metallic arsenic, he cannot be sure that 

 the arsenic really proceeded from the suspected substance ; 

 for the impurity of the zinc or sulphuric acid might have 

 been the cause of its appearance. It is therefore the 

 practice of chemists to make what they call a Hind ex 

 periment, that is to try whether arsenic appears in the 

 absence of the suspected substance. The same precaution 

 ought to be taken in all important analytical operations. 

 Indeed, it is not merely a precaution, it is an essential 

 part of any experiment. If the blind trial be not made, 

 the chemist merely assumes that he knows what would 

 happen. Whenever we assert that because A and X are 

 found together A is the cause of X, we imply and assume 

 that if A were absent X would be absent. But wherever 

 it is possible, we ought clearly not to leave this as a 

 mere assumption, or even as a matter of inference. Ex 

 perience is ultimately the basis of all our inferences, 

 but if we can with care bring immediate experience 

 to bear upon the point in question we should not trust 

 to anything more remote and liable to error. When 

 Faraday examined the magnetic properties of the bearing 

 apparatus, in the absence of the substance to be ex 

 perimented on, he really made a blind experiment (see 



vol. ii. p. 41). 



We ought also, whenever we can, to test the sufficiency 

 and accuracy of any method of experiment by introducing 

 known amounts of the substance or force to be detected. 

 Thus a new analytical process for the quantitative esti 

 mation of an element should be tested by performing it 



