TRUSTWORTHINESS OF SCIENTIFIC KNOWLEDGE. 149 



the latter only its degree or relative quantity. Experi- 

 ments may be crude in a quantitative sense, and yet be 

 trustworthy as qualitative tests ; and a chemist may 

 deserve trust, but yet not be accurate ; for instance, he 

 may obtain a pure and complete precipitate of a substance 

 in analysis, and wash and dry it most perfectly, but failing 

 to weigh it with exactitude, the result he obtains is not 

 accurate ; or he may be accurate and yet may err ; he may, 

 for example, find a magnetic substance in tea, and may 

 weigh it with the greatest precision, and set its weight 

 down as being that of metallic iron ; but if he has not 

 proved it to be iron, his result, although accurate in 

 weight, is not to be depended upon. Strictly speaking, 

 however, a man who may not be depended upon as to 

 qualitative matter of fact cannot be accurate, because, if 

 he cannot be relied upon for the fact itself, he cannot be 

 certain in any of his quantitative statements respecting it. 

 It is of but little use to measure a thing, unless we know 

 what it is we are measuring ; and it is of less value to 

 measure an effect without knowing and measuring the 

 cause and conditions of it. Priestley was an example of 

 a chemist who was trustworthy, but not accurate. He 

 was a great qualitative investigator ; he discovered many 

 new substances, and his discoveries were real, as subse- 

 quent experience has proved ; but his experiments were 

 crude in a quantitative sense, he rarely made use of the 

 balance, and was unable to make quantitative analyses. 



In determining a qualitative fact, measurement is 

 usually unnecessary, as we continually see in the art of 

 qualitative chemical analysis. In other cases, however, by 

 means of a measurement, we obtain both a qualitative and 

 a quantitative result at the same time ; and in a few cases 

 the only method we know of obtaining a qualitative result 

 is by means of a quantitative measurement, or under 



