xix.] EXPERIMENT. 433 



of germs, even after the most careful efforts to exclude 

 them, and in the case of many diseases, both of animals 

 and plants, germs which we have no means as yet of de- 

 tecting are doubtless the active cause. It has long been 

 a subject of dispute, again, whether the plants which spring 

 from newly turned land grow from seeds long buried in 

 that land, or from seeds brought by the wind. Argument 

 is unphilosophical when direct trial can readily be applied ; 

 for by turning up some old ground, and covering a portion 

 of it with a glass case, the conveyance of seeds by the 

 wind can be entirely prevented, and if the same plants 

 appear within and without the case, it will become clear 

 that the seeds are in the earth. By gross oversight some 

 experimenters have thought before now that crops of rye 

 nad sprung up where oats had been sown. 



Blind or Test Experiments. 



Every conclusive experiment necessarily consists in the 

 comparison of results between two different combinations 

 of circumstances. To give a fair probability that A is the 

 cause of X, we must maintain invariable all surrounding 

 objects and conditions, and we must then show that where 

 A is X is, and where A is not X is not. This cannot really 

 be accomplished in a single trial. If, for instance, a 

 chemist places a certain suspected substance in Marsh's 

 test apparatus, and finds that it gives a small deposit of 

 metallic arsenic, he cannot be sure that the arsenic really 

 proceeds from the suspected substance ; the impurity of the 

 zinc or sulphuric acid may have been the cause of its 

 appearance. It is therefore the practice of chemists to 

 make what they call a Hind experiment, 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 assume that if A were absent X would be absent. 

 But wherever it is possible, we ought not to take this 

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



f V 



