THE SCIENTIFIC WORK OF TYNDALL. 667 



wave length, the laws governing the dispersion of the light are 

 simple. Tyndall pursued the investigation to the case where the 

 particles have grown beyond the limit above indicated, and 

 found that the polarization of the dispersed light was effected in 

 a peculiar and interesting manner. 



Atmospheric dust, especially in London, is largely organic. If, 

 following Tyndall, we hold a spirit lamp under the track of the 

 light from the electric lamp, the dark spaces, resulting from the 

 combustion of the dust, have all the appearance of smoke. 



In confined and undisturbed spaces the dust settles out. I 

 have here a large flask which has been closed for some days. If I 

 hold it to the lamp, the track of the light, plainly visible before 

 entering and after leaving the flask, is there interrupted. This, 

 it will be evident, is a matter of considerable importance in con- 

 nection with organic germs. 



The question of the spontaneous generation of life occupied 

 Tyndall for several years. He brought to bear upon it untiring 

 perseverance and refined experimental skill, and his results are 

 those now generally accepted. Guarding himself from too absolute 

 statements as to other times and other conditions, he concluded 

 that under the circumstances of our experiments life is always 

 founded upon life. The putrefaction of vegetable and animal 

 infusions, even when initially sterilized, is to be attributed to the 

 intrusion of organic germs from the atmosphere. 



The universal presence of such germs is often regarded as a 

 hypothesis difficult of acceptance. It may be illustrated by an 

 experiment from the inorganic world. I have here, and can pro- 

 ject upon the screen, glass pots, each containing a shallow layer 

 of a supersaturated solution of sulphate of soda. Protected by 

 glass covers, they have stood without crystallizing for forty-eight 

 hours. But if I remove the cover, a few seconds or minutes will 

 see the crystallization commence. It has begun, and long needles 

 are invading the field of view. Here it must be understood that 

 with a few exceptions, the crystalline germ required to start the 

 action must be of the same nature as the dissolved salt ; and the 

 conclusion is that small crystals of sulphate of soda are uni- 

 versally present in the atmosphere. 



I have now completed my task. With more or less success I 

 have laid before you the substance of some of Tyndall's contribu- 

 tions to knowledge. What I could not hope to recall was the 

 brilliant and often poetic exposition by which his vivid imagina- 

 tion illumined the dry facts of science. Some reminiscences of 

 this may still be recovered by the reader of his treatises and 

 memoirs ; but much survives only as an influence exerted upon 

 the minds of his contemporaries, and manifested in subsequent 

 advances due to his inspiration. 



