168 FRAGMENTS OF SCIENCE. 



abolish this cloud by drying the breath previous to its 

 entering the beam ; or, still more simply, by warming 

 the glass tube. The luminous track of the beam is for 

 a time uninterrupted by the breath, because the dust 

 returning from the lungs makes good, in great part, the 

 particles displaced. After a time, however, an obscure 

 disk appears in the beam, the darkness of which in- 

 creases, until finally, towards the end of the expiration, 

 the beam is, as it were, pierced by an intensely black 

 hole, in which no particles whatever can be discerned. 

 The deeper air of the lungs is thus proved to be abso- 

 lutely free from suspended matter. It is therefore in 

 the precise condition required by Professor Lister's 

 explanation. This experiment may be repeated any 

 number of times with the same result. I think it must 

 be regarded as a crowning piece of evidence both of the 

 correctness of Professor Lister's views and of the im- 

 potence, as regards vital development, of optically pure 

 air. 1 



Application of Luminous Beams to Water. 



The method of examination here pursued is also 

 applicable to water. It is in some sense complementary 

 to that of the microscope, and may, I think, materially 

 aid enquiries conducted with that instrument. In 

 microscopic examination attention is directed to a small 

 portion of the liquid, and the aim is to detect the in- 

 dividual particles. By the present method a large portion 

 of the liquid is illuminated, the collective action of the 

 particles being revealed, by the scattered light. Care 



1 Dr. Burden Sanderson draws attention to the important 

 observation of Brauell, which shows that the contagium of a preg- 

 nant animal, suffering from splenic fever, is not found in the blood 

 of the foetus ; the placental apparatus acting as a filter, and holding 

 back the infective particles. 



