PUTEEFACTION AND INFECTION. 77 



days ago, for example, an inverted bell-jar was filled 

 with distilled water, into which, while it was briskly 

 beaten by a glass rod, was dropped a solution of mastic 

 in alcohol. The proportion was less than that employed 

 by Briicke, being about ten grains of the gum to 1 ,000 

 grains of the alcohol. The jar was placed under a sky- 

 light, at the height of the eye above the floor. It was 

 of a beautiful cerulean hue, this colour arising whoUy 

 from the light scattered by the mastic particles. 

 Looked at horizontally through a Nicol's prism, with its 

 shorter diagonal vertical, the blue light passed freely 

 to the eye. Turning the long diagonal vertical, the 

 scattered light was wholly quenched, and the jar 

 appeared as if filled with ordinary pure water. 



I tried the effect of a powerful filter upon those parti- 

 cles, and found that they passed sensibly unimpeded 

 through forty layers of the best filtering-paper.' 



The liquid containing them was examined by a 

 microscope magnifying 1 ,200 diameters. The suspended 

 mastic particles entirely eluded this power, the medium 

 in which they swam being as uniform as distilled water 

 in which no mastic whatever had been precipitated. 



The optical deportment of the floating matter of 

 the air proves it to be composed, in part, of particles 

 of this excessively minute character. The concentrated 

 beam reveals them collectively, long after the micro- 

 scope has ceased to distinguish them individually. 

 In London rooms, moreover, they are for the most part 

 organic particles, which may be removed from the air 

 by combustion. In presence of such facts, any argu- 

 ment against atmospheric germs, based upon their being 

 beyond the reach of the microscope, loses all validity. 



We are here brought face to face with a question 



• There are filters, however, which stop them ; but of this 

 immediately. 



