pn the last edition of the "Fragments of Science," an essay on **Dust and 



Disease" followed here; but, as almost all my writings on the "Germ Theory'* 

 are now collected in a single volume entitled "Essays on the Floating Matter of 

 the Air," "Dust and Disease" no longer appears in the "Fragments." In its 

 place I venture to introduce a short article written early last year for an impor- 

 tant American magazine.] 



THE SKY' 



INVITED to write for tlie ** Forum*' an article that 

 would have brought me face to face with *' problems 

 of life and mind,'* for which I was at the moment 

 unprepared, and unwilling to decline a request so cour- 

 teously made, I offered, if the editor cared to accept it, to 

 send him a contribution on the subject here presented. 



I mentioned this subject, thinking that, in addition to 

 its interest as a fragment of '* natural knowledge," it might 

 permit of a glance at the workings of the scientific mind 

 when engaged on the deeper problems which come before 

 it. In the house of Science are many mansions, occupied 

 by tenants of diverse kinds. Some of them execute with 

 painstaking fidelity the useful work of observation, re- 

 cording from day to day the aspects of Nature or the indi- 

 cations of instruments devised to reveal her ways. Others 

 there are who add to this capacity for observation a power 

 over the language of experiment, by means of which they 

 put questions to Nature, and receive from her intelligible 

 replies. There is, again, a third class of minds that can- 

 not rest content with observation and experiment, whose 

 love of causal unity tempts them perpetually to break 



> From "The Forum," February, 1888. 



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