THE MICROSCOPE. 53 



quires a certain concentration of the nutritive fluid, in order to ac- 

 complish the spore-formation. 



7. If flakes, containing spores, are taken from a watch-glass 

 (paragraph 5, above), containing putrid, but still virulent blood, and, 

 placed in a test-tube, full of distilled water, the virulence is not de- 

 stroyed, but is retained for weeks unchanged. 



8. Such flakes may also be dried, and, after a certain time, 

 moistenen with water, and again dried, and this repeated, indefinite- 

 ly, without destruction of the virulence. 



9. A watch-glass of fresh carbon blood, placed in a room at 

 8° (46. 4*^.) remains virulent for only three days. The rods, at this 

 time, have not formed spores, and show the granular, disintegrating 

 appearance which indicates their death. — J. B. M., in Medical 

 Herald. 



On the Grains of SIlica and Micrococci of the Atmos- 

 phere. — At the period of the great debate on spontaneous genera- 

 tion between M. Pasteur and Pouchet, the latter was the first to 

 draw attention to the fact that some of the minute spherical 

 granulations discovered by the microscope in dust deposited from 

 the air in various regions of the globe, were essentially composed of 

 silica. 



That they had often been mistaken for eggs of infusoria or for 

 micrococci was very evident; but when the dust was submitted to 

 complete calcination in a platinum crucible the same grains were 

 still visible, with the same forms and dimensions as before. 



There is no doubt that the dust of the atmosphere reveals to 

 the microscope, besides the larger mineral fragments mostly of an 

 angular shape, exceedingly minute spherical or circular bodies, being 

 often not more than o-ooi of a millimetre in diameter, and very 

 similar in size and shape, which I'csist the action of a white heat in 

 contact with the air, and that of strong hydrochloric acid. In some 

 of my observations they were remarkably numerous. Both before 

 and after the action of heat they are more or less transparent. 

 What can be the origin of these singular objects? 



The same experiments repeated with siliceous algae, such as 

 those belonging to the large family of the Diatoviacece and with the 



