APPENDIX. 401 



entertained in respect to the unassailable exactness of your 

 conclusions. 



" For the first time in the history of science, we are justified 

 in cherishing confidently the hope that, as far as epidemic 

 diseases are concerned, medicine will soon be delivered from 

 empiricism, and placed on a real scientific basis ; when that 

 great day shall come, humanity will, in my opinion, recognise 

 the fact that the greatest part of its gratitude will be due to 

 you. 



" Believe me, ever very faithfully yours, 



"JOHN TYNDALL." 



"We need scarcely say that we read this letter with the 

 liveliest gratification, and were delighted to learn that our 

 studies had received the support of one renowned in the scientific 

 world alike for the rigorous accuracy of his experiments as for 

 the lucid and picturesque clearness of all his writings. The 

 reward as well as the ambition of the man of science consists in 

 earning the approbation of his fellow-workers, or that of those 

 whom he esteems as masters. 



Mr. Tyndall has observed this remarkable fact, that in a box, 

 the sides of which are coated with glycerine, and the dimensions 

 of which may be variable and of considerable size, all the 

 particles of dust floating in the air inside fall and adhere to the 

 glycerine in the course of a few days. The air in the case is 

 then as pure as that in our double-necked flasks. Moreover, a 

 transmitted ray of light will tell us the moment when this 

 purity is obtained. Mr. Tyndall has proved, in fact, that to 

 an eye rendered sensitive by remaining in darkness for a little, 

 the course of the ray is visible as long as there are any floating 

 particles of dust capable of reflecting or difl'using light, and 

 that, on the other hand, it becomes quite obscure and invisible 

 to the same e3'"e as soon as the air has deposited all its solid 

 particles. When it has done this, which it will do very quickly 



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