132 FRAGMENTS OF SCIENCE 



would darken the air, and on the assumed impossibility 

 of their existence in the requisite numbers, without inva- 

 sion of the solar light, an apparently powerful argument 

 has been based by believers in spontaneous generation. 

 Similar arguments have been used by the opponents of 

 the germ theory of epidemic disease, who have triumph- 

 antly challenged an appeal to the microscope and the 

 chemist's balance to decide the question. Such argu- 

 ments, however, are founded on a defective acquaintance 

 with the powers and properties of matter. Without com- 

 mitting myself in the least to De la Rive's notion, to the 

 doctrine of spontaneous generation, or to the germ theory 

 of disease, I would simply draw attention to the demon- 

 strable fact that, in the atmosphere we have particles 

 which defy both the microscope and the balance, which 

 do not darken the air, and which exist, nevertheless, in 

 multitudes sufficient to reduce to insignificance the Israel- 

 itish hyperbole regarding the sands upon the seashore. 



The varying judgments of men on these and other 

 questions may perhaps be, to some extent, accounted for 

 by that doctrine of Eelativity which plays so important a 

 part in philosophy. This doctrine affirms that the impres- 

 sions made upon us by any circumstance, or combination 

 of circumstances, depend upon our previous state. Two 

 travellers upon the same height, the one having ascended 

 to it from the plain, the other having descended to it 

 from a higher elevation, will be differently affected by 

 the scene around them. To the one nature is expanding, 

 to the other it is contracting, and impressions which have 

 two such different antecedent states are sure to differ. 

 In our scientific judgments the law of relativity may also 



