38 THE FLOATING-MATTER OF THE AIR. 



The foregoing essay, as far as it relates to the 

 theory which ascribes epidemic disease to the develop- 

 ment of low parasitic life within the human life, was 

 embodied in a discourse delivered before the Eoyal 

 Institution in January 1870. In June 1871, after a 

 brief reference to the polarization of light by cloudy 

 matter, I ventured to recur to the subject in these 

 terms : What is the practical use of these curiosities ? 

 If we exclude the interest attached to the observation of 

 new facts, and the enhancement of that interest through 

 the knowledge that facts often become the exponents of 

 laws, these curiosities are in themselves worth little. 

 They will not enable us to add to our stock of food, or 

 drink, or clothes, or jewellery. But though thus shorn 

 of all usefulness in themselves, they may, by carrying 

 thought into places which it would not otherwise have 

 entered, become the antecedents of practical conse- 

 quences. In looking, for example, at our illuminated 

 dust, we may ask ourselves wliat it is. How does it 

 act, not upon a beam of light, but upon our own bodies ? 

 The question then assumes a practical character. We 

 find on examination that this dust is mainly organic 

 matter — in part living, in part dead. There are among 

 it particles of ground straw, torn rags, smoke, the pollen 

 of flowers, the spores of fungi, and the germs of other 

 things. But what have they to do with the animal 

 economy ? Let me give you an illustration to which 

 my attention has been lately drawn by Mr. George 

 Henry Lewes, who writes to me thus : 



' I wish to direct your attention to the experiments 

 of Von Kecklingshausen, should you happen not to know 

 them. They are striking confirmations of what you say 

 of dust and disease. Last spring, when I was at his 

 laboratory in Wiirzbiirg, I examined with him blood 

 that had been three weeks, a mouth, and five weeks, out 



