AIR-GERMS AND SPONTANEOUS GENERATION. 95 



without water, and the yolk of egg, grew putrid as rapidly as in com- 

 mon air. 



The result of these experiments is, that there are spontaneous de- 

 compositions of organic substances which require nothing but the 

 presence of oxygen gas to cause them to commence; while others 

 need, besides oxygen, the presence in the atmospheric air of those 

 unknown things, which are destroyed by heat or sulphuric acid, or 

 are retained by the cotton. 



The two observers do not then decide on the nature of these things, 

 and do not assert categorically that they are germs, and, in reality, 

 nothing allows us to draw these conclusions. 



M. Pasteur's experiments have advanced the question another 

 step, by proving that they are really germs of ferments and infusoria, 

 which are destroyed by heat, or arrested by the sulphuric acid or cot- 

 ton in the experiments alluded to above. 



M. Pasteur made a hole in a window-shutter, several metres above 

 the ground, and through this he passed a glass tube .iy6 inch in 

 diameter, and containing a plug of soluble cotton .39 inch in length, 

 kept in its place by a spiral platinum wire. One of the ends of this 

 tube passed into the street ; the other communicated with a continuous 

 aspirator. When the air had passed for a sufficient time, the plug of 

 cotton, more or less soiled by the dust which it had intercepted, was 

 placed in a small tube with the mixture of alcohol and ether, which 

 dissolves gun-cotton. It was left for the space of a day. All the 

 dust was deposited at the bottom of the tube, where it is easy to 

 wash it by decantation, without any loss, if care is taken to separate 

 each washing by an interval of repose of from twelve to twenty hours. 

 The deposit, and the liquid which covers it, are put in a watch-glass 

 together ; after the evaporation of the alcohol, the remainder is placed 

 in water, and examined with the microscope. M. Pasteur also made 

 use of ordinary sulphuric acid in order to moisten the dust. This 

 agent had the effect of separating the grains of starch and calcium 

 carbonate, which are always found in greater or less quantities in 

 deposits collected on the plug of cotton. 



m^m 





Figs. 1 and 2. Organic Corpuscles of Dust, mixed with Amorphous Particles. 



Figs. 1 and 2 represent organic corpuscles, associated with amor- 

 phous particles, as seen through the microscope, under a power of 350 

 diameters; the liquid containing them was common sulphuric acid. 



