688 THE POPULAR SCIENCE MONTHLY. 



another planet beyond. But I trust that I have now made it evident 

 to you that this confident expectation is not justified by any absolute 

 necessity of N.ature, but arises entirely out of our belief in her uni- 

 formity ; and into the grounds of this and other primary beliefs, which 

 serve as the foundation of all scientific reasoning, we shall presently 

 inquire. 



There is another class of cases, in which an equal certainty is gen- 

 erally claimed for conclusions that seem to flow immediately from ob- 

 served facts, though really evolved by intellectual processes; the 

 apparent simplicity and directness of those processes either causing 

 them to be overlooked or veiling the assumptions on which they are 

 based. Thus Mr. Lockyer speaks as confidently of the sun's chromo- 

 sphere of incandescent hydrogen, and of the local outbursts which 

 cause it to send forth projections tens of thousands of miles high, as 

 if he had been able to capture a flask of this gas, and had generated 

 water by causing it to unite with oxygen. 



Yet this confidence is entirely based on the assumption that a cer- 

 tain line which is seen in the spectrum of a hydrogen-flame means 

 hydrogen also when seen in the spectrum of the sun's chromosphere ; 

 and, high as is the probability of that assumption, it cannot be re- 

 garded as a demonstrated certainty, since it is by no means incon- 

 ceivable that the same line might be produced by some other substance 

 at present unknown. And so, when Dr. Huggins deduces, from the 

 different relative positions of certain lines in the spectra of different 

 stars, that these stars are moving from or toward us in space, his train 

 of reasoning is based on the assumption that these lines have the same 

 meaning that is, that they represent the same elements in every 

 luminary. That assumption, like the preceding, may be regarded as 

 possessing a sufficiently high probability to justify the reasoning based 

 upon it ; more especially since, by the other researches of that excel- 

 lent observer, the same chemical elements have been detected as va- 

 pors in those filmy cloudlets which seem to be stars in an early stage 

 of consolidation. But, when Frankland and Lockyer, seeing in the 

 spectrum of the yellow solar prominences a certain bright line not 

 identifiable with that of any known terrestrial flame, attribute this to 

 an hypothetical new substance which they propose to call Helium, it 

 is obvious that their assumption rests on a far less secure foundation, 

 until it shall have received that verification which, in the case of Mr. 

 Crooke's researches on Thallium, was afforded by the actual discovery 

 of the new metal, the presence of which had been indicated to him by 

 a line in the spectrum not attributable to any substance then known. 



In a large number of other cases, moreover, our scientific interpre- 

 tations are clearly matters of judgment ; and this is eminently a per- 

 sonal act, the value of its results depending in each case upon the 

 qualifications of the individual for arriving at a correct decision. The 

 surest of such judgments are those dictated by what we term "com- 



